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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

7

Networks and Data Sources

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Federal Aviation Administration GPS Augmentation Systems

Loni Czekalski

Office of the Associate Administrator for Research and Acquisition

Federal Aviation Administration

MISSION AND USERS

The FAA has the statutory mission to establish, operate and maintain navigation systems for the safe operation of aircraft in the United States. Navigation capabilities with appropriate accuracy, integrity, and reliability allow airspace users to safely travel from one location to another and operate in the different phases of flight: approach/departure, terminal, enroute and landing. At the present time the FAA has a ground based air navigation system. The agency is beginning to transition to a space based system because it will likely provide equal or better service and be more cost effective. This is important for the users, because the FAA is currently in the process of being reinvented into an agency supported by user fees and off the federal budget. The cost of operating and maintaining the ground based system is believed to be approximately $200M per year, and a space based system will work well and cost less. The purpose of FAA GPS augmentation systems is to provide the navigation capabilities required in a cost effective manner. When they become operational they will replace existing radio navigation aids to support instrument navigation in the enroute environment and many instrument approach procedures. The WAAS signal is not secured or controlled and is available to anyone with a suitable receiver. To date, a number of antennas and aircraft receivers are under development by various companies.

WIDE AREA AUGMENTATION SYSTEM (WAAS)

The WAAS system augments the DoD provided GPS Standard Positioning System (SPS) signal in space. The WAAS will provide sufficient accuracy for precision approaches, availability and continuity of service for sole means navigation requirements as well as improvements in GPS integrity. Through a ground station network linked by FAA communications systems the WAAS will provide navigation corrections to airborne users by means of geostationary satellites.

A contract was awarded in August 1995 to Wilcox Corp. with teammates Hughes and TRW to develop and furnish the WAAS. The system is currently under development and is intended to provide a signal by early 1998.

An initial WAAS system will begin operations in late 1998 and the end state full system is planned to become operational in 2001.

The WAAS acquisition strategy was to build an expandable system with a ground component that uses already developed software with new software for integrity, availability and accuracy in independent modules. The phased introduction of the system would start with a functional verification system for testing, and use the FAA's terrestrial communications network to link ground facilities. The space component of the system will use leased communications satellite service to allow for expansion of the coverage area, new technology and phased implementation of improvements to meet performance requirements. The standard broadcast format for the WAAS augmentation message will allow for global utilization.

SYSTEM DESCRIPTION

As shown in figure 1 the WAAS is made up of eight functional parts. These functions are performed at reference stations and master stations to generate the WAAS message for broadcast to users. Each WAAS reference station (WRS) collects independent sets of data including geosynchronous satellite observables, GPS satellite observables and local troposphere observables and transmits the data to each master station. Independence of data sets is ensured by gathering observable parameters through independent sets of hardware. Data is collected at a rate consistent with the expected level of variation. For example, slowly changing troposphere data is collected less frequently than GPS satellite data. Prior to transmitting data to the master station each reference station verifies the reasonableness of the data. All data is time tagged. WAAS master stations (WMSs) provide correction processing, satellite orbit determination, verification/validation and generate the WAAS messages. WMSs collect all data received from WRSs and process it once per second. After the 250 bit WAAS message is generated it is transmitted via an FAA private data network using T-1 lines at 56KB/sec to ground earth

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 1 WAAS functions.

stations (GESs) for transmission to satellites for retransmission to users. The coverage area to the non-precision approach level of capability is the US domestic airspace except for the Alaskan peninsula west of 160 degrees longitude. This coverage is obtained by using INMARSAT geosynchronous satellites Atlantic Ocean Region East, Atlantic Ocean Region West and Pacific Ocean Region. Locations of WRSs, WMSs and satellite communications earth stations are shown in figure 2.

WAAS MESSAGE DATA

The WAAS message contains correction data for users to apply to data obtained from GPS satellites. Message data are:

  • use/don't use

  • verified satellite fast corrections

  • verified satellite long-term clock and ephemeris corrections

  • verified ionospheric grid point (IGP) locations

The data verification function verifies the integrity of all data provided to WAAS users prior to transmission and validates that data.

WAAS MESSAGE BROADCAST

The WAAS message is computed at the reference stations and master stations and transmitted over existing FAA private data network communications systems. At each GES a geosynchronous uplink subsystem (GUS) selects one WMS as it's message source and encodes the received message using 1/2 rate forward error correcting convolutional code. The resultant 500 symbol per second message is modulated on a C band signal and uplinked to the geosynchronous communications satellite for broadcast to the WAAS users on the GPS L-1 channel (1575.42 MHz). The WAAS message contains the correction data for the users of GPS satellite data. The maximum latency for the fast correction data is 5.2 seconds.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 2 WAAS concept.

PERFORMANCE REQUIREMENTS

Performance requirements for the WAAS are described for initial system, enroute through nonprecision approach and precision approach environments. The following performance requirements are for the enroute through nonprecision approach regimes with the initial WAAS requirements noted if they are different.

Availability. The total system has a required availability of .99999. Navigation system and GPS/WAAS signal in space subparts also require .99999 availability. Initial WAAS has a required availability of .999.

Accuracy. Horizontal positioning accuracy for the navigation system is .054 NM (100m) at 95% confidence and .27 NM (500m) at 99.999 % confidence.

Integrity. The probability of hazardously misleading information (HMI) coming from the GPS/WAAS signal in space is a maximum of 10 −7 per hour. Time to alarm for the total system will be no greater than 10 seconds.

Continuity of Function. Continuity of navigation services will be at least (1 - 10−5) per hour for each of the total system, navigation system and airborne elements. Continuity for the GPS/WAAS signal in space is at least (1-10−8) per hour. Continuity of fault detection, excluding outages of less than 5 minutes, is 1−(2 x 10−5) per hour for each element and the total system.

Maximum Latency of Fast Correction. The maximum latency is specified for the GPS/WAAS signal in space as 5.2 seconds.

SYSTEM OPERATION AND MAINTENANCE DATA

System monitor, control and maintenance collects all transmitted WAAS message outputs including the WAAS signal quality, ionospheric grid definition, list of GPS and WAAS satellites, UTC, and any additional manually collected data. This data will be collected and maintained for system maintenance and quality control.

The system as currently specified does not include provisions for access by the general community to data for post processing corrections. Because the communications systems used for WAAS carry surveillance and other safety of flight and security data the FAA does not allow other parties access to the system. Data would be available through the Notices to Airmen (NOTAM) system and possibly manually through the servicing maintenance organization on a case by case basis. An agreement for providing data at a port for government agency users is in negotiation.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Scientific activities with suggestions for making this data available, such as type, frequency, quantity, media/method are welcome to contact the FAA GPS/ Navigation Product Team, AND-500, at 800 Independence Ave SW, Washington, DC 20591.

GEODETIC CONSIDERATIONS

The WAAS contractor is required to provide the surveys for site location and has already begun this activity. Monuments for locating the GPS and geosynchronous communications satellite receive antennas will be US Geodetic Survey Federal Base Network Point or equivalent. The accuracy requirements specified are:

  • Horizontal: 5 cm

  • Ellipsoidal Height 10 cm

  • Orthometric Height 10 cm NAVD 88

Antenna placement errors relative to the local monument are specified to be within 1 cm horizontal and 2 cm vertical.

LOCAL AREA AUGMENTATION SYSTEM (LAAS)

The Local Area Augmentation System (LAAS) is intended as a system to provide more precise positioning capability for precision approaches. Local area in this case means from 25 to 30 miles. The FAA is in the process of conducting the analysis necessary to make a decision on the LAAS system. Because of the capital investment required and the budget environment the FAA would build the system only if the system is cost beneficial over it's expected service life. Current work is devoted to integrity/continuity, pseudolite proof of concept and system architecture. Later, specifications will be prepared and more detailed modeling and analysis conducted and a contract awarded to build the system. If all activities area completed as planned and if resources are available system specifications could be complete by the end of 1998.

REFERENCES

FAA (Federal Aviation Administration). 1995. Federal Aviation Administration Specification Wide Area Augmentation System (WAAS) FAA-E-2892A.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
U.S. Coast Guard Differential GPS Navigation Service

Gene Hall

U. S. Coast Guard

ABSTRACT

The United States Coast Guard provides a Differential Global Positioning System (DGPS) service for the Harbor and Harbor Approach (HHA) phase of marine navigation. DGPS technology is the first to economically offer geodetic accuracy meeting the Federal planning requirement of sub10 meters for harbor and harbor approach navigation. The DGPS service coverage area includes the coastal United States, Great Lakes, Puerto Rico, and most of Alaska and Hawaii. This DGPS service is available to the public navigator as an all-weather navigation sensor to supplement traditional visual, radar, and depth sounding techniques.

The design process for the United States Coast Guard's DGPS service began with efforts to define system operational requirements. The goal of these requirements was to ensure the same level of user integrity provided by present Coast Guard electronic navigation aids (Loran-C and Omega). Refinement of operational requirements by risk analysis of specific harbor navigation scenarios was then conducted. The final system architecture evolved to meet the defined requirements under traditional restraints of current technology, present and future economics, and the flexibility to adapt to future requirements.

The operational doctrine to define DGPS service parameters and the service management infrastructure has been developed. The DGPS operations phase has begun. This paper provides a brief history on the evolution of DGPS and describes the operation of the DGPS service including technical information and broadcast site specifications.

DISCLAIMER- The views expressed herein are those of the author and are not to be construed as official or reflecting the views of the Commandant or of the U.S. Coast Guard.

BACKGROUND

The U.S. Coast Guard is mandated by Federal law (14 USC 81) to implement, maintain, and operate electronic navigation aids that meet maritime needs of the U.S. armed forces and/or U.S. commerce. The U.S. Coast Guard's expertise in enhancing maritime safety through the utilization of radio (electronic) navigation services dates to 1921 with the first operational radiobeacons. In the last two decades, the U.S. Department of Defense (DOD) has led technology from terrestrial to space-based radionavigation systems, first with TRANSIT, and then the prototype NAVSTAR Global Position System (GPS).

In 1987, the U.S. Coast Guard Research and Development Center in Groton, Connecticut, began conducting research and testing of differential techniques to enhance GPS accuracy. Simply stated, the differential technique involves installing navigation equipment at a precisely known location. The equipment receives the GPS signal and compares the position solution from the received signal to its known location. The result of this comparison is then generated in the form of a correction message and sent to local users via radiobeacon broadcast. The received correction is applied by the user's GPS equipment to reduce the system position error, thereby improving the user's absolute accuracy. This effort was coordinated through the Special Committee (SC) 104 created by the Radio Technical Commission for Maritime Services (RTCM).

The differential effort was driven by the search for a system with the capability to meet the accuracy requirement for Harbor/Harbor Approach navigation as had been defined in the Federal Radionavigation Plan (FRP). The FRP identifies that accuracy on the order of less than 10 meters (2drms)1 is required for the HHA phase of navigation [FRP 94]. The FRP also states requirement for the Coastal and Ocean phases for maritime navigation which have respectively been satisfied with Loran-C and Omega services.

In 1989, the U.S. Coast Guard modified the existing marine radiobeacon located at Montauk Point, New York to broadcast differential corrections in the RTCM SC-104 format. The Montauk Point field tests demonstrated that Minimum Shift Keying (MSK)2 modulation of an existing radiobeacon signal was effective in transmission

1  

2drms means twice the distance of the root mean square error. In practice, any position fix obtained using the given system has a 95% probability of having a radial error equal to or less than the 2drms value expressed.

2  

Minimum Shift Keying is a special form of frequency modulation. MSK involves utilizing the smallest possible frequency shift of the carrier frequency to relay digital information. A shift up in frequency from the carrier relays a digital “1” and down to “0”. The actual shift in frequency is 1/4th the data transmission rate.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

of RTCM SC-104 format corrections. The MSK modulation technique could be utilized with no adverse effect on the automatic direction finding receivers of traditional marine radiobeacon users. Important to both the U.S. Coast Guard and the public, MSK technology is economical to implement at existing radiobeacons and within user receivers. By January 1990, the RTCM published the SC-104 format version 2.0 document. With a formal U.S. industry differential GPS correction standard and the initial radiobeacon broadcast success, Montauk Point began the first continuous public U.S. DGPS broadcast on August 15, 1990. This transmission marks the beginning of the U.S. Coast Guard transition from DGPS research and development towards implementation of a U.S. maritime differential GPS service.

DGPS ARCHITECTURE

The DGPS service architecture is shown in Figure 1. The functional elements of the U.S. Coast Guard DGPS Navigation Service include:

FIGURE 1 DGPS service architecture.

  • Reference Station - Precisely located GPS receiving equipment which calculates satellite range corrections based on a comparison of the satellite navigation message to its known location.

  • Integrity Monitor - Precisely located GPS receiver and MSK radiobeacon receiver which applies differential corrections. The corrected position is compared to its known location to determine if the correction broadcast from the Reference Station is in tolerance.

  • Broadcast Site - A marine radiobeacon transmitting correction data in the 285 to 325 kHz band.

  • Control Station - Site for human centralized control of the DGPS service elements. DGPS performance data processing and archiving is accomplished here. The East Coast Control Station is located at the USCG Navigation Center in Alexandria, Virginia. The West Coast Control Station is located at the Navigation Center Detachment in Petaluma, California. Both sites are manned 24 hours per day.

  • Communicative Network - An X.25 packet-switched service providing connectivity between broadcast sites and control stations.

  • DGPS User Equipment - Consists of two interfaced receivers with a display; a radiobeacon receiver for MSK demodulation and a GPS receiver capable of applying differential corrections.

TECHNICAL CHARACTERISTICS

GPS correction data based on NAD-83 coordinates is provided for both real-time and post processing applications. Real-time correction data is broadcast to the user via radiobeacon only for satellites at an elevation angle of 7.5 degrees or higher through use of the type 9-3 message in the RTCM SC-104 format. The official GPS coverage provided is based on elevation angles of ten degrees or higher. Satellites at elevation angles lower than 7.5 degrees are adversely affected by spatial decorrelation, multipath, and minimal processing time between acquisition and actual use. Corrections for a maximum of nine satellites will be broadcast. If more than nine satellites are above 7.5 degree elevation angle, a situation which occurs less than one percent of the time, then corrections are broadcast for the nine satellites with the highest elevation angles [USCG Broadcast Standard].

The latency of this information is determined by the baud rate at which it is transmitted. There are 210 bits in a type 9-3 message (three satellites corrected) including the message header. Therefore, at 100 baud the latency is 2.1 seconds. Naturally, this time is cut in half when transmitting at 200 baud. In reality, latencies on the order of 2-5 seconds are realized depending on the number of satellites in use. Other factors contributing to latency include partial decoding techniques, parity checking, and the receiver's internal processing.

GPS satellites data consisting of CA code, P1 and P2 Range, and L1 and L2 Carrier Phase information is collected every 30 seconds by the National Geodetic Survey (NGS) from both Reference Stations at each broadcast site. NGS processes the data and makes it available to the public for post processing applications. A

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

benefit to this arrangement is that NGS provides monument stability for each DGPS site by continually checking and updating geodetically surveyed antenna positions and reporting their findings to the Coast Guard. The output interval of 30 seconds for this data is set by the Control Station watchstander. Because the X.25 network is used for control, monitoring, and remote data access, limits must be set on the mount of data, not the type of data, shared. Otherwise, remote user access could interfere with and delay control station alarms. Only authorized users are allowed access to the DGPS X.25 network.

Each DGPS broadcast site houses dual Ashtech Z-12/R Reference Stations to provide redundancy. Geodetic GPS antennas are used with built in low noise amplifiers to provide the necessary RF signal gain (35 dB) for the receiver to work properly with an antenna cable up to 30 meters long [Ashtech].

Each DGPS broadcast site houses dual Trimble 4000IM MSK Integrity Monitors to provide redundancy. The Integrity Monitor MSK antenna is a near field passive loop antenna. The GPS antenna includes an omni-directional L1 GPS receiving antenna [Trimble].

SYSTEM PERFORMANCE [Broadcast Standard]

-Accuracy- The position accuracy of the USCG DGPS Service is within 10 meters (2drms) in all specified coverage areas. A reasonable approximation for determining the achievable accuracy at a given point is to take the typical error at a short baseline from the reference station (approximately 0.5 meters), add an additional meter of error for each 150 kilometers of separation from the reference station broadcast site, and add an additional 1.5 meters for the user equipment. Some high-end user sets are achieving pseudorange measurement accuracies of less that 30 centimeters in the absence or the abatement of multipath. Hence, the user with high-end equipment who is within 300 kilometers from a given broadcast can achieve accuracy better than 3 meters (2drms).

The continuous velocity accuracy of the system (i.e. the vessel's speed over ground) is better than 0.1 knots rms in VTS areas which utilize Dependent Surveillance.3

-Availability- This is defined as the percentage of time in a one month period during which a DGPS Broadcast site transmits healthy pseudorange corrections at its specified output level. The DGPS Navigation Service was designed for, and is operated to, maintain a broadcast availability level which exceeds 99.7%, assuming a complete and healthy satellite constellation is in place (i.e. HDOP<2.3). Any DGPS area of coverage that falls within a Vessel Traffic Service region which utilizes ‘dependent surveillance' for vessel tracking will maintain a signal availability in the coverage area of 99.9%. A signal availability will be higher than a broadcast availability if a coverage area receives more than one broadcast.

-Integrity- System integrity is built upon the foundation of the monitor stations. The Integrity Monitors will ensure the correction broadcast and signal strengths are in tolerance. Users are alarmed within 10 seconds if an out-of tolerance condition exists. The user equipment suite plays a significant role in assuring that the integrity of the system is preserved. It should be capable of automatically selecting the appropriate radiobeacon. A satisfactory broadcast is one which is classified as healthy, is presently monitored, and the pseudorange time out limit of 30 seconds for at least four satellites has not been reached. The user need not be within the advertised range of the broadcast for it to be satisfactory.

-Reliability- This is the probability that the service, if useable at the beginning of a mission segment (maneuver), will remain available over the course of the maneuver. Reliability is the frequency with which failures occur and is measured in the number of outages per million hours of operation as shown in Table 1.

TABLE 1

MANEUVER CATEGORY

RELIABILITY (Outages/Mhr)

<140 sec

2000

140 to 280 sec

1000

280 to 560 sec

500

-Coverage- The USCG DGPS Navigation Service is designed to provide coverage at the specified levels for all “Harbor and Harbor Approach Areas” and other “Critical Waterways” for which the U.S. Coast Guard provides aids to navigation. Due to the omni-directions nature of the broadcasts, and that a high power radiobeacon may cover more than one harbor, coverage often extends into

3  

Dependent Surveillance is any Technology which depends on active participation between the mariner and the Vessel Traffic Service to control the flow of traffic.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

additional areas. As a result, complete coverage of the coast line of the continental United States is provided out to 50 nautical miles. Coverage is also provided for the Great Lakes, most of Hawaii, Alaska, and Puerto Rico.

SITE MAP

Of the 51 sites shown transmitting corrections (Figure 2), all but for (Millers Ferry, Sallisaw, Rock Island, and Alma) are presently controlled and monitored by the Control Station. Site specific information is provided in Table 2.

PRESENT STATUS AND FUTURE PLANS

On November 1, 1995, the Coast Guard DGPS system began operation under a ‘Preoperational phase'. This phase was used to operationally test and evaluate system performance. As a result, much was learned and many improvements to the DGPS service will be made over the next few years.

On January 30, 1996, DGPS entered a ‘Initial Operational Capability' (IOC) phase in which the service is available for positioning and navigation. During IOC, enhancements to Control Station software and hardware will be accomplished, radiobeacon antennas will be upgraded to meet mission goals, transmitters will be replaced with new state-of-the-art equipment which operate with battery backup, and the DGPS service will undergo validation. All the while, coverage will be provided throughout North America with high time availability. Upon completion of IOC, the DGPS service will be declared ‘Full Operational Capability ' (FOC) meeting all availability, accuracy, integrity, and reliability performance requirements.

Discussions are ongoing with other Federal agencies for additional sites west of the Mississippi to provide coverage for navigable portions of the Missouri and Arkansas Rivers. The Walla Walla site is established in support of the Federal Railroad Administration's “Positive Train Control” study. Present needs and plans do not call for utilization of signals from GLONASS or a geostationary system such as WAAS.

FIGURE 2 DGPS sites as of March 1, 1996.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
SUMMARY

The primary mission of the DGPS service is to provide sub-10 meter accuracy for the harbor/harbor approach phase of marine navigation. This is the most important issue we face as DGPS service providers. However, other users have found innovative ways to utilize DGPS services and where feasible, the Coast Guard DGPS network has expanded to meet their needs. NOAA is locating GPS receiving equipment at some of our Broadcast sites to predict GPS signal delays caused by the neutral atmosphere. The Coast Guard encourages sharing its resources with other agencies, academia, and the scientific community as the overall cost is reduced and everyone benefits from the valuable lessons learned.

The U.S. Coast Guard will continue to fully cooperate on international fronts with the International Association of Lighthouse Authority (IALA) and the International Maritime Organization (IMO) to achieve global DGPS commonality. Nationally, the U.S. Coast Guard is consulting with other agencies to adapt the DGPS service to meet their needs. Agencies active in DGPS include the National Geodetic Survey (NGS) for inland surveying, the National Oceanic and Atmospheric Administration (NOAA) and the National Fish and Wildlife Association for hydrographic surveying, the Army Corps of Engineers (ACE) for dredging and coastal construction, the Department of Interior for natural resource mapping, the Federal Highway and Federal Railroad Administrations to name just a few.

REFERENCES

Federal Radioavigation Plan 1994, U.S. Department of Defence, DOD-4650.5 and U.S. Department of Transportation, DOT-VNTSC-RSPA-95-1, National Technical Information Service, Springfield, VA, May 1995.

Broadcast Standard For The USCG DGPS Navigation Service, COMDTIMST M16577.1, April 1993, Commandant (G-OPN), U.S. Coast Guard Headquarters, Washington, DC.

Ashtech Z-12-R Differential GPS Reference Station Technical Reference Manual, 2nd Draft, 1 November 1994.

Trimble Technical Reference Manual CDRL #A008, Final Version October 24, 1995.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

TABLE 2 United States Coast Guard DGPS Site Information

Atlantic and Gulf Coasts

Broadcast Site

Frequency (kHz)

Trans Rate (BPS)

Latitude(N)

Longitude(W)

Range (NM)

Radiobeacon ID

NAS Brunswick, ME

316

100

43 53.70

69 56.28

115

800

Portsmouth Harbor, NH

288

100

43 04.26

70 42.59

100

801

Chatham, MA

325

200

41 40.27

69 57.00

95

802

Montauk Point, NY

293

100

41 04.03

71 51.63

130

803

Sandy Hook, NJ

286

200

40 28.29

74 00.71

100

804

Cape Henlopen, DE

298

200

38 46.61

75 05.26

180

805

Cape Henry, VA

289

100

36 55.58

76 00.45

130

806

Fort Macon, NC

294

100

34 41.84

76 40.99

130

807

Charleston, SC

298

100

32 45.45

79 50.57

150

808

Cape Canaveral, FL

289

100

28 27.60

80 32.60

200

809

Miami, FL

322

100

25 43.97

80 09.61

75

810

Key West, FL

286

100

TBD

TBD

110

811

Egmont Key, FL

312

200

27 36.03

82 45.65

210

812

Puerto Rico

295

100

18 27.77

67 04.01

125

817

Mobile Point, AL

300

100

30 13.65

88 01.45

170

813

English Turn, LA

293

200

29 52.74

89 56.50

170

814

Galveston, TX

296

100

29 19.79

94 44.21

180

815

Aransas Pass, TX

304

100

27 50.30

97 03.53

180

816

Great Lakes Region

Broadcast Site

Frequency (kHz)

Trans Rate (BPS)

Latitude (N)

Longitude (W)

Range (SM*)

Radiobeacon ID

Wisconsin Point, WI

296

100

46 42.60

92 01.40

40

830

Upper Keweenaw, WI

298

100

47 13.70

88 37.50

130

831

Sturgeon Bay, WI

322

100

44 47.70

87 18.80

110

832

Milwaukee, WI

297

100

43 01.60

87 53.31

140

833

Whitefish Point, MI

318

100

46 46.28

84 57.48

80

834

Neebish Island, MI

309

200

46 19.28

84 09.04

60

835

Cheboygan, MI

292

200

45 39.21

84 27.94

110

836

Saginaw Bay, MI

301

100

43 37.72

83 50.27

85

837

Detroit, MI

319

200

42 17.84

83 05.72

100

838

Youngstown, NY

322

100

43 13.871

78 58.20

150

839

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Inland River Region

Broadcast Site

Frequency (kHz)

Trans Rate (BPS)

Latitude (N)

Longitude (W)

Range (SM *)

Radiobeacon ID

Vicksburg, MS

313

200

32 19.88

90 55.19

115

860

Memphis, TN

310

200

35 27.94

90 12.34

115

861

St Louis, MO

322

200

38 36.67

89 45.50

115

862

Rock Island, IA

311

200

42 00.50

90 14.00

150

863

Alma, MN

317

200

44 18.25

91 54.23

150

864

Millers Ferry, AL

320

200

32 05.40

87 23.73

150

865

Sallisaw, OK

299

200

35 22.00

94 49.00

100

866

Kansas City, MO

305

200

39 07.07

95 24.88

100

867

Louisville, KY

TBD

         

West Coast Region

Broadcast Site

Frequency (kHz)

Trans Rate (BPS)

Latitude (N)

Longitude (W)

Range NM

Radiobeacon ID

Cold Bay, AK

289

100

55 11.41

162 31.90

180

898

Kodiak, AK

313

100

57 37.13

152 11.35

180

897

Kenai, AK

310

100

60 40.10

151 21.00

170

896

Potato Point, AK

298

100

61 03.00

146 42.00

100

895

Cape Hinchinbrook, AK

292

100

60 14.30

146 38.80

120

894

Gustavus, AK

288

100

58 25.50

135 41.80

170

892

Annette Island, AK

323

100

55 04.33

131 36.50

170

889

Whidbey Island, WA

302

100

48 18.76

122 41.77

90

888

Robinson Point, WA

323

200

47 23.25

122 22.49

60

887

Walla Walla, WA

TBD

         

Fort Stevens, OR

287

100

46 12.29

123 57.36

180

886

Cape Mendocino, CA

292

100

40 26.40

124 24.40

180

885

Point Blunt, CA

310

200

37 51.18

122 25.14

60

884

Pigeon Point, CA

287

100

37 11.22

122 23.40

180

883

Point Arguello, CA

321

100

34 34.70

120 38.60

180

882

Point Loma, CA

302

100

32 39.92

117 14.58

180

881

Kokole Point, HI

300

200

21 59.00

159 45.50

300

880

Upolu Point, HI

285

100

20 14.80

155 53.20

170

879

*

Great Lakes and Western Rivers DGPS sites indicate radiobeacon ranges in statute milea, all others are in nautical miles.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Bay Area Regional Deformation (BARD) Array

Nancy King and William Prescott

U. S. Geological Survey

INTRODUCTION

In 1993, the Bay Area Regional Deformation (BARD) permanent Global Positioning System (GPS) array began operation. The ten BARD stations span three major parts of the Pacific - North American plate boundary: the San Andreas, Hayward - Rodgers Creek, and Calaveras - Green Valley faults. Previous geodetic and geologic observations imply 38 mm/yr across the central Bay Area, with 20 mm/yr of deep slip on the San Andreas fault, 9 mm/yr of deep slip on the Hayward and Calaveras faults, and 5 mm/yr of surface creep on the Hayward fault (Lienkaemper et al. 1991; Lisowski et al., 1991; Savage and Lisowski 1993). The BARD stations provide a 233 km long profile across the plate boundary, with stations concentrated near the Hayward fault in the San Francisco Bay Area. Station FARB, on the Farallon Islands on the Pacific plate, is as far west of the San Andreas fault as it is possible to be in northern California. Station CMBB ties FARB and the Bay Area stations to the Sierra Nevada foothills.

The scientific goals of the BARD array are precise measurement of crustal deformation over distances not possible before GPS, observation of signals related to earthquakes, analysis of error (including the current problem of monument noise), and the development of the capability to monitor deformation in near real-time.

SITES, MONUMENTS, AND OPERATION

The U.S. Geological Survey (USGS), the Seismographic Station at the University of California, Berkeley (UCB), Trimble Navigation, and Stanford University jointly operate the BARD array. Table 1 lists the operator, receiver type, and installation date for each station. All sites have dual-frequency P (now Z) code receivers. The sampling interval is 30 seconds. Data is stored in receiver memory until the close of the UTC day, and then downloaded over telephone lines.

UCB operates the Northern California Earthquake Data Center (Romanowicz et al., 1994), at which all BARD data are available in both raw and RINEX format. Usually, data for a given day is on-line about 18 hours later. The World Wide Web page is at http://www.quake.geo.berkeley.edu.

Two USGS stations (CHAB and WINT) began operation in September 1991. The monuments are in alluvium. Each of the 4 monument legs is a 3/4 inch (19 mm) stainless steel rod. One vertical rod and 3 rods inclined at about a 30 degree angle, each driven to a depth of about 12 m, are welded together at the top. Since these sites are in an urban area, all equipment is inside a fiberglass enclosure. King et al. (1995) describe the design and history of these sites.

USGS and Trimble installed two stations (MOLA and NUNE) in October 1993. These receivers are Trimble SSE4000's. MOLA is in rock, NUNE in alluvium. The original monuments were like those at CHAB and WINT. Later modifications included the replacement of fiberglass enclosure lids with plexi-glass, the re-setting of each monument to bring the antenna as close as possible to the top of the en-closure, and adjustments at NUNE to accommodate possible slope instability. These modifications caused data gaps and offsets.

UCB installed Ashtech Z-12 receivers (one of them contributed by Lawrence Livermore National Laboratory) at BRIB, CMBB, FARB, HOPB, and TIBB were installed in 1993-1995. Each monument is a stainless-steel rod set into a drill hole in competent rock. Each antenna screws directly onto the top of the monument. A plexiglass cover protects the antenna. The receiver and auxiliary equipment are in nearby buildings.

The Earth Science and Aeronautics departments at Stanford University installed a Trimble SSE4000 receiver at station SUAA in April 1994. This station is on top of a building, and may not be stable enough to measure tectonic deformation.

USGS processes the data daily, using precise orbits from IGS, with the GIPSY-OASIS II software developed by the Jet Propulsion Laboratory (Webb and Zumberge, 1993). Solutions and plots are available on the USGS-maintained BARD World Wide Web page (http://quake.wr.usgs.gov/QUAKES/geodetic/bard). In order to make the BARD array useful to other agencies, the National Geodetic Survey (NGS) has positioned several of the stations in the NAD83 reference frame. For more information, ftp to the NGS computer (proton.ngs.noaa.gov).

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

TABLE 1 Stations of the BARD Array

Station

Operator

Receiver

Start Date

BRIB

UC Berkeley

Ashtech Z-12

Aug. 14, 1993

CHAB

USGS

Ashtech Z-12

Sep. 12, 1991

CMBB

UC Berkeley

Ashtech Z-12

Dec. 09, 1993

FARB

UC Berkeley

Ashtech Z-12

Jan. 30, 1994

HOPB

UC Berkeley

Ashtech Z-12

Aug. 26, 1995

MOLA

USGS/Trimble

Trimble SSE4000

Oct. 15, 1993

NUNE

USGS/Trimble

Trimble SSE4000

Oct. 15, 1993

SUAA

Stanford

Trimble SSE4000

Apr. 21, 1994

TIBB

UC Berkeley

Ashtech Z-12

Jun. 18, 1994

WINT

USGS

Ashtech Z-12

Sep. 12, 1991

RESULTS

For daily measurements from September 1993 through December 1994, the rms scatter about the best linear fit is typically 10 to 20 mm for vertical, and 2 to 3 mm for north, east, and length. Relative to the Farallon Islands on the Pacific plate, the velocity of Columbia, in the Sierra foothills, is 42.4 ± 1.0 mm/yr, oriented S42o ± 2oE. This implies that the San Andreas fault zone accommodates approximately 90 percent of the Pacific-North America relative plate motion estimated by the NUVEL-1 model (Demets et al., 1990).

Equipment changes sometimes caused offsets. These changes included the addition of a line amplifier at BRIB and antenna swaps at CHAB and WINT. However, not all antenna swaps caused offsets. This disturbing result emphasizes the importance of continuity at permanent stations, and documentation whenever changes are necessary.

IMPROVEMENTS TO THE BARD ARRAY

The BARD array will soon double in size. UC-Berkeley plans to install 13 new sites in 1996. One will be at Parkfield in central California, one will be at Yreka in northern California, and the others will fill in the existing profile. Several will be collocated at broadband seismic stations operated by UC Berkeley. USGS recently installed a station near Mammoth Lakes, California.

This year, we plan to replace the antennas at existing BARD sites with choke-ring models. At CHAB and WINT the antennas are currently inside fiberglass enclosures. Data collected under the fiberglass lids does not appear to be noisier than data collected in the open. However, we will soon replace the fiberglass enclosure lids with plexiglass covers.

There is now a delay of about 18 hours between the end of the UTC day and the availability of that day's data at the Northern California Earthquake Data Center. This delay is not a problem now, since we process 24 hours of data at a time and the IGS orbits are not available for a week or two. However, eventually we plan to process data in near real-time, and other agencies may also want BARD data quickly. Therefore, we plan to upgrade our data collection. UC Berkeley plans to install frame relay telemetry at several sites. This will allow real-time data down-loading. At sites without frame relay telemetry, the crucial factor controlling data availability is the time it takes to download data over the telephone, one site at a time, to the controlling computer at each institution. Additional computers and faster modems would improve data availability significantly. We will try to make such improvements as soon as we, or other agencies, need faster access to BARD data.

In September 1995, we provided 5-second data for the National Geodetic Survey's San Francisco Bay Project. With the equipment and software we now use, this was not easy or routine. The improvements described above, plus more receiver memory, would make the collection of 5-second data practical. Eventually, we want to use such data for monitoring deformation in near real time. We would make this data available for, say, 30 days, and then store 30-second data in the archive. In the meantime we may be able to collect data at a high sampling rate, upon request, for special projects.

ACKNOWLEDGMENTS

We thank M.H. Murray, R. Clymer, B. Romanowicz, and B. Frohring, for their contributions.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
REFERENCES

DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein, 1990, Current plate motions, Geophys. J. Int, 101, 425-478.

King, N. E., J. L. Svarc, E. B. Fogleman, W. K. Gross, K. W. Clark, G. D. Hamilton, C. H. Stiffler, and J. M. Sutton, 1995, Continuous GPS observations across the Hayward fault, California, 1991-1994, Journal of Geophysical. Research., 100, 20,271-20,283 , 1995.

Lienkaemper, J. J., G. Borchardt, and M. Lisowski, 1991, Historic creep rate and potential for seismic slip along the Hayward Fault, California, Journal of Geophysical. Research., 96, 18,261-18,283.

Lisowski, M., J.C. Savage, and W.H. Prescott, 1991, The velocity field along the San Andreas fault in central and southern California, Journal of Geophysical. Research., 96, 8369-8389.

Romanowicz, Barbara, Douglas Neuhauser, Barbara Bogaert, and David Oppenheimer, 1994, Accessing northern California earthquake data via Internet, Eos, Transactions, American Geophysical Union, 75, 257,259-260.

Savage, J.C., and M. Lisowski, 1993, Inferred depth of creep on the Hayward Fault, California, Journal of Geophysical. Research., 98, 787-793.

Webb, F.H., and J.F. Zumberge, 1993, An Introduction to GIPSY/OASIS-II, PL Publication D-11088, Jet Propulsion Laboratory, Pasadena, CA.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Scripps Orbit and Permanent Array Center (SOPAC) and Southern California Precision GPS Geodetic Array (PGGA)

Yehuda Bock, Jeff Behr, Peng Fang, Jeff Dean, Rosemary Leigh

SOPAC, IGPP, Scripps Institution of Oceanography

INTRODUCTION

The Scripps Orbit and Permanent Array Center (SOPAC) is the central operating center for the Southern California Permanent GPS Geodetic Array (PGGA) and serves as one of seven IGS Analysis Centers producing improved ephemerides for the GPS constellation. In addition to these duties SOPAC serves as one of three primary archives of IGS global tacking data as well as a principal archive of continuous GPS data from the Southern California Integrated GPS Network (SCIGN).

CONTINUOUS GPS IN SOUTHERN CALIFORNIA

Beginning in 1990, researchers from Scripps Institution of Oceanography, the Jet Propulsion Laboratory and the Massachusetts Institute of Technology initiated the Permanent GPS Geodetic Array (PGGA) (Bock, 1991). By the summer of 1992, 7 stations had been deployed: GOLD, PIN1, PIN2, JPLM, ROCH, SIO1, VNDP and HARV (Table 1). The initial array successfully recorded region-wide, coseismic and postseismic displacements caused by the June 28, 1992 Landers earthquake sequence (Blewitt et al., 1993; Bock et al., 1993; Wdowniski et al., 1996). By mid-January 1994 the PGGA numbered 13 sites and had been geographically extended to the north and east with the addition of continuously operating sites at QUIN, CASA, PVEP, MATH, and BLYT (Table 1). Examination of continuous data following the January 17, 1994 Northridge earthquake indicated coseismic displacements occurred at only PVEP and JPLM which terminate a baseline spanning the Los Angeles Basin (Zhang et al., 1994).

A renewed burst of site development followed the extensive damage associated with the Northridge earthquake. The primary focus of the expansion was the study of strain accumulation in the more complex blind-thrust environment of the LA Basin. By the end of 1994, an additional 13 sites had been deployed in what has come to be known as the LA Basin Dense GPS Geodetic Array, the densely-distributed counterpart to the PGGA. Both of these arrays now comprise the cooperative Southern California Integrated GPS Network (SCIGN) maintained by the researchers of the Southern California Earthquake Center (SCEC). (See the Acknowledgments section for a list of SCIGN collaborators).

Notwithstanding the success in measuring coseismic displacements, the primary purpose of the PGGA is to define the nature of interseismic crustal motions in California and to serve as a regional reference frame for field GPS measurements. While the last two years of continuous GPS development in Southern California have focused on deformation measurement in the LA Basin, the PGGA is expanding to the east and southeast of the LA Basin to monitor deformation associated with the North American/Pacific plate boundary, especially that associated with the San Andreas fault and the Salton trough. The SCIGN array is made up of Ashtech, Rogue, Trimble and TurboRogue P-code receivers. While the PGGA now consists of 20 sites, it is expected to increase by an additional 7 sites in southern California, Baja California and western Arizona by the summer of 1996 (Figure 1).

PGGA SITES

The principal requirement when installing a GPS receiver to study crustal deformation is that the site represent, as well as possible, the actual movement of the crustal block on which it is placed. Researchers at Scripps have tried to achieve this goal by either deploying monuments in bedrock or on presumably stable structures, or by using one of a number of deep-referenced monument designs. Examples of the first case are sites such as COSO and MATH which use steel rods cemented into drill holes in bedrock or PVEP which uses a stainless steel rod cemented into a hole in the top wall of a former Nike missile silo. A similar method is used at the CRFP site where a stainless steel rod is cemented into the roof of a two-story concrete building.

In the second case, monument designs reflect research that indicates that the greater the depth from the surface of the earth, the more stable the point. A comparison study of ~2 years of two-color geodimeter data to two sets of co-located shallow and deep-referenced monuments indicates an order-of-magnitude decrease in random walk motion of the deep-referenced

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

monument (Langbein et al., 1995). This method has two requirements: greater depth of attachment to the earth and isolation of the monument legs from near-surface noise. To achieve this, two designs have been implemented (Wyatt et al., 1989). The first (Type A), at sites PIN1, SIO3 and VNDP, employ 3 sets of 3 steel legs installed to ~10m depth (one vertical, the others at ~90° to one another, ~35° from vertical) at the vertices of an equilateral triangle. By introducing compliant material around the upper 3-4 m of the anchoring legs, they are insulated from near-surface ground displacements. Each of the three sets of rods or pipes are terminated in a single weld point at ground surface and a concrete pad is built upon, and tied into, the weld points. A monument marker is placed at the center of the pad. Upon this pad is installed a ~1.75 m-high tripod, each leg of the tripod mounted on one of the three deep-referenced piers.

TABLE 1 Site Information for the Southern California Permanent GPS Geodetic Arraya

Site Name

Site ID

Date Installed

Receiver Type

Operating Agency

SOPAC Download

Monument Type

Goldstone

GOLD

12/15/89

Rogue SNR-8

JPL

Internet

25' Steel Microwave Tower

Pinyon Flat 1

PIN1

2/9/90 A

Ashtech Z-XII

SIO

Telephone

Deep-referenced A

Pinyon Flat 2

PIN2

3/3/90

Trimble 4000SST

SIO

Telephone

Deep-referenced B

JPL

JPLM

3/20/90

Rogue SNR-8000

JPL

Internet

Concrete slab w/level plate

Pinemeadow

ROCH

4/19/91

Trimble 4000SST

SIO

Telephone

Stainless Rod in Granite

Scripps 3 b

SIO3

7/8/93

Ashtech Z-XII

SIO

Telephone

Deep-referenced A

Vandenberg

VNDP

5/5/92

Ashtech Z-XII

SIO-MIT-JPL

Telephone

Deep-referenced A

Harvest Platform

HARV

6/17/92

Rogue SNR-8000

JPL

Internet

Oil Platform

Quincy

QUIN

9/6/92

Rogue SNR-8000

JPL

Internet

Concrete slab w/level plate

Mammoth Lakes

CASA

1/20/93

Rogue SNR-8000

JPL

Internet

FLINN Concrete Pillar

Lake Mathews

MATH

4/14/93

Trimble 4000SSE

RCFC-SIO

Cellular

Stainless Rod in Bedrock

Palos Verdes

PVEP

5/17/93

Trimble 4000SSE

CIT-SIO

Telephone

Stainless Rod in Bunker

Blythe

BLYT

1/13/94

Ashtech Z-XII

RCS-SIO

Telephone

Driven Stainless Tripod

Monument Peak

MONP

3/31/94

Ashtech Z-XII

SIO-NASA

PC/Internet

Deep-referenced B

Bommer Canyon

TRAK

6/2/94

Ashtech Z-XII

OC-SIO

Telephone

Driven Stainless Tripod

Yucaipa

CRFP

6/15/94

Ashtech Z-XII

SIO-UCLA

Telephone

Stainless Rod in Rooftop

Ensenada, Mex.

CICE

3/28/95

Rogue SNR-8000

JPL-INEGI

Internet

Concrete Pillar (20'x20'x20')

Parkfield

CARR

5/31/95

Rogue SNR-8000

JPL

Internet

Antenna Mast w/level plate

China Lake

COSO

8/16/95

Ashtech Z-XII

GPO-SIO-MIT

RF modem

Stainless Rod in Granite

Durmid Hill

DHLG

4/24/96

Ashtech Z-XII

SIO

Telephone

Deep-referenced B (Driven)

a Receivers listed are those installed before May 1996; check individual site logs for complete site history. Institutions overseeing or assisting in the development of sites or subsets of PGGA sites: SIO, Scripps Institution of Oceanography; JPL, Jet Propulsion Laboratory; MIT, Massachusetts Institute of Technology; RCFC, Riverside County Flood Control; RCS, Riverside County Surveyors Office; OC, Orange County Environmental Management Office; GPO, Geothermal Program Office of the China Lake Naval Weapons Center; CIT, California Institute of Technology; UCLA, University of California, Los Angeles; INEGI, Mexican National Survey Office. The COSO site was downloaded over a combination µwave-fiber optic-telephone link until switched to an RF modem-telephone link in April 1996.

b SIO3 follows continuous site SIO1 installed February 10, 1990

The advantage of this design is that there is an identifiable ground-surface mark which can be used to site in an antenna or reference to other survey points. A disadvantage is the expense and difficulty in installing such a mark. A second type of deep-referenced monument (Type B) uses 4-5 steel legs (one vertical, 3-4 at 35° to vertical) terminated in a single weld point 1-2 m above ground level. The antenna is mounted directly above the weld point. In this case there is no ground pad nor strictly defined horizontal survey marker except for the antenna screw mount itself. The vertical reference point is a drill hole in the side of the vertical leg defining

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

the elevation of the mark. This monument has been installed at PIN2 and MONP (See Table 1) and has been adopted for the planned expansion of the SCIGN network in 1996 (Schematic diagrams for monument type B can be found on WWW URL http://www-pfo.ucsd.edu).

All PGGA sites are downloaded within 0-3 hours of the end of each day, twelve by Scripps and 7 by JPL. SIO data download is executed via public telephone, cellular telephone, radio modem or directly to an on-site PC connected to Scripps by Internet. All downloads are automatic, initiated by PC-based scripts at 00:05 UTC. Once the data arrive at Scripps they are translated from raw receiver files into the RINEX file format. Both raw and RINEX files are moved to UNIX hard disk and copied to the SOPAC Garner Archive where they may be accessed over the Internet via anonymous ftp (Appendix A). All PGGA sites utilize on-site trickle-charged batteries or uninterruptible power supply for receiver and communications power backup.

FIGURE 1. Map of the Southern California Permanent GPS Geodetic Array (PGGA). Solid triangles over open circles represent existing PGGA sites, open triangles over solid circles represent proposed sites to be installed before fall 1996. Sites QUIN and CASA not shown. Two and three-parameter meteorological packages (P and T, or P, T and relative humidity) will be deployed at several of these sites as well as a number of sites in the dense component of the Southern California Integrated GPS Network (not shown) during 1996. The five boxes trending across the southern portion of southern California represent individual Synthetic Aperture Radar (SAR) images. Several of the proposed sites are projected for regions of SAR image overlap.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Over the last two years, significant advances have been made in the science of GPS based estimation of precipitable water. SOPAC will capitalize on these advances in the spring of 1996 by deploying 18 meteorological packages at PGGA and DGGA sites. These packages employ quartz-crystal barometers with resolution better than 1 µbar accurate to 0.01% of reading. Barometers will be stable to better than 0.1 mbar/yr. Temperature data will be recorded to 0.1°C with a total accuracy of 1°C. The daily data from these packages will be incorporated into the SOPAC analysis routines to calculate half-hourly estimates of the total precipitable water content in the atmosphere over southern California.

In addition to activities associated with the PGGA, SOPAC has worked in support of U.S. Geological Survey researchers on the development of sites in the dense component of the SCIGN array in the Los Angeles Basin. SOPAC assisted with the development and download of new sites while the USGS download infrastructure was put in place. SOPAC has provided researchers at the USGS in Pasadena, CA, USGS, Menlo Park, CA, the University of Washington and the University of Hawaii with the Data Acquisition Software Suite (DASS), PC-based command files which interface with remote download software developed by the manufacturers of the Ashtech, Trimble and Rogue receivers to run automatic download of continuously operating stations. The DASS package is available to all interested parties via anonymous ftp to the SOPAC archive.

SOPAC GPS PROCESSING

Since November 1991, the SOPAC Analysis Center has been analyzing global GPS data to provide precise satellite ephemerides to the GPS community. The SOPAC analysis group now operates as one of 7 IGS analysis center. Since July 1995, SOPAC has been providing single day, near-real-time precise ephemerides for rapid analysis of southern California tectonic motions and as development towards real-time precipitable water estimation for weather forecasting (Fang and Bock, 1995). The near-real-time orbits are of two types, rapid and predicted ephemerides. The rapid orbits are calculated using a subset of global sites which show rapid download times and frequent availability and provide strong geographic control over the satellite arcs. The orbit is commonly available on the SOPAC archive within 12-14 hours of the end of the modeled Julian day. The predicted orbits are calculated for Julian day, J, from a Kalman filter projection of a three-day arc of days J-3 through J-1. This solution, still somewhat experimental, is being driven strongly by the requirements of weather forecasting scientists using precipitable water estimates determined using GPS meteorology (Bevis et al., 1992; Duan et al., 1996).

In addition to global processing, SOPAC also analyses upwards of 60 sites per day in subnetworks of ~15-30 sites. Subnetworks are selected primarily on the basis of geography but have also been designated on the basis of application (Central U.S. GPS Met, LA Basin tectonics) or receiver type. Presently, the four subnets are the European network, the CORS network (a set of ~17 sites equipped with meteorological equipment in the central and eastern U.S., used to estimate precipitable water in the atmosphere), the DGGA and the PGGA subnets of the SCIGN array.

To analyze this GPS data, SOPAC uses the GAMIT/GLOBK software package developed at MIT in the late 1980s (King and Bock, 1996; Herring 1996). Because SOPAC processes so many sites each day (~110 sites per day, not including rapid and predicted orbits) we use a method of distributed processing (Zhang et al., 1995) to relieve the computational burden and minimize processing times. Four regional and two global subgroups are analyzed using the method of least-squares adjustment (GAMIT) providing improved orbital ephemerides, earth-orientation parameters and site coordinates. Final estimates of ephemerides and site coordinates are calculated using a Kalman filter (GLOBK) to combine variance-covariance matrices from each of the 6 solutions for a seven day span of data, one final solution per GPS week output in the new SINEX format. There is normally a delay of approximately 5 days between the end of a GPS week and the completion of that weeks combination solution in GLOBK. After each weekly solution, the time series of positions and baseline time series are posted to SOPAC's Homepage on the World Wide Web (Appendix A). They are then examined for outliers and anomalous signals and used to quantify the effects of site equipment changes or disturbances. Finally, the time series are statistically analyzed to assess the magnitude of random walk and random error for each site as a measure of site/monument stability. (Zhang et al., 1996)

The Scripps orbits show good agreement with those of the other analysis centers, displaying rms repeatability of ~10 cm/day with respect to the final IGS orbit generated in a combination of all 7 analysis center orbits. Initial predicted orbits have shown rms repeatability on the order of 30-50 cm. In addition, the time series of site positions for the PGGA, DGGA and BARD arrays in California have shown precision on the order of 1-2 mm/yr in the horizontal and 4-5 mm/yr in the vertical.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
SOPAC GARNER ARCHIVE

The SOPAC Garner Archive holds RINEX data from 1991 to the present for the continuous GPS sites of the IGS Global Network. The SOPAC archive is one of three primary archives (with the CDDIS and the IGN) for IGS data. IGS data are downloaded to SOPAC automatically over the Internet, usually within 2 hours of their posting on one of several regional archives. In addition to the global data set, the Scripps archive has, over the past three years, become a primary or secondary archive for raw and RINEX data from the SCIGN, BARD and CORS arrays. All raw RINEX data for these networks are available on the Garner archive through either anonymous ftp or the World Wide Web (Appendix A). In addition to RINEX data, SOPAC archives daily, receiver-image files for the 23 sites of the SCIGN array maintained by Scripps and the US Geological Survey.

In addition to raw receiver files, the SOPAC archive holds precise ephemerides for the GPS constellation calculated by the 7 primary IGS analysis centers. For more near-real time needs, the SOPAC analysis center provides both a rapid and a predicted orbit in it's “combinations ” directory.

The SOPAC archive presently utilizes a magneto-optical jukebox with storage slots for 32 1.2 GB platters, plus ~10 GB in hard disk space to improve access times to the most recently downloaded and processed data. In all, this provides a continuously accessible archive capacity of ~50 GB. As a consequence of the limited number of slots in the jukebox, not more than 120 days of the most recent global and regional RINEX data are normally kept on-line. To get around this constraint we take requests from users for specific data spans, placing requested data on-line in the order requests are received. It is usually less than a one day delay between the time a request is received and the time that data are on-line. Users are notified by electronic mail when their data are available. It is expected that SOPAC will be acquiring new hardware with more than 150 GB of storage capacity by fall 1996. This should allow continuous unattended access to all global and regional RINEX data as well as a wide variety of global and regional solution files. Increase in the SOPAC archive capacity will certainly be necessary should the archive access continue to increase in the coming years as it did during 1995.

FUTURE RESEARCH AT SOPAC

The Scripps group continues to be involved in new areas of GPS research. At present, the analysis wing is reprocessing all global and regional data from 1991 through 1995 to take advantage of improvements in the GAMIT software. It is expected that position estimates from this reanalysis will better illustrate the southern California velocity field, especially within the densely instrumented LA Basin. Additionally, characterization of the PGGA and DGGA sites on the basis of the resulting time series may lead to the development of standards for continuous GPS site installation.

Continuing effort will be made to determine the fidelity and utility of GPS estimates of the total zenith delay in the field of GPS meteorology. By summer 1996, eighteen of the SCIGN sites should be instrumented with barometers and thermometers so that precipitable water may be modeled. Researchers from SOPAC, the University of Hawaii and NOAA 's Forecast System's Laboratory will continue to work together on modeling precipitable water for the purpose of weather forecasting over the central US. Attempts are being made to bring more data from the Southern Hemisphere into analysis centers sooner to provide greater global coverage of the satellite orbits and improve the rms of predicted ephemerides. Should such orbits continue to fit within 20-30 cm rms with respect to the final IGS orbits, the possibility for the use of near-real-time estimates of precipitable water as an input into meteorological models will be enhanced.

The newest project being addressed by SOPAC and other Scripps researchers is the integration of GPS with Interferometric Synthetic Aperture Radar (INSAR) to determine interseismic deformation with dense spatial resolution and GPS accuracy. Precisely positioned GPS sites within SAR image sidelap zones (Figure 1) will be equipped with radar reflectors in order to calibrate and minimize satellite orbital (‘baseline') errors. Continuous GPS sites equipped with meteorological sensors and data from meteorological satellites will be used to interpolate atmospheric water vapor over southern California and correct SAR interferograms for artifacts caused by tropospheric refraction.

ACKNOWLEDGMENTS

We thank Paul Tregoning and Jie Zhang for their contributions. We would like to thank our many colleagues participating in the Southern California Integrated GPS Network (SCIGN), the Crustal Deformation Working Group of the Southern California Earthquake Center, local and state agencies in California, and the International GPS Service for Geodynamics (IGS) for their support. There are several groups which have taken part in the development of the PGGA and the SCIGN array. We would like to take a few lines to list the institutions which have contributed over the years to this project:

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Scripps Institution of Oceanography

Scripps/IGPP Pinyon Flat Geophysical Observatory

NASA-Jet Propulsion Laboratory

U.S. Geological Survey

Massachusetts Institute of Technology

Orange County Environmental Management Agency

Riverside County Flood Control and Water Conservation District

Riverside County Surveyor

Vandenberg Air Force Base

Geothermal Program Office, China Lake Naval Air Weapons Station

California Institute of Technology

University of California, Los Angeles

Crafton Hills Community College

This work was supported by NASA NAGW-2641 and NAG-5-1917, SCEC PO 569930, USGS 14-08-0001-G1673, 1434-92-G2196 and 1434-95-G2629, NSF EAR 92 08447 and EAR 94 16338, Riverside County Flood Control and Water Conservation District, Riverside County Transportation Division, and the California Dept. of Transportation (Caltrans).

SCEC Reference Number 328

REFERENCES

Bevis, M., S. Businger, T. Herring, C. Rocken, R. Anthes and R. Ware. 1992. GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning System, J. Geophys. Res., 97, pp. 15,787-15,801.

Blewitt, G., M.B. Heflin, K.J. Hurst, D.C. Jefferson, F.H. Webb and J.F. Zumberge. 1993. Absolute far-field displacements from the 1992 Landers Earthquake Sequence, Nature, 361, pp. 340-342.

Bock, Y. 1991. Continuous Monitoring of Crustal Deformation, GPS World, June 1991, pp. 40-47.

Bock, Y. 1994. Crustal Deformation and Earthquakes, Geotimes, 39, pp. 16-18.

Bock, Y., C.C. Agnew, P. Fang, J.F. Genrich, B.H. Hager, T.A. Herring, K.W. Hudnut, R.W. King, S. Larsen, J.B. Minster, K. Stark, S. Wdowinski and F.K. Wyatt. 1993. Detection of Crustal Deformation from the Landers Earthquake Sequence Using Continuous Geodetic Measurements, Nature, 361, pp. 337-340.

Bock, Y., S. Wdowinski, P. Fang, J. Zhang, J. Behr, J. Genrich, D.C. Agnew, et al.Southern California Permanent GPS Geodetic Array: Continuous Measurements of Crustal Deformation, submitted to J. Geophys. Res., 1996.

Duan, J., M. Bevis, P. Fang, Y. Bock, S. Chiswell, S. Businger, C. Rocken, F. Solheim, T. Van Hove, R. Ware, S. McClusky, T.A. Herring and R. King. GPS Meteorology: Direct Estimation of the Absolute Value of Precipitable Water, J. Appl. Meteor., in press 1996.

Fang, P. and Y. Bock. 1995. Scripps Orbit and Permanent Array Center Report to the IGS, 1994 Annual Report, International GPS Service for Geodynamics, J.F. Zumberge, R. Liu and R.E. Neilan, eds., IGS Central Bureau, Jet Propulsion Laboratory, Pasadena, pp. 213-233.

Herring, T.A. 1996. Documentation of the GLOBK Software, v. 4.0, Mass. Inst. of Technology.

King, R.W. and Y. Bock. 1996. Documentation of the GAMIT GPS Analysis Software, v. 9.4, Mass. Inst. of Technology and Scripps Inst. of Oceanography.

Langbein, J., F. Wyatt, H. Johnson, D. Hamann and P. Zimmer. 1995. Improved Stability of a Deeply Anchored Geodetic Monument, Geophys. Res. Lett., 22, pp. 3533-3536.

Wdowinski, S., Y. Bock, J. Zhang, and P. Fang. Southern California Permanent GPS Geodetic Array: Spatial Filtering of Daily Positions for Estimating Coseismic and Postseismic Displacements Induced by the 1992 Landers Earthquake, submitted to J. Geophys. Res., 1996.

Wyatt, F., B. Bolton, S. Bralla and D.C. Agnew. 1989. New Designs of Geodetic Monuments for Use With GPS, EOS Trans. Amer. Geophys Union, 70, pp. 1054.

Zhang, J., Y. Bock, P. Fang, J. Behr, J. Genrich and K. Hudnut. 1994. Surface Deformation in the Northridge Earthquake and the Los Angeles Basin from the PGGA Time Series , EOS Trans. Amer. Geophys Union, 75, pp. 166.

Zhang, J., Y. Bock, P. Fang and J. Behr. 1995. Distributed Analysis of GPS Networks, IUGG XXI General Assembly, July 2-14, 1995, Boulder, CO, pp. A32.

Zhang, J., Y. Bock, H. Johnson, P. Fang, S. Wdowinski, J. F. Genrich and J. Behr, Southern California Permanent GPS Geodetic Array: Error Analysis of Daily Position Estimates and Site Velocities, submitted to J. Geophys. Res., 1996.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
APPENDIX A
THE SOPAC GARNER ARCHIVE

This is a short introduction to those who wish to access IGS and regional raw and RINEX data and improved orbits from the SOPAC archive. We have two levels of interaction and access to our data holdings, the GARNER ftp archive at toba.ucsd.edu, and the SOPAC home page on the World Wide Web at http://toba.ucsd.edu.

  1. The GARNER Archive

Using ftp, log in to the archive:

% ftp toba.ucsd.edu

login% anonymous

passwd% your email address, i.e..

pgga@pgga.ucsd.edu

This will place you at the root of our ftp directory tree. From here you will be able to access raw or RINEX data, collect SOPAC precise orbits, or download site logs for all sites archived. There are some “README...” notices at this level which we encourage you to read as well as some other information on the archive structure and services in the directory “info”.

However, data access may be done following these basic guidelines:

  1. Select your data type and ‘cd' to that directory. Data types are raw, RINEX, global (global GPS solutions from which precise orbits are generated), regional (regional GPS solutions, California and Europe) and products and combinations, which contain precise, rapid and predicted ephemerides.

  2. Select the year and day (for regional, global, RINEX, raw, i.e.. 94data/333) or week (for products, ie. 0834) of the data you wish to download. For example,

% cd rinex/95data/355

to get to the RINEX data for Julian day 355 of 1995, or % cd combinations/0845

to get to the GLOBK combinations directory for GPS week 845, perhaps to download precise ephemerides in GAMIT g-file format.

  1. WWW Access to SOPAC Data Holdings

SOPAC has, since November 1995, provided users with a new and efficient method to check on data availability and to request off-line data via our World Wide Web homepage. To access the SOPAC homepage, direct your Web browser to:

http://toba.ucsd.edu

Once connected, click on our “On-line Data Request” which, if you are using a forms-compatible Web browser, will take you to the SOPAC data request form. Completely fill out the menu provided in that form. When you have submitted this form, the archive will be searched to determine whether or not the data you desire are on or off-line. If the data are on-line you will be allowed to go to the directory which contains those data and download selected files.

If your data are off-line you will be prompted to submit a data request to archive administrators. As soon as your request is handled, you will be notified via email that your data are on-line.

For further information on the SOPAC Web interface, please download “README.http” in the root directory of our anonymous ftp at toba.

  1. Additional Documentation

In response to user requests, we have made some documents more accessible, namely the IGS site logs. Please cd to “docs/site_logs” to access the latest logs for GPS stations archived at SOPAC.

Also in the “docs” directory are weekly download “newsletters” which show the SOPAC data holdings for each GPS week. You may want to search here first to see if the data you seek are archived for any particular site-day.

Please email any questions or comments on the SOPAC archive to Jeff Behr at behr@pgga.ucsd.edu or Jeff Dean at jdean@pgga.ucsd.edu. Requests for more information regarding SOPAC operations may be made by sending email to pgga@pgga.ucsd.edu or by calling (619) 534-0229, FAX (619) 534-9873.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Shared Use of DGPS Data for NOAA Weather Forecasting and Climate Monitoring

Seth Gutman, Russell Chadwick, Daniel Wolfe, Anthony Simon

NOAA Environmental Research Laboratories

INTRODUCTION

The ability to make significant improvements in cloud and precipitation forecasts is limited by the lack of information about the temporal and spatial variability of water vapor in the atmosphere (USWRP, 1995). This is a difficult problem (AGU, 1995), but overcoming it is essential for an improved understanding of the hydrological cycle at all scales (local to global), time frames (hourly to seasonal), and locations (land and ocean.) In a practical sense, improved water vapor observations are not only necessary for weather forecasting and climate monitoring, but for other applications such as improved GPS surveying and navigation. This is because the GPS signal is delayed by the presence of water vapor in the troposphere, and water vapor has the greatest variability of all natural atmospheric constituents.

Alternatives for addressing the problem of what water vapor observations are required, and how best to acquire them, are under investigation as part of a new National Oceanic and Atmospheric Administration -National Science Foundation program called the North American Atmospheric Observing System - NAOS. Because of practical constraints, no single observing system can provide all of the information needed by modern numerical weather prediction models to take full advantage of their capabilities. As a consequence, the concept of a composite observing system, consisting of modernized radiosondes and commercial aircraft observations 1 supplemented with satellite and surface-based observations of total precipitable water vapor2 (in some yet-to-be determined) configuration has been proposed. The routine assimilation of these observations into numerical weather prediction models is expected to provide the necessary information about the 4-dimensional distribution of water vapor in the atmosphere.

One of the most promising new water vapor observing systems to be developed in recent years utilizes the delay in the GPS signal caused by the neutral atmosphere, normally considered a nuisance parameter by surveyors and navigators, and relating it to the total (integrated) quantity of precipitable water vapor present.

This paper describes some of the opportunities provided by the growing network of Differential GPS (DGPS) sites belonging to the Continuously Operating Reference Station (CORS) network (W. Strange, this volume) to develop a practical water vapor monitoring system for NOAA. An integral part of this activity will be objective and subjective evaluations of the impact these data make on weather forecasting (R. McPherson, this volume) and climate monitoring (J. Curry, this volume). Some other potential applications of a surface-based GPS water vapor monitoring system will also be discussed.

BACKGROUND

In recent years, techniques have been developed to use GPS to measure the signal delay caused by the constituents of the neutral atmosphere, and relate it to the total amount of precipitable water vapor (PWV) in the troposphere (Bevis et al. 1992, Businger et al. 1996; Rocken et al. 1993, 1995). These measurements can be made with high reliability under virtually all weather conditions, with accuracies comparable to other observing systems such as radiosondes and microwave water vapor radiometers (Gutman et al. 1994, 1995; NOAA 1995). When supplemented with surface meteorological data, the GPS receivers incorporated into the network of CORS sites managed by NOAA National Geodetic Survey are adequate to make these measurements.

Although GPS provides us with an opportunity to develop a water vapor observing system at comparatively modest cost, the impact of total (integrated) PWV observations on forecast accuracy has not yet been determined. Significant improvements in weather forecast accuracy are anticipated however (Kuo et al. 1993), and techniques to assimilate GPS PWV data into operational numerical weather prediction models are being developed and tested at the NOAA Forecast Systems Laboratories (FSL) in Boulder, Colorado.

1  

Providing direct observations of the vertical distribution of water vapor, but with relatively course horizontal and temporal resolution.

2  

Providing remote observations with high temporal and horizontal resolution, but course vertical resolution.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Before the decision to implement an operational water vapor observing system using this technique can be made however, the data must be shown to make a significant positive impact on forecast accuracy, and the benefits derived from these improvements must be shown to justify the total cost of developing, implementing, and operating the system.

While simulated observations are useful for some of these activities, it is essential that verification and evaluation studies be carried out using real GPS data. Unfortunately, the number and distribution of GPS water vapor observing systems operated by NOAA are currently insufficient to carry out these evaluations.

EXPANDING THE NUMBER OF WATER VAPOR OBSERVING SITES

While searching for the best method(s) to address this deficiency, we identified the guidance provided by the Deputy Under Secretary for Oceans and Atmospheres, and the recommendations of the General Accounting Office regarding Federal agency joint development and use of GPS3.

To calculate the amount of water vapor in the atmosphere using GPS, it is necessary to have three things: 1) specialized GPS receivers; 2) software to calculate the error in position caused by the delay of the GPS signal as it passes through the atmosphere; and 3) accurate surface meteorological sensors that permit the contribution of water vapor to the “total tropospheric delay” to be identified. We determined that a growing number of GPS receivers operated by other Federal agencies such as the U.S. Coast Guard (albeit for reasons other than atmospheric remote sensing) are capable of making the necessary GPS measurements, but they lack the surface meteorological sensors necessary to calculate precipitable water vapor.

Figure 1 shows the locations of the Environmental Research Laboratories (ERL) GPS water vapor systems that have been operating continuously for more than one year, and the network of U.S. Coast Guard (USCG) and U.S. Army Corps of Engineers (ACOE) differential GPS (DGPS) sites designed to facilitate ship navigation and enhance public safety well into the next century. Sites like these provide NOAA with an opportunity to increase the number of water vapor observing sites at very low cost, simply through the addition of a low cost surface meteorological sensor package.

FIGURE 1 Continuously Operating Reference Station (CORS) sites available for water vapor monitoring.

3  

GAO/RCED-94-280, September 1994.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

ERL and the National Weather Service (NWS) National Data Buoy Center at Stennis Space Center, MS, have developed such a package called GSOS - GPS Surface Observing System (Figure 2). In collaboration with the owner-agencies of these sites, NOAA plans to install GSOS at CORS facilities on a strictly non-interference basis.

FIGURE 2 GSOS installation at a typical USCG DGPS site.

ADVANTAGES TO SHARED USE OF DGPS SITES

This approach provides us with an innovative solution to the problem of how to increase the number of GPS water vapor observing sites at very low cost, while simultaneously enhancing the value of these facilities to their users and the American Taxpayer. In particular, it leverages the $13 billion investment by DoD in the space and ground system segments of GPS, as well as the investments in GPS receivers and communications infrastructure made by other federal agencies such as USCG. In addition, the use of surface-based GPS to monitor atmospheric water vapor is a totally unforeseen dual-use application of the Global Positioning System that promises to benefit both the civilian and National defense communities.

UTILITY OF GPS WATER VAPOR MONITORING

GPS water vapor data are expected to make significant contributions in the areas of weather forecasting, climate monitoring, and improved positioning, as described below.

Weather Forecasting - GPS water vapor monitoring is expected to lead to improved mesoscale and local area cloud and precipitation forecasts. Improvements in predicting the time of onset, duration, location, and quantity of rain or snow fall will lead to more accurate flood predictions and warnings. An improved description of the quantity and distribution of water vapor in the atmosphere will provide valuable information in a number of areas including: prediction of severe storms including thunderstorms, tornadoes, and hurricanes; anticipation of fire hazards and fire weather support; air quality, air pollution monitoring and prediction; disposal of hazardous materials; agriculture and crop damage forecasts; and national defense. In addition, this improved description of the water vapor field can be used to calibrate satellite water vapor sensors over land, provide validation of satellite measurements, improve physical retrievals of temperature and humidity, and generally contribute to improved satellite data quality.

Climate Monitoring - GPS observations can contribute to improvements in the long-term observations of atmospheric water vapor that are necessary for the prediction and detection of changes in the global climate. Water vapor plays a fundamental part in global climate processes, since it is the principle mechanism by which moisture and latent heat are transported, causing “weather”. Water vapor is transported by the circulation of the atmosphere where it can condense to form liquid water or ice crystals in clouds. Water vapor is by far the most abundant greenhouse gas, and its concentration is extremely sensitive to the quantity of carbon dioxide (CO2) in the atmosphere.

Improved Positioning - From the ERL/NWS perspective, the primary goal of implementing a GPS PWV observing system is to improve our understanding of the 3-dimensional distribution of water vapor in the atmosphere at any point in time. Once this is accomplished, however, it is possible to use these data, along with a description of the temperature and pressure fields, to estimate the total tropospheric signal delay at any grid point in the model. Such a product is called a “Total Delay Nowcast” since it is a prediction of the current and very near term GPS signal delays caused by the neutral atmosphere. Tropospheric delays can be expressed as a differential correction, and broadcast along with other DGPS corrections to improve real-time GPS positioning accuracy. Some applications of the total delay nowcast include: vehicle navigation; engineering, surveying & mapping; agriculture; national defense; and monitoring the stability of structures such as dams and power plants in areas prone to hazards such as earthquakes, land slides, or volcanoes.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
REFERENCES

AGU, 1995. Water Vapor in the Climate System Special Report, IBSB No. 0-87590-865-9, American Geophysical Union, Washington, D.C., 1995.

Bevis, M., S. Businger, T.A. Herring, C. Rocken, R.A. Anthes, and R.H. Ware, 1992. GPS Meteorology: Remote sensing of atmospheric water vapor using the Global Postioning System. J. Geophys. Res., 97, 15, 787-801.

Businger, S., S.R. Chiswell, M. Bevis, J. Duan, A.A. Anthes, C. Rocken, R.H. Ware, M. Exner, T. VanHove, and F.S. Solheim, 1996. The promise of GPS in atmospheric monitoring. Bull. Amer. Meteor. Soc. 77, 5-18.

Curry, J., Webster, P., 1997. Precipitable Water Vapor Requirements for Climate Modelling, this volume.

GAO (General Accounting Office). 1994. Global Positioning Technology - Opportunities for Greater Federal Agency Joint Development and Use, GAO /RCED-94-280. Washington, D.C.: GAO, September 1994.

Gutman, S.I., R.B. Chadwick, D.E. Wolfe, A. Simon, T. VanHove, and C. Rocken, 1994. Toward and operational water vapor observing system using GPS. FSL Forum, Forecast Systems Laboratory, Boulder, CO, September 1994, 12-19.

——, D.E. Wolfe, A. Simon, 1995. Development of an operational water vapor remote sensing system using GPS: a progress report. FSL Forum, Forecast Systems Laboratory, Boulder, CO, December 1995, 21-32.

Kuo, Y.-H., Y.-R. Guo, and E.R. Westwater, 1993. Assimilation of precipitable water vapor into a mesoscale numerical model. Mon. Wea. Rev., 121, 1215-1238.

McPherson, R., Kalnay, E. and Lord, S., 1997. The Potential Role of GPS/MET Observations in Operational Numerical Weather Prediction, this volume.

NOAA, 1995. Precipitable water vapor comparisons using various GPS processing techniques. Document No. 1203-GP-36, 35 pp. (Available through NOAA ERL FSL Demonstration Division, Boulder, CO.).

Rocken, C., R. Ware, T. VanHove, F. Solheim, C. Alber, J. Johnson, M. Bevis, and S. Businger, 1993. Sensing atmospheric water vapor with the Global Positioning System . Geophys. Res. Lett., 20, 2631-2634.

——, T. Vanhove, F. Solheim, R.H. Ware, M. Bevis, S. Businger, and S.R. Chiswell, 1995. GPS/STORM - GPS sensing of atmospheric water vapor for meteorology. J. Atmos. Oceanic Technol., 12, 468-478.

Strange, W., and Weston, N., 1997. The National Geodetic Survey Continuously Operating Reference System (CORS), this voulme.

USWRP, 1995. Report of the First Prospectus Development Team - U.S. Weather Research Program, Office of the USWRP Lead Scientist, Mesoscale and Microscale Meteorology Division and the National Center for Atmospheric Research, Boulder, CO, May 1995.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

ATTACHMENT 1.

QUESTIONS FOR NETWORK OPERATORS & SERVICE PROVIDERS

Contact: Seth I. Gutman

NOAA ERL/FSL/DD

(303) 497-7031 gutman@fsl.noaa.gov

  1. Overview of network or service and its technical characteristics, to include:

    Current and planned

    Type of receiver and antenna - NOAA ERL is currently using Trimble SSE receivers and fixed ground plane (ST) antennas. Future systems will be Trimble SSI receivers and choke ring antennas.

    Typical site description - NOAA ERL GPS CORS facilities are usually established at NOAA Profiler Network (NPN) sites located primarily in the Mid-West. NPN sites are located in relatively flat terrain. These locations are generally free of obstructions and away form natural and man-made sources of interference. Antennas are permanently mounted on a corner post of a 8' chain link security fence. The receiver is permanently located in an electronics rack in the radar shelter.

    Photos - see image and attached drawing

    Map and list of sites - see attached documentation

    Types of data and differential corrections provided - Half-hour data, 30-sec samples. No differential corrections provided.

    Method and latency of data correction dissemination - Data processed daily using SIO rapid orbits. We do not disseminate data corrections.

    Network Configuration map depicting communications, data rates, locations, etc - see attached documentation

    Communications

    type - Dedicated FTS-2000

    cost - N/A

    reliability - 98%

    latency - 0

    frequency (every hour, day, etc.) - Half hour

    brief tech description (vsat, sat link, inmarsat etc.) ATT/Paradyne CSU/DSU

    Data Access & Archiving details - Daily RINEX files sent to NGS.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
  1. Who are the primary users and customers of your network or service? - ERL (FSL and ETL), NWS (NCEP and WSFO's), NESDIS, NGS, DOE, UNAVCO, UH, SIO.

  2. Does your organization encourage scientific use of your network or service? -This is the principal use of our data.

  3. What do you consider to be the costs and benefits of meeting the needs of scientific users? -This is the sole justification for our activities.

  4. Are you currently satisfied with the Federal Government's GPS/DGPS policy and management as it affects your operations? If not, why? - Yes.

  5. What do you feel is the most important issue you face as a DGPS network operator or service provider? - We are presently not a DGPS service provider. We are collaborating with NGS and USCG to acquire meteorological data at USCG DGPS sites that will be available to the CORS community.

TYPICAL SITE CONFIGURATION

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

ANTENNA INSTALLATION AT PLATTEVILLE

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

PRINCIPAL FACTS FOR NOAA/ERL GPS SITES

Principal Facts - Lamont

Station ID: LMNO

Station Coord: 36.69 - 97.48

Receiver Model Number: Trimble 4000SSE

Receiver Part Number: 18292-41

Receiver Serial Number: 3327A03450

Antenna Part Number: 14632-00

Antenna Serial Number: 3328A68604

Antenna Height: 2.752m (~ above ground)

Pressure Height: 2.1944m

Installed: 11-22-94

Magnetic Declination: 6.5833 E

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Principal Facts - Platteville

Station ID: PLTC

Station Coord: 40.18 -104.72

Receiver Model Number: 4000SSE

Receiver Part Number: 18292-41

Receiver Serial Number: 3327A03457

Antenna Part Number: 14632-00

Antenna Serial Number: 3328A68606

Antenna Height: 2.698 m (~ above ground)

Pressure Height: 1.71 m

Installed: 11-3-94

Magnetic Declination: 10.8667 E

Principal Facts - Hillsboro

Station ID: HBRK

Station Coord: 38.31 -97.30

Receiver Model Number: 4000SSE

Receiver Part Number: 18292-01

Receiver Serial Number: 3504A09467

Antenna Part Number: 23903-00

Antenna Serial Number: 0220011899

Antenna Height: 2.869 m (~ above ground)

Pressure Height: 0.560 m

Installed: 4-12-95

Magnetic Declination: 6.11667 E

Principal Facts - Vici

Station ID: VCIO

Station Coord: 36.07 -99.22

Receiver Model Number: 4000SSE

Receiver Part Number: 18292-01

Receiver Serial Number: 3503A09385

Antenna Part Number: 23903-00

Antenna Serial Number: 0220003683

Antenna Height: 2.785 m (~ above ground)

Pressure Height: 0.591 m

Installed: 4-13-95

Magnetic Declination: 7.2833 E

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Principal Facts - Haskell

Station ID: HKLO

Station Coord: 35.81 -95.78

Receiver Model Number: 4000SSE

Receiver Part Number: 18292-01

Receiver Serial Number: 3504A09470 (DOA 4-17 returned to Trimble

4-20-95) Receiver Serial Number: 3515A10453 (Installed 4-19-95)

Antenna Part Number: 23903-00

Antenna Serial Number: 0220003686

Antenna Height: 2.676 m (~ above ground)

Pressure Height: 0.585 m

Installed: 4-14-95

Magnetic Declination: 5.31667 E

Principal Facts - White Sands

Station ID: WSMN

Station Coord: 32.41 -106.35

Receiver Model Number: 4000SSE

Receiver Part Number: 18292-01

Receiver Serial Number: 3451A09192 (Failed 5-5-95 returned to

Trimble 5-8-95) Receiver Serial Number: 3446A08830 (Replaced 5-8-95)

Antenna Part Number: 23903-00

Antenna Serial Number: 0220011898

Antenna Height: 4.322 m (~ above ground)

Pressure Height: 2.022 m

Installed: 4-28-95

Magnetic Declination: 10.31667 E

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Principal Facts - Purcell

Station ID: PRCO

Station Coord: 34.98 -97.52

Receiver Model Number: Trimble 4000SSE

Receiver Part Number: 18292-41

Receiver Serial Number: 3343A04269

Antenna Part Number: 14632-00

Antenna Serial Number: 3328A68603

Antenna Height: 3.175m (~ above ground)

Pressure Height: 2.301 m

Installed: 11/28/95

Magnetic Declination: 8.200 E

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Large Permanent GPS Networks in Japan

Christian Rocken

University NAVSTAR Consortium

Takeshi Sagiya and Hiromichi Tsuji,

Geographical Survey Institute of Japan

INTRODUCTION

Japanese government agencies and Universities operate several of the worlds largest permanent GPS networks. The Geographical Survey Institute (GSI) of Japan alone has been operating two GPS networks with over 210 GPS receivers for over one year. These two networks, called GRAPES and COSMOS, will be combined and expanded, and a 610-station permanent GPS network will assume operation in Spring 1996 (Figure 1). The Japanese arrays deploy the latest in dual frequency receiver technology and data are analyzed daily with state-of-the-art GPS processing software. The main purpose of the arrays is the determination of crustal strain in the Japanese island arc system. There are also plans to make the data available to the public for geodetic surveys and other applications. Future use of the GSI network for monitoring atmospheric water vapor is currently discussed between scientists from GSI and the Japanese Meteorological Agency. This paper provides an overview of the GSI network.

FIGURE 1 GSI permanent GPS network - Spring 1996. Island sites to the south are not shown.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
GSI NETWORK DESCRIPTION

The 610-station permanent station GPS network operated by GSI will go into operation in Spring 1996. This network will be composed of the 100-station nationwide “GRAPES” network, the 110-station “COSMOS” network which is centered around the Tokyo metropolitan area with additional sites in Kobe, plus 400 new sites distributed over the entire country. Average spacing between sites will be ~ 30 km for all of Japan. Dual frequency P-code GPS receivers are used, GRAPES operates with Ashtech Z-XII, COSMOS with Trimble 4000 SSE, and the new receivers are Trimble 4000 SSi.

Figure 2 shows a GSI monument and the installed equipment. The monument is 5 meters high and anchored in the ground with a 2.5 meter thick concrete slab.

FIGURE 2 GSI permanent GPS monument and installed GPS equipment.

Data from the sites are transmitted to the central processing facility in Tsukuba. Sites are accessed mostly via ISDN, but public telephone lines are used as well. Data are downloaded for processing once per day. GRAPES sites collect data for 24 hours/day at a 30 seconds sampling rate, most COSMOS sites collect data for 12 hours/day with 60 second sampling. The new 610 station network will collect and download at 30 seconds for 24 hours/day resulting in ~ 1 GByte/day of data in the uncompressed standard Receiver Independent Exchange (RINEX) format. High-rate 1-second data is buffered at 120 stations and will be downloaded in case of an earthquake.

DATA ANALYSIS

The primary purpose of the GSI GPS network is the determination of crustal strain in Japan. Figure 3 shows the estimated station velocities and associated uncertainties after one year of analysis of data from the COSMOS network. Figure 4 shows coseismic displacements caused by the devastating 1995 Hyougoken Nanbu (or Kobe) earthquake. Additional interesting results from the GRAPES network can be found on the GSI Homepage at http://www.gsi-mc.go.jp/gsihome-e.html.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 3 shows deformation velocities, and corresponding 1-sigma error ellipses for the COSMOS GPS network. Crustal deformation velocities for the Tokyo/Yokohama metropolis and surroundings were estimated from 1 year of data with the Bernese Processing Engine. (Figure courtesy Takeshi Sagiya, GSI.).

Three high accuracy software packages are currently in use at GSI. These three packages are (1) Bernese/BPE, developed at the University of Bern, Switzerland, (2) GAMIT/GLOBK developed at MIT and SIO, and (3) GIPSY, developed at JPL. GSI plans to use all three packages for the analysis of data from the new 610-station GPS network. UNAVCO scientists are coordinating the operation of the three software packages in collaboration with teams from GSI, Hitachi Zosen Information System Co., LTD., the University of Bern, MIT, SIO and JPL.

All three software packages will be operated from the same user interface, and three solutions will be computed for the entire network. These three solutions shall be compared and combined to form a “best” solution. The approach of using more than one software package at GSI mirrors the successful approach by the International GPS Service for Geodynamics (IGS). The IGS combines results computed with several different GPS analysis packages to obtain the best possible products. GSI selected this approach to benefit from the different strengths and new developments in each of the three software packages, and to ensure that scientific competition and collaboration of the software groups will help to improve GSI products in the future.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 4 Horizontal coseismic displacements and 99% confidence ellipses due to the Kobe (Hyougo-ken Nanbu) earthquake estimated from a 4-minth time series of the coordinates. These solutions were computed from GRAPES data with GAMIT software at GSI (Courtesy, H. Tsuji, GSI).

SOLUTIONS AND PRODUCTS

The following is a list of products and solutions that GSI plans to provide from the analysis of its 610-station network.

  1. GSI precise GPS orbits will be computed from its own network plus data from the Global IGS network.

  2. Solutions for the entire 610-station network will be computed with GSI orbits after a 2-day delay.

  3. Solutions for the entire network will be computed with IGS orbits after a 10-day delay.

  4. Solutions for COSMOS will be computed several hours after the end of each day.

  5. High-rate 1-second kinematic solutions for the determination of ground motion will be computed for areas stricken by an earthquake within hours of the earthquake (120 selected stations will buffer 1-second data for use in case of an earthquake).

  6. Real-time-kinematic (RTK) solutions will be computed for selected stations that have ISDN connections in case of an earthquake to monitor ground motion in real-time.

  7. Public data for additional research projects can be included in the processing at GSI.

All three software packages are also required to provide tropospheric delay estimates. In addition GSI is planning to provide data to the public for surveying, scientific applications, and other uses.

FUTURE PLANS

Current plans call for further densification of the GSI network. GSI is also discussing the use of its network for weather prediction with Japanese meteorological authorities. This use would require more frequent than once per day downloading of the data, and the availability of additional meteorological data such as temperature, pressure and humidity for the GPS sites.

The current size of its networks and ambitious future plans clearly have established Japanese agencies as one of the world leaders in permanent GPS network applications.

ACKNOWLEDGMENTS

We want to thank the many scientists and engineers at GSI, HZS, JPL, SIO, MIT, UCAR/UNAVCO, and the University of Bern that are contributing to make the GSI network a success. Data analysis and network operation is funded by the GSI, Japan. UCAR/UNAVCO facilities were used for preparation of this document.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
International GPS Service for Geodynamics: Mission, Development, Network, Structure, Products, and Current Projects

Gerhard Beutler,

Astronomical Institute, University of Bern, Switzerland

Jan Kouba,

Geodetic Survey of Canada, Natural Resources Canada

Ruth Neilan,

International GPS Service for Geodynamics, Central Bureau, Jet Propulsion Laboratory

THE IGS OBJECTIVES AND MISSION

According to the IGS Terms of Reference (IGS Colleague Directory) the primary objective of the IGS is to provide a service to support, through GPS data products, geodetic and geophysical research activities. Cognizant of the immense growth in GPS applications the secondary objective of the IGS is to support a broad spectrum of operational activities performed by governmental or selected commercial organizations. The service also develops the necessary standards/specifications and encourages international adherence to its conventions.

IGS collects, archives and distributes GPS observation data sets of sufficient accuracy to satisfy the objectives of a wide range of applications and experimentation. These data sets are used by the IGS to generate the following products:

  • high accuracy GPS satellite ephemerides

  • earth rotation parameters

  • coordinates and velocities of the IGS tracking stations

  • GPS satellite and tracking station clock information

  • atmosphere information.

The IGS accomplishes its mission through the following components:

  • Network of tracking stations

  • Data Centers

  • Analysis Centers and Associate Analysis Centers

  • Analysis Coordinator

  • Governing Board.

In view of the above objectives it is clear that there must be a close relationship of the IGS to the International Earth Rotation Service (IERS). This link is formally established by an IERS representative in the IGS Governing Board and by the fact that the IGS is the GPS technique coordinator for the IERS.

DEVELOPMENT OF THE IGS

The primary motivation in planning the IGS was the recognition in 1989 that the most demanding users of the GPS satellites, the geophysical community, were purchasing receivers in exceedingly large numbers and using them as more or less black boxes, using software packages which they did not completely understand, mainly for relative positioning (Mueller, 1993).

The IAG Planning Committee for the IGS with Ivan I. Mueller as chairman issued the Call for Participation on 1 February 1991. More than 100 scientific organizations and governmental institutions announced their participation either as an observatory (part of the IGS network), as an analysis center, as a data center, as analysis center coordinator, or as Central Bureau. At the 20th General Assembly of the IUGG in Vienna, August 1991, the IAG Planning Committee was restructured and renamed as IGS Campaign Oversight Committee. This committee organized the 1992 events, namely the 1992 IGS Test Campaign and Epoch'92.

The 1992 IGS Test Campaign, scheduled from 21 June to 23 September 1992, focused on the routine determination of high accuracy orbits and ERPs; it was to serve as the proof of concept for the future IGS. Epoch'92 on the other hand was scheduled as a two-week campaign for the purpose of serving as a first extension of the that part of the relatively sparse IGS Network analyzed on a daily basis by the IGS Analysis Centers.

The 1992 IGS Test Campaign was successful beyond expectation. This was why data collection and analysis continued after the official end of the campaign, first on a

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

best effort basis, then within the formally established IGS Pilot Service. The 1992 Campaigns were evaluated at the 1993 IGS Workshop in Bern (Beutler and Brockmann, 1993). Based on this evaluation the IGS was restructured again and established as an official IAG Service on January, 1, 1994. For more information related to the development of the IGS we refer to (Beutler et al, 1996).

FIGURE 1 The IGS network in January 1996.

THE IGS NETWORK

The present realization of the IGS network is given in Figure 1. Unnecessary to say that the network was growing considerably since 1992, when the IGS Test Campaign started.

Table 1 (Attachment) gives an overview of the observational data available through the IGS on a daily basis. The Table is extracted from the summary information made available through CDDIS, one of three IGS Global Data Centers (Noll, 1996). The 13 stations marked with (*) are either kept fixed or tightly constrained on the official ITRF coordinates and station velocities to ensure one and the same realization of the ITRF within the IGS. These stations are either collocated with VLBI and/or SLR.

Table 1 also illustrates that the IGS network still is essentially a Rogue/Turborogue network. A single-receiver type network has its advantages in the analysis (receiver specific biases due to the antenna cancel out to some extent). The drawback has to be seen in the fact that receiver specific problems (e.g. operation under AS, Anti-Spoofing) will harm the entire network.

The data of all IGS stations in principle should arrive on the level of the IGS Global Data Centers a few hours after the last observation epoch. There is of course data transmission via dial-up modems, but it is fair to state that Internet is the primary component for data transmission. At the IGS data centers all data are available in the RINEX-format (Gurtner, 1994). Using 30-second data sampling (current IGS standard), about

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

0.5 Mbytes of data have to be transferred in compressed form per station and day.

THE PRESENT STRUCTURE OF THE IGS

The essential components of the IGS were already listed in the first section, the IGS network was introduced in the previous section. Let us now briefly discuss the essential IGS components (except the network) in 1996.

The IGS Central Bureau is located at Jet Propulsion Laboratory (JPL). It is responsible for the general management of the IGS. The primary function of the CB is to facilitate communications, coordinate IGS activities, establish and promote compliance to IGS network standards, monitor network operations and quality assurance of data, maintain documentation, organize reports, meetings and workshops, and insure compatibility of IGS and IERS.

The IGS CB makes available the IGS reports and the IGS mail messages through an electronic mailbox. The Central Bureau Information System (CBIS) is the primary source of information about the IGS today (Liu et al, 1994). It contains, among other items, general information about the IGS and the GPS, an up-to-date documentation of all IGS stations, the IGS-report and IGS-mail series, and the official IGS orbits.

The IGS Colleague Directory is regularly updated and distributed, the first Annual Report of the IGS (Zumberge et al, 1995) was issued in fall 1995, the Annual Report for 1995 is expected for mid 1996. Short information about the IGS is available through the CBIS, too.

There are currently seven IGS Analysis Centers, namely

  • CODE, Center for Orbit Determination in Europe, located at the University of Bern, Switzerland,

  • EMR, Natural Resources, Ottawa, Canada,

  • ESA, at European Space Operation Center (ESOC), Darmstadt, Germany,

  • GFZ, GeoForschungsZentrum, Potsdam, Germany,

  • JPL, Jet Propulsion Laboratory, Pasadena, USA,

  • NGS, National Geodetic Survey, Silver Spring, USA,

  • SIO, Scripps Institution of Oceanography, San Diego, USA.

Six different software developments based on rather different principles are used by these institutions. Friendly competition and cooperation of the Analysis Centers is an essential aspect and a key-driver for an amazing quality improvement over the last four years (Beutler et al, 1996), (Mireault et al, 1996).

The IGS Analysis Center Coordinator plays an essential role within the IGS. His main responsibility is to monitor the Analysis Centers' activities. Quality control, performance evaluation, and continued development of appropriate analysis standards are important issues.

Probably the most important task, however, consists of the generation of the official IGS products, in particular the official IGS Orbits (made available currently in the ITRF93), the IGS Clocks, and the IGS Earth Rotation Parameters. At present the IGS Orbits are made available on a weekly basis with a delay of (at maximum) 11 days. The weekly analyses may be found in the IGS-Report series, see e.g. (Kouba, 1996). In summary one may state that today the individual IGS series as produced by the analysis centers range between 5 cm to 20 cm rms per satellite coordinate, between 0.7 ns and 5 ns rms for satellite clocks, and between 0.2 mas to 0.5 mas for the pole coodinates. The official IGS products are expected to be on the level of the best contributions. More information is available in (Mireault et al, 1996).

IGS Data Centers are either operational, regional or global in nature. Operational data centers are in direct contact with the tracking sites, regional data centers reduce traffic on electronic networks. The Global Data Centers are the main interface to the Analysis Centers and the IGS user community (concerning IGS tracking data and products). There are at present three IGS Global Data Center, namely

  • CDDIS at Goddard Space Flight Center, Greenbelt,

  • IGN (Institut Géographique National), Paris,

  • SIO (Scripps Institution of Oceanography), San Diego

which are also responsible for archiving IGS data and products.

The IGS Governing Board (GB) exercises general control over the activities of the Service including modifications to the organization that would be appropriate to maintain efficiency and reliability, while taking full advantage of the advances in technology and theory. The IGS GB consists of 15 members. The current members of the GB may be found in (IGS Colleague Dirctory, 1996).

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
NEW IGS PROJECTS
Densification of the ITRF using GPS

The densification of the ITRF through regional GPS analyses was a key issue in 1994 and 1995. The guidelines for such a densification were defined at the IGS Workshop in December 1994 (Zumberge and Liu, 1995). The conclusions of the workshop and the development since may be summarized as follows:

  • The densified IGS network should consist of about 200 globally well distributed sites.

  • The densification shall be realized in cooperation with other groups setting up permanent GPS networks.

  • Regional analyses will be performed by (new) IGS Associate Analysis Centers. These will make extensive use of the global products (orbits, erps, ITRF coordinates and velocities of sites well established in the global network). These AACs produce free network solutions on a weekly basis. The seven IGS Analysis Centers play the role of such AACs, too.

  • The weekly free network solutions are combined by special IGS Analysis Centers, where in a pilot phase JPL, University of Newcastle and MIT serve as such special centers.

  • The Pilot Project started in September 1995, where in the first phase only the products of the seven Analysis Centers were used as input for solution combinations.

The pilot phase of this project is in full progress now. It may be assumed that the ultimate goal, a relatively dense global GPS network with spacings between points of about 1000 km – 2000 km permanently available for regional geodynamics will be realized in future. The relative accuracy of the network will be clearly sub-centimeter for the horizontal, about one centimenter for the vertical components.

Rapid Orbits

There was considerable pressure on the IGS to come out with a very rapid product available about 12-24 hours after the last observation epoch. It was decided by the IGS Governing Board in 1995 that the IGS should actually move into this direction.

Since January 1, 1996 up to six ACs (CODE, EMR, ESA, GFZ, JPL, SIO) have been taking part in the IGS Preliminary orbit/clock pilot project. Despite the initial difficulties, the persistent data delivery delays, and continuing INTERNET problems it is fair to say that the IGP project exceeds all expectations. The time ACs have to wait is driving the ACs solution precision (rather than the number of stations), as the remote stations, vital to station geometry are also the most prone to data delivery delays. Again the products delivered by the IGS Analysis Centers are analysed and combined into a (semi-)official product by the IGS Analysis Center Coordinator. The quality check is even more important for these quick orbits than for the normal products because quality checks at the Analysis Centers are more problematic due to the limited amount of time and data available.

Atmosphere Modeling

The focus at the IGS Workshop on Special Topics and New Directions in Potsdam (May 1995, proceedings in press) was on one hand on processing aspects (fascinating orbit modeling aspects and antenna problems were discussed) on the other hand on possible new products like ionosphere models, station-specific precipitable water content values with a high temporal resolution using the IGS network.

All IGS Analysis Centers have to model tropospheric refraction. (Gendt and Beutler, 1996) showed that the consistency of estimates stemming from different IGS Analysis Centers is relatively high. It should thus be possible to extract on a routine basis the precipitable water content for the entire IGS network with a high temporal resolution (two hours or finer) – provided high accuracy barometers are deployed in the IGS network . Even if the IGS products are available only about two weeks after the observations, the results are still most valuable for climatological studies. Should very rapid IGS products become routinely available, the same estimates might be used for weather prediction, a topic of highest interest to meteorologists (Bevis et al, 1992). A session is devoted to troposphere related topics at the IGS Analysis Center Workshop in Silver Spring (March 1995).

The motivation to produce GPS derived ionosphere models is manyfold: Local or regional models may be used to remove (reduce) biases in single frequency surveys; regional or global models may be used to calibrate altimeter measurements (ERS-1 is an example) or to calibrate radio signals from space vehicles in the planetary system; last but not least there are pure research projects (ionosphere maps, extraction of geomagnetic indices). At present we may not yet speak of a high degree of consistency of ionosphere models produced by different IGS (and other) Analysis Centers. This aspect is studied right now. Another important aspect for the future development is to define an interface

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

between the IGS and the ionosphere research community. What exactly is needed in ionospheric research, what kind of products should be made available? The IGS definitely needs the help of the mentioned research community.

SUMMARY AND OUTLOOK

If we review the development of the IGS since the foundation of the IGS planning committee in 1989 we conclude that the IGS accomplishes today (spring 1996) its mission with respect to global and regional geodynamics:

  • Orbits are available with an accuracy allowing for regional geodynamics without further orbit refinement and a delay (11 days) which is sufficient for most purposes.

  • The IGS pole positions x and y are very important contributions to the IERS poles (Bulletins A and B).

  • Through the annual solutions of the IGS Analysis Centers the IGS contributes more and more to the realization of the ITRF. From now on this contribution will even be more significant and reliable through the project densification of the ITRF using GPS.

  • The clock information distributed in the IGS orbit files allows e.g. a point positioning with sub-meter accuracy using state of the art dual band GPS receiver.

We hope (and assume) that similar statements will be tree in a few years time with respect to atmosphere physics:

  • Several IGS Analysis Centers (CODE, ESA, JPL) started producing ionosphere models using significantly different procedures and observables. Other organizations like DLR Neustrelitz and University of New Brunswick started such activities, too. The next step will consist of comparing these regional and global models. First results will be presented at the IGS Analysis center Workshop in Silver Spring.

  • The troposphere estimates of the IGS processing centers are consistent on the level of a few mm. A regular IGS comparison and combination activity might now be built up. These topics will be addressed at the Silver Spring IGS Analysis Center Workshop.

For the atmosphere (and other) applications it is essential that IGS orbits are available in nearly real time. Such IGS products already are available on an experimental basis right now.

Let us conclude this review of IGS activites with some important characteristics of the IGS:

  • The IGS actually is an International and a multi-agency service.

  • Although the IGS is a Service with the goal to facilitate research a considerable amount of research is taking place within the IGS. In this respect it is essential that there is a high redundancy in all IGS components (network, data centers, analysis centers).

  • The IGS products (even those of the analysis centers) always were freely available to the scientific community. This led e.g. to the detection of biases in the official IERS erp-series.

  • The IGS always was open in cooperating with other groups operating permanent networks. IGS standards for data formats, station monumentation are observed by many other network operators today.

REFERENCES

Beutler, G. and E. Brockmann (1993). International GPS Service for Geodynamics. Proceedings of the 1993 IGS Workshop, 369 pages, Druckerei der Universität Bern, available through IGS Central Bureau.

Beutler, G., I.I. Mueller, R.E. Neilan (1996). The International GPS Service for Geodynamics (IGS): The Story. IAG Symposium No. 115, in Series International Association of Geodesy Symposia, ed. W. Torge, pp. 3-13 , Springer-Verlag, 1996, pp. 3-13.

Bevis, M., S. Businger, T.A. Herring, Ch. Rocken, A. Anthes, R.H. Ware (1992). GPS Meteorology: Remote Sensing of Atmospheric Water Vapor using the Global Positioning System. Journal of Geophysical Research, Vol. 97, No D14, pp. 15787-15801.

Gendt, G, G. Beutler (1995). Consistency in the Troposphere Estimations Using the IGS Network., IGS Workshop on Special Topics and New Directions, GeoForschungsZentrum Potsdam, May 1995. In press.

Gurtner, W. (1994). RINEX: The Receiver-Independent Exchange Format. GPS World, Vol. 5, pp. 48-52.

IGS Colleague Directory (1996). Available through the IGS Central Bureau.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Kouba, J. (1996). AC Coordinator: Week 840 IGS Rapid Orbits. IGSREPORT No. 2439, Central Bureau Information System (CBIS), February 1996.

Liu, R., W. Gurtner, J.F. Zumberge, R.E. Neilan (1994). Introducing the Central Bureau Information System of the International GPS Service for Geodynamics., IGS Colleague Directory, December 1994, IGS Central Bureau (JPL).

Mireault, Y., J. Kouba, F. Lahaye (1996). IGS Combination of Precise GPS Satellite Ephemerides an Clocks. IAG Symposium No. 115, in Series International Association of Geodesy Symposia, ed. W. Torge, pp. 14-23, Springer-Verlag, 1996, pp. 3-13.

Mueller I.I. (1993). Planning an International Service using the Global Positioning System (GPS) for Geodynamic Applications, Proc. IAG Symp. No. 109 on Permanent Satellite Tracking Networks for Geodesy and Geodynamics, Springer Verlag.

Noll, C. (1996). Wk 0841 CDDIS Data Holdings, IGSREPORT No. 2440. Central Bureau Information System (CBIS), February 1996.

Zumberge, J.F., R. Liu, R.E. Neilan (1995). International GPS Service for Geodynamics Annual Report 1994., IGS Central Bureau., 329 pages.

Zumberge, J.F., R. Liu (1995). Densification of the IERS Terrestrial Reference Frame through Regional GPS Network. Proceedings of the December 1994 IGS Workshop in Pasadena, available through the IGS Central Bureau.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
ATTACHMENT 1: CURRENT IGS GPS DATA HOLDINGS AT GLOBAL DATA CENTERS

IGS Electronic Report Tue Feb 27 1996,11:43:09 PST, Message Number 2440

Author: CDDIS/Carey Noll

Subject: Wk 0841 CDDIS Data Holdings

TABLE 1 Current IGS GPS Data Holdings (as of 27-Feb-96 14:36), IGS Operational Week 112/GPS Week 0841 (February 18 through February 24, 1996; Days 96049 through 96055)

 

Mon.

 

049

050

...

054

055

No.

Site Name

Name

Receiver

0218

0219

...

0223

0224

Days

 

Albert Head

ALBH

Rogue SNR-8000

X

X

...

X

X

7

Algonquin*

ALGO

Rogue SNR-8000

X

X

...

X

X

7

Ankara

ANKR

Rogue SNR-8000

X

X

...

X

X

7

Annapolis

USNA

Rogue SNR-8000

X

X

...

X

7

 

AOA, Westlake

AOA1

Rogue SNR-8000

X

X

...

X

X

7

Arequipa

AREQ

Rogue SNR-8000

X

X

...

X

X

7

Auckland

AUCK

Rogue SNR-8000

X

X

...

X

X

7

Bangalore

IISC

Rogue SNR-8000

X

X

...

5

Bermuda

BRMU

Rogue SNR-8000

X

X

...

X

X

7

Bishkek

POL2

Rogue SNR-8000

X

X

...

X

X

7

Bogota

BOGT

Rogue SNR-8000

X

X

...

-

3

Borowiec

BOR1

Rogue SNR-8000

X

X

...

X

X

7

Brussels

BRUS

Rogue SNR-8000

X

X

...

X

X

7

Carr Hill

CARR

Rogue SNR-8000

X

X

...

X

X

7

Catalina Island

CAT1

Rogue SNR-8000

X

X

...

X

X

7

Chatham Island

CHAT

Rogue SNR-8000

X

X

...

X

X

7

CIT, Pasadena

CIT1

Rogue SNR-8000

X

X

...

X

X

7

Davis

DAV1

Rogue SNR-8100

X

X

...

-

-

3

Easter Island

EISL

Rogue SNR-8000

X

X

...

X

X

7

Ensenada

CICE

Rogue SNR-8000

X

X

...

X

X

7

Fairbanks*

FAIR

Rogue SNR-8

X

X

...

-

-

5

Fortaleza

FORT

Rogue SNR-8000

-

X

...

X

X

6

Goldstone*

GOLD

Rogue SNR-8

X

-

...

-

-

1

Grasse

GRAS

Rogue SNR-8100

X

X

...

X

X

7

Graz

GRAZ

Rogue SNR-8

X

X

...

X

X

7

Greenbelt

GODE

Rogue SNR-8100

X

X

...

X

X

7

Guam

GUAM

Rogue SNR-8000

X

X

...

X

X

7

Hartebeesthoek*

HART

Rogue SNR-8

X

X

...

X

X

7

Harvest Platform

HARV

Rogue SNR-8000

X

X

...

X

X

7

Herstmonceux

HERS

Rogue SNR-8A

X

X

...

X

X

6

Horn Point

HNPT

Rogue SNR-12

X

X

...

X

X

7

Irkutsk

IRKT

Rogue SNR-8000

X

X

...

X

X

7

Jozefoslaw

JOZE

Trimble 4000SSE

X

X

...

X

X

7

Kellyville

KELY

Rogue SNR-8000

X

-

...

-

-

1

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
 

Kerguelen

KERG

Rogue SNR-8C

X

X

...

X

X

7

Kiruna

KIRU

Rogue SNR-8100

X

X

...

X

X

7

Kitab

KIT3

Rogue SNR-8000

X

X

...

-

X

6

KokeePark*

KOKB

Rogue SNR-8

X

X

...

X

X

7

Kootwijk*

KOSG

Rogue SNR-12

X

X

...

X

X

7

Kourou

KOUR

Rogue SNR-8C

X

X

...

X

X

7

L'Ebre

EBRE

Trimble 4000SSE

X

X

...

-

-

5

La Plata

LPGS

Rogue SNR-8000

X

X

...

X

X

7

Long Beach

LBCH

Rogue SNR-8000

X

X

...

X

X

7

Madrid*

MADR

Rogue SNR-8

X

X

...

X

X

7

Malindi

MALI

Rogue SNR-8C

X

X

...

X

X

7

Mammoth Lakes

CASA

Rogue SNR-8000

X

X

...

X

X

7

Maspalomas

MAS1

Rogue SNR-8100

X

X

...

X

X

7

Matera

MATE

Rogue SNR-8

X

X

...

X

X

7

McDonald

MDO1

Rogue SNR-8000

X

X

...

X

X

7

McMurdo

MCM4

Rogue SNR-8000

X

X

...

X

X

7

Mendeleevo

MDVO

Trimble 4000SSE

X

X

...

-

X

6

Metsahovi

METS

Rogue SNR-8100

X

X

...

X

X

7

Monument Peak

MONP

Ashtech LPZ-XIID

X

X

...

X

X

7

Mount Wilson

WLSN

Rogue SNR-8000

X

X

...

X

X

7

North Liberty

NLIB

Rogue SNR-8000

X

X

...

X

X

7

Ny-Alesund

NYAL

Rogue SNR-8

X

X

...

X

X

7

O'Higgins

OHIG

Rogue SNR-8000

X

X

...

X

X

7

Oatt Mountain

OAT2

Rogue SNR-8000

X

X

...

X

X

7

Onsala

ONSA

Rogue SNR-8000

X

X

...

X

X

7

Palos Verdes

PVEP

Trimble 4000SSE

X

X

...

X

X

7

Pamate

PAMA

Rogue SNR-8100

X

X

...

X

X

7

Pasadena

JPLM

Rogue SNR-8000

X

X

...

X

X

7

Penticton

DRAO

Rogue SNR-8000

X

X

...

X

X

7

Perth

PERT

Rogue SNR-8100

X

X

...

X

X

7

Pie Town

PIE1

Rogue SNR-8000

X

X

...

X

X

7

Pinyon Flat

PIN1

Ashtech Z-XIID

X

X

...

X

X

7

Potsdam

POTS

Rogue SNR-8000

X

X

...

X

X

7

Quincy

QUIN

Rogue SNR-8000

X

X

...

X

X

7

Reykjavik

REYK

Rogue SNR-8000

X

X

...

X

X

7

Richmond

RCM5

Rogue SNR-8000

X

X

...

X

X

7

Saddle Peak

SPK1

Rogue SNR-8000

X

X

...

X

X

7

Saint John's

STJO

Rogue SNR-8000

X

X

...

X

X

7

Santiago*

SANT

Rogue SNR-8

X

X

...

X

X

6

Scripps

SIO3

Ashtech Z-XII3

X

X

...

X

X

7

Shanghai

SHAO

Rogue SNR-8100

X

X

...

X

X

7

Solomons Island

SOL1

Trimble 4000SSE

X

X

...

X

X

7

St. Croix

CRO1

Rogue SNR-8000

X

X

...

X

X

7

Taejon

TAEJ

Trimble 4000SSE

X

X

...

X

X

7

Taiwan

TAIW

Rogue SNR-800

X

X

...

X

X

7

Thule

THU1

Rogue SNR-8000

X

X

...

X

X

7

Tidbinbilla*

TIDB

Rogue SNR-8

X

X

...

X

X

7

Tromso*

TROM

Rogue SNR-8

X

X

...

X

X

7

Tsukuba

TSKB

Rogue SNR-8100

X

X

...

X

X

7

UCLA, Los Angeles

UCLP

Rogue SNR-8000

X

X

...

X

X

7

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
 

USC, Los Angeles

USC1

Rogue SNR-8000

X

X

...

X

X

7

Usuda

USUD

Rogue SNR-8000

X

X

...

X

X

7

Vandenberg

VNDP

Ashtech LPZ-XIID

X

X

...

X

X

7

Villafranca

VILL

Rogue SNR-8100

X

X

...

X

X

7

Westford

WES2

Rogue SNR-8000

X

X

...

X

X

7

Wettzell*

WETT

Rogue SNR-800

X

X

...

X

X

7

Wettzell

WTZR

Rogue SNR-8000

X

X

...

X

X

7

Whittier College

WHC1

Rogue SNR-8000

X

X

...

X

X

7

Whittier Library

WHI1

Rogue SNR-8000

X

X

...

X

X

7

Yaragadee*

YAR1

Rogue SNR-8

X

X

...

X

X

7

Yellowknife*

YELL

Rogue SNR-8000

X

X

...

X

X

7

Zimmerwald

ZIMM

Trimble 4000SSE

X

X

...

X

X

7

Zvenigorod

ZWEN

Rogue SNR-8000

X

X

...

X

X

7

Totals:

97 stations

95

95

95

...

90

648

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Flow, Archiving, and Distribution of Global GPS Data and Products for the IGS and the Role of the Crustal Dynamics Data Information System (CDDIS)

Carey Noll

NASA Goddard Space Flight Center

Maurice Dube

Hughes/STX

INTRODUCTION

The International GPS Service for Geodynamics (IGS) was formed by the International Association of Geodesy (IAG) to provide GPS data and highly accurate ephemerides in a timely fashion to the global science community to aid in geophysical research. This service has been operational since January 1994. The GPS data flows from a global network of permanent GPS tracking sites through a hierarchy of data centers before they are available to the user at designated global and regional data centers. A majority of these data flow from the receiver to global data centers within 24 hours of the end of the observation day. Common data formats and compression software are utilized throughout the data flow to facilitate efficient data transfer. IGS analysis centers retrieve these data daily to produce IGS products (e.g., orbits, clock corrections, Earth rotation parameters, and station positions). These products are then forwarded to the global data centers by the analysts for access by the IGS Analysis Coordinator, for generation of the rapid and final IGS orbit product, and for access by the user community in general. To further aid users of IGS data and products, the IGS Central Bureau Information System (CBIS) was developed to provide information on IGS sites and participating data and analysis centers. The CBIS, accessible through ftp and the World Wide Web (WWW), provides up-to-date data holding summaries of the distributed data systems. The IGS, its data flow, and the archival and distribution at one of its data centers will be discussed.

THE INTERNATIONAL GPS SERVICE FOR GEODYNAMICS (IGS)

The purpose of this international service is to provide data from a global network of GPS tracking sites as well as derived products, such as highly accurate ephemerides, Earth rotation parameters, and a global reference frame, to the international science community to further understanding in geodetic and geophysical research. In 1991, a call for participation was issued, seeking participation from groups and agencies to install GPS sites and to serve as data centers and/or data analysis centers. The first IGS campaign was held mid-1992; a pilot service continued after this successful test period and the service became operational in January 1994. In general, the GPS tracking data are delivered, archived, and publicly available within 24 hours after the end of observation day. Derived products, including an official IGS orbit, are available within ten days.

During the IGS planning stages, it was realized that a distributed data management system was vital to the success of the service. A distributed system would provide for rapid turnaround of data from the global GPS network as well as ensure system backup and redundancy should a particular data center become unavailable for some period of time. Furthermore, establishment of standards in data formats and compression were necessary to ensure the efficient and timely flow of data and products. Lastly, a centralized information system was established to provide the user community with general information about the IGS and the current status of the receiver network, as well as the data holdings at the distributed data centers.

IGS DATA AND PRODUCTS
GPS Tracking Data

Raw receiver data are downloaded on a daily basis by operational data centers and converted into RINEX, Receiver INdependent EXchange format (Gurtner, 1994). GPS tracking data from the IGS network are recorded at a thirty second sampling rate. The GPS data unit typically consists of two daily files, starting at 00:00:00 UTC and ending at 23:59:30 UTC; one file contains the range observations, a second file contains the GPS broadcast ephemerides for all satellites tracked. These two RINEX

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

data files form the smallest unit of GPS data for the IGS and after format conversion, are forwarded to a regional or global data center for archival and distribution. The format of the file names at the data centers is ssssddd#.yyt_Z, where ssss is the site name, ddd is the day of year, # is the file sequence number for the day (typically set to 0 to indicate a single file per site per day), yy is the year, and t is the file type (O for observation and N for navigation). The _Z designation denotes the file is in compressed format. Each site produces approximately 0.6 Mbytes of data per day in compressed RINEX format.

At the global data center, the ephemeris data files for a given day are merged into a single file, which contains the orbit information for all GPS satellites for that day. This daily ephemeris data file is also stored in compressed form and named BRDCddd0.yyN_Z (where ddd is the day of year and yy is the year). Users can thus download this single file instead of all broadcast ephemeris files from the individual stations.

IGS Products

Seven IGS data analysis centers (ACs) retrieve the GPS tracking data daily from the global data centers to produce IGS data. These products consist of daily precise satellite ephemerides, clock corrections, Earth rotation parameters, and station positions. The files are sent to the IGS global data centers by these analysis centers in uncompressed ASCII (in general), using NGS SP3 format (Remondi, 1989) for the precise ephemerides and Software Independent Exchange Format, SINEX, (Blewitt et. al., 1995) for the station position solutions. The Analysis Coordinator for the IGS, located at NRCan, then accesses one of the global data centers on a regular basis to retrieve these products to derive the combined IGS orbits, clock corrections, and Earth rotation parameters as well as to generate reports on data quality and statistics on product comparisons (Beutler et. al., 1993). Furthermore, users interested in obtaining precision orbits for use in general surveys and regional experiments can also download these data from the global data centers. The time delay of the IGS rapid products is dependent upon the timeliness of the individual IGS analysis centers; on average, the combined orbit is generated within two to three days of receipt of data from all analysis centers (typically within ten days).

FLOW OF IGS DATA AND INFORMATION

The flow of IGS data (including both GPS data and derived products) as well as general information can be divided into several levels (Gurtner and Neilan, 1995) as shown in Figure 1:

  • Tracking Stations

  • Data Centers (operational, regional, and global)

  • Analysis Centers

  • Analysis Center Coordinator

  • Central Bureau (including the Central Bureau Information System, CBIS)

  • Governing Board

These components of the IGS will be discussed in more detail below.

FIGURE 1 Flow of IGS data.

Tracking Stations

The global network of GPS tracking stations are equipped with precision, dual-frequency, P-code receivers operating at a thirty-second sampling rate. The IGS currently supports over 100 globally distributed stations. These stations are continuously tracking and are accessible through phone lines, network, or satellite connections thus permitting rapid, automated download of data on a daily basis. Any station wishing to participate in the IGS must submit a completed station log to the IGS Central Bureau, detailing the receiver, site location, responsible agencies, and other general information. These station logs are accessible through the CBIS. The IGS has established a hierarchy of these 100 sites since not all sites are utilized by every analysis center (Gurtner and Neilan, 1995). A core set of approximately forty or

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

fifty sites are analyzed on a daily basis by most centers; these sites are called global sites. Sites used by one or two analysis centers for densification on a regional basis are termed regional sites. Finally, sites part of highly dense networks, such as one established in southern California to monitor earthquake deformation, are termed local sites. This classification of IGS sites determines how far in the data center hierarchy the data are archived. For example, only global sites should flow to the global data center level, where regional sites would be archived at a regional data center only.

Data Centers

The IGS has also established a hierarchy of data centers to distribute data from the network of tracking stations: operational, regional, and global data centers. Operational data centers are responsible for the direct interface to the GPS receiver, connecting to the remote site daily and downloading and archiving the raw receiver data. The quality of these data are validated by checking the number of observations, number of observed satellites, date and time of the first and last record in the file. The data are then translated from raw receiver format to a common format and compressed. Both the observation and navigation files are then transmitted to a regional or global data center within a few hours following the end of the observation day.

Regional data centers gather data from various operational data centers and maintain an archive for users interested in stations of a particular region. These data centers forward data from designated global sites to the global data centers within at most 24 hours of receipt. IGS regional data centers have been established in several areas, including Europe and Australia.

The IGS global data centers are ideally the principle GPS data source for the IGS analysis centers and the general user community. Global data centers are tasked to provide an on-line archive of at least 150 days of GPS data in the common data format, including, at a minimum, the data from all global IGS sites. The global data centers are also required to provide an on-line archive of derived products, generated by the seven IGS analysis centers. There are currently three IGS global data centers:

  • Crustal Dynamics Data Information System (CDDIS), at NASA's Goddard Space Flight Center in Greenbelt Maryland

  • Institut Geographique National (IGN) in Paris, France

  • Scripps Institution of Oceanography (SIO) in La Jolla California

These data centers equalize holdings of global sites and derived products on a daily basis. The three global data centers provide the IGS with a level of redundancy, thus preventing a single point of failure should a data center become unavailable. Users can continue to reliably access data on a daily basis from one of the other two data centers. Furthermore, three centers reduce the network traffic that could occur to a single geographical location.

Analysis Centers

Seven IGS data analysis centers (ACs) retrieve the GPS tracking data daily from the global data centers to produce daily orbit products and weekly Earth rotation parameters and station position solutions; the IGS ACs are:

  • Center for Orbit Determination (CODE) at the Astronomical Institute of Berne (AIUB) Switzerland

  • European Space Agency (ESA) in Darmstadt Germany

  • Geodatisches ForschungsZentrum (GFZ) in Potsdam Germany

  • National Resources of Canada (NRCan) (formerly Energy, Mines, and Resources, EMR) in Ottawa Canada

  • Jet Propulsion Laboratory (JPL) in Pasadena California

  • National Geodetic Survey (NGS) in Rockville Maryland

  • Scripps Institution of Oceanography (SIO) in La Jolla California

These solutions, along with summary files detailing data processing techniques, station and satellite statistics, etc., are then submitted to the global data centers within one week of the end of the observation week.

Analysis Center Coordinator

The Analysis Center Coordinator, located at NRCan, retrieves the derived products and produces a combined IGS orbit product based on a weighted average of the seven individual analysis center results. The combined orbit is then made available to the global data centers and the IGS CBIS within ten days following the end of the observation week.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Central Bureau

The Central Bureau, located at JPL, sees to the day-to-day operations and management of the IGS. The Central Bureau facilitates communication within the IGS community through several electronic mail services. The Central Bureau also has created, operates, and maintains the Central Bureau Information System (CBIS) (Liu, et. al., 1995), designed to disseminate information about the IGS and its participants within the community as well as to other interested parties. The CBIS was developed to provide a central source for general information on the IGS as well as pointers to the distributed data centers, guiding users to the most efficient access to data and product holdings. In addition, the CBIS contains general information about the current status of the GPS constellation, the IGS global network, and the various data and analysis centers associated with the IGS. The CBIS contains information about:

  • IGS organization and operation

  • global network of GPS tracking sites

  • general descriptions of GPS receivers and antennas

  • access information and data holdings summaries for the IGS data centers

  • descriptions of GPS data flow

  • up-to-date data and product availability charts

  • GPS system status

  • IGS electronic mail archives

  • software for general use (e.g., UNIX-compatible compress/decompress routines for various platforms)

  • IGS combined orbit product archive

The CBIS server is accessible over the Internet, via anonymous ftp, and the WWW.

Governing Board

The IGS Governing Board, consisting of fifteen elected members from the IGS participants, is responsible for the overall management of the IGS and recommending modifications to the organization of the service in order to improve its efficiency, reliability, etc.

OPTIMIZATION OF THE FLOW OF DATA AND INFORMATION

During the IGS design phases, it was realized that a distributed data flow and archive scheme would be required. The network of fixed GPS receivers could easily grow to over 200. Therefore, the volume of data transmitted must be optimized in order to make efficient use of electronic networks in place around the world. Furthermore, a centralized data information system would be required to monitor the flow of data and provide general information on data holdings and status of the IGS in general.

The network of IGS sites is composed of GPS receivers from a variety of manufacturers. To facilitate the analysis of these data, raw receiver data are downloaded on a daily basis by operational data centers and converted into the standard format (RINEX). Data products generated by IGS analysis centers are also available in standard formats, developed by the GPS user community.

A second area of standards employed by the IGS is in data compression. The daily GPS data in RINEX format from a single site are approximately 2.0 Mbytes in size; with a network of nearly 100 sites, this totals 200 Mbytes per day. Thus, to lessen electronic network traffic as well as storage at the various data centers, a data compression scheme was promoted from the start of the IGS test campaign. It was realized that the chosen software must be executable on a variety of platforms (e.g., UNIX, VAX/VMS, and PC) and must be in the public domain. After testing several packages, UNIX compression was the software of choice and executables for VAX/VMS and PC platforms were obtained and distributed to data and analysis centers. This data compression algorithm reduces the size of the distributed files by approximately a factor of three; thus daily GPS files average 0.6 Mbytes per site, or a total of 60 Mbytes per day.

The Central Bureau Information System (CBIS), discussed earlier, is an electronic service accessible via Internet and WWW for distributing information to the IGS user community. Although the CBIS is a central data information system, the underlying data are updated via automated queries to the distributed data centers. These queries update the CBIS data holdings information as well as GPS status reports and IGS electronic mail archives several times per day. Other data, such as station configuration logs and the official IGS product archives, are deposited when new or updated information is generated.

DATA ARCHVING AND DISTRIBUTION AT THE CDDIS

The Crustal Dynamics Data Information System (CDDIS) (Noll, 1993) has been operational since September 1982, serving the international space geodesy and geodynamics community. This data archive was initially conceived to support NASA's Crustal Dynamics Project (Smith and Baltuck, 1993); since the end of this

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

successful program in 1991, the CDDIS has continued to support the science community through NASA's Space Geodesy Program (SGP). The main objectives of the CDDIS are to store all geodetic data products acquired by NASA programs in a central data bank, to maintain information about the archival of these data, and to disseminate these data and information in a timely manner to authorized investigators and cooperating institutions. Furthermore, science support groups analyzing these data submit their resulting data sets to the CDDIS on a regular basis. Thus, the CDDIS is a central facility providing users access to raw and analyzed data to facilitate scientific investigation. A portion of the CDDIS data holdings is stored on-line for remote access. Information about the system is also available via remote download or via the WWW at the Uniform Resource Locator (URL) address:

http://cddis.gsfc.nasa.gov/cddis.html .

The CDDIS began archiving GPS tracking data in early 1992 in support of NASA programs and expanded this user community to include the IGS at the start of the test campaign in mid-1992. As stated previously, the role of the CDDIS in the IGS is to serve as one of three global data centers. In this capacity, the CDDIS is responsible for archiving and providing access to both GPS data from the global IGS network as well as the products derived from the analysis of these data.

Computer Architecture

The CDDIS is operational on a dedicated Digital Equipment Corporation (DEC) VAX 4000 Model 200 running the VMS operating system. This facility currently has nearly thirty Gbytes of on-line magnetic disk storage and 650 Mbytes of on-line rewriteable optical disk storage. The CDDIS is located at NASA's Goddard Space Flight Center and is accessible to users 24 hours per day, seven days per week. The CDDIS is available to users globally through electronic networks using TCP/IP (Transmission Control Protocol/Internet Protocol) and DECnet (VAX/VMS networking protocol), through dial-in service (300-, 1200-, 2400- and 9600-baud) and through the GTE SprintNet system.

Currently, two magnetic disk drives, totaling nearly eight Gbytes in volume, are devoted to the storage of the daily GPS tracking data; a third drive is used for storing GPS products, special requests, and supporting information. A dual-drive, rewriteable optical disk system provides additional on-line disk storage for GPS data. This unit contains two 5.25 inch optical disk drives with a capacity of 325 Mbytes per platter. These disks also serve as the long-term archive medium for GPS data on the CDDIS. Approximately one week of GPS tracking data (with a network of eighty sites) can be stored on a single side of one of these platters. The older data continues to be stored on these optical disks and can easily be requested for mounting and downloading remotely by the user. Alternatively, if the request is relatively small, data are downloaded to magnetic disk, providing temporary on-line access.

CDDIS GPS Archive

The IGS data are retrieved and/or transmitted daily by operational and regional data centers to the global data centers. For the CDDIS, the Australian Survey and Land Information Group (AUSLIG) in Belconnen Australia, NOAA's Cooperative International GPS Network (CIGNET) Information Center (CIC) in Rockville Maryland, ESA, GFZ, the Geographical Survey Institute (GSI) in Tsukuba Japan, JPL, and NRCan make data available to the CDDIS from selected receivers on a daily basis. In addition, the CDDIS accesses the remaining two global data centers, SIO and IGN, to retrieve (or receive) data holdings not routinely transmitted to the CDDIS by a regional data center. Table 1 lists the data sources and their respective sites that are currently transferred daily to the CDDIS. These data are summarized and archived to public disk areas in daily subdirectories; the summary and inventory information are also loaded into an on-line data base. Status files are also updated reflecting the current data holdings and time delays in data delivery; these files are automatically uploaded to the IGS CBIS.

The IGS data archive of the CDDIS consists of the GPS observation and navigation files in compressed RINEX format as well as summaries of the observation files used for data inventory and reporting purposes. These data are organized on disk by GPS day and by file type (observation, navigation, summary). Under the current ninety to one hundred station network configuration, approximately 150 days worth of GPS data are available on-line to users at one time. During 1994, the CDDIS archived data on a daily basis from an average of sixty stations; during 1995, this number increased to over ninety stations by the end of the year. Each site produces approximately 0.6 Mbytes of data per day; thus, one day's worth of GPS tracking data, including the CDDIS inventory information, totals nearly 60 Mbytes. For 1995, the CDDIS GPS data archive totaled over eighteen Gbytes in volume; this represents data from over 30K observation days. Of the ninety or more sites archived each day at the CDDIS, not all are of “global” interest; some, such as those in Southern California, are

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

regionally oriented. The CDDIS receives data from these sites as part of its NASA archiving responsibilities.

TABLE 1 Sources of GPS Data Transferred to the CDDIS

Source

   

Sites

   

No. Sites

AUSLIG

CAS1

DAV1

HOB2

MAC1

   

4

CIGNET

BRMU

FORT

HNPT

KELY

RCM5

SOL1

9

 

USNA

WES2

WUHN

       

NRCan

ALBH

ALGO

DRAO

STJO

YELL

 

5

ESA

KIRU

KOUR

MALI

MAS1

PERT

VILL

6

GFZ

KIT3

LPGS

POTS

ZWEN

   

4

GSI

TAIW

TSKB

       

2

IGN

ANKR

BOR1

BRUS

EBREa

GRAS

GRAZ

24

 

HART

HERS

IRKT

JOZE

KERG

KOSG

 
 

MATE

MDVO

METS

NYAL

OHIG

ONSA

 
 

PAMA

REYK

TROM

WETT

WTZR

ZIMM

 

JPL

AOA1

AREQ

AUCK

BOGT

CARR

CASA

42

 

CAT1

CHAT

CICE

CIT1

CRO1

EISL

 
 

FAIR

GODE

GOLD

GUAM

HARV

IISC

 
 

JPLM

KOKB

LBCH

MADR

MCM4

MDO1

 
 

NLIB

OAT2

PIE1

QUIN

SANT

SEY1

 
 

SHAO

SNI1

SPK1

THU1

TIDB

UCLP

 
 

USC1

USUD

WHC1

WHI1

WLSN

YAR1

 

KOREA

TAEJ

         

1

SIO

MONP

PIN1

PVEP

SIO3

VNDP

 

5

UNAVCO

POL2

         

1

Totals:

     

103 sites from 11 data centers

a EBRE data currently delivered by ICC but will be delivered by IGN upon resolution of communication issues.

IGS Data and Product Archive Procedures

Software was written at the CDDIS to automatically process the incoming IGS data on a daily basis. The programs are queued to run automatically at certain times of the day to check the receiving areas, i.e., the operational data center accounts on the CDDIS, for new IGS data. The new GPS data files are then processed: the data are transferred to the correct daily subdirectories on the public disk areas, the observation data are then decompressed and summarized, and finally the summary information is loaded into the CDDIS data base. The summary process performs elementary data checking and extracts information that can be used to inventory the data in the CDDIS. Figure 2 illustrates the data flow, from station to public archive on the CDDIS. Typically, the archiving routines on the CDDIS are executed several times a day for each source in order to coincide with their automated delivery processes. In general, the procedures for archiving the GPS tracking data are fully automated, requiring occasional monitoring only, for replacement data sets or re-execution because of system or network problems.

The CDDIS generates weekly reports utilizing the summary information available in the on-line data base. Additional reports summarize the GPS data on a yearly basis. The CDDIS also maintains an archive of and indices to IGS Mail and Report messages. These reports are distributed via the IGS reporting mechanisms and stored on-line for ftp download or browsing through the WWW.

The derived products from the IGS Analysis Centers are also delivered to the CDDIS, usually within seven days of the end of the observation week; the combined products produced by the IGS Analysis Coordinator are available a few days following the arrival of all product files from the individual ACs, typically within ten days. Automated routines to copy the IGS products to the public archives are also executed on the CDDIS several times per day. The files from all analysis centers as well as the combined orbit are stored on disk by GPS week. All IGS products generated since the start of the IGS test campaign in June 1992 are available on-line on the CDDIS.

Access Procedures

The CDDIS is an open archive which serves a wide user community interested in space geodesy and geodynamics data. Therefore, any interested users are permitted access to the archive of GPS data and products.

As stated previously, the data archives on the CDDIS are accessible remotely through Internet, DECnet, and dial-up phone lines. Potential users of the CDDIS are asked to request the user account name and password information since the GPS archives are not accessible through an open or “anonymous” account. The CDDIS permits both remote file transfer and direct connections through Internet (i.e., ftp or telnet) and DECnet (i.e., COPY over the network or SET HOST). Dial-up users can run KERMIT or XMODEM software on the CDDIS to upload GPS data and products to their remote hosts. General information about the CDDIS and the GPS data availability, as well as a link to the IGS CBIS and other related resources, are accessible through the CDDIS home page on the WWW.

Users interested in data from time periods currently not on-line can submit a special request to the CDDIS. The staff will then attempt to satisfy this request in the most efficient method possible. A significant amount of staff time is expended on fielding inquiries about the IGS and the CDDIS data archives as well as identifying and making data available from the off-line archives. To satisfy requests for off-line data, the CDDIS copies data from optical disk archive to an on-line magnetic disk area, or for larger requests, mount the optical disks in a

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

scheduled fashion, coordinating with the user as data are downloaded.

Statistics On Data Flow

An important goal of the IGS is to provide analysis centers timely access to the GPS tracking data. The initial equirements stated that data from global sites was to be available for analysis through the global data centers within 72 hours of the end of the observation day. The IGS Pilot Service tightened this requirement and had requested that data be made available within 48 hours. To meet this objective it is imperative that regional data collection centers retrieve data from the receivers as regularly and in as automated a fashion as possible. Data retrieval and transmission must be routine and programmed to allow for consistent flow of data on a daily basis. Upon arrival at the CDDIS, these data must be processed quickly and made available to the user community. The CDDIS typically processes data within hours of receipt via automated procedures.

FIGURE 2 Flow of GPS data from IGS site to the CDDIS.

A majority of the data delivered to and archived on the CDDIS since the start of the IGS operational service in 1995 were available to the user community within 24 hours after the observation day. Figure 3 shows that seventy-five percent of the data from all sites delivered to the CDDIS were available within one day of the end of the observation day; nearly ninety percent were available within two days. Figure 4 shows these statistics in hours for the current set of “global stations”, those stations processed by three or more IGS Analysis Centers on a daily basis.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 3 CDDIS GPS data availability statistics (all stations).

FIGURE 4 CDDIS GPS data availability statistics (global stations only).

As can be seen, approximately eighty percent of the data are available to users within 36 hours. These statistics were derived from the results of the daily archive report utilities (Gurtner and Neilan, 1995) developed by the IGS Central Bureau and executed several times each day on the CDDIS. The IGS tracking stations and data centers hope to further improve these statistics by testing new procedures to push data to the global data centers even faster. This already remarkable statistic would not have been possible, however, without the adoption of standards in data formatting and compression.

The chart shown in Figure 5 summarizes the monthly usage of the CDDIS for retrieval of GPS data during 1994 and 1995. The data reflected in this figure was produced daily by automated routines that peruse the log files created by each network access of the CDDIS. In total, nearly 640K files where transferred, amounting to approximately 160 Gbytes in volume for 1994. Averaging these figures for that year, users transferred 53K files per month, totaling 13 Gbytes in size. In 1995, nearly 1.25 million files were transferred totaling 320 Gbytes in volume. Users transferred on average 104K files, 27 Gbytes in volume, per month in 1995. The chart in Figure 6 details the total number of host accesses per month with the number of distinct (i.e., unique) hosts (i.e., users) per month shown as an overlay. Here, a host access is defined as an initiation of an ftp or remote DECnet copy session; this session may list directory contents only, or may transfer a single file or many files. Figure 7 illustrates the profile of users accessing the CDDIS during 1995; these figures represent the number of distinct hosts in a particular country or organization. Nearly half of the users of GPS data available from the CDDIS come from U.S. government agencies, universities, or corporations. As can be seen, the system usage continues to grow as GPS technology is utilized by an increasingly diverse user community.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 5 Data volume number and of files transferred to/from the CDDIS during 1994 and 1995 in support of the IGS.

FIGURE 6 Number of host access to the CDDIS during 1994 and 1995 in support of the IGS.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 7 Distribution of IGS users of the CDDIS.

CONCLUSIONS

The IGS has shown that near real-time availability of GPS data is a reality. The hierarchy that was established in both tracking stations and data centers has streamlined data flow, with the global data center serving as the main interface between the data and the user. Standards in data formats and compression software are essential to the successful operation of the IGS. Furthermore, automation in data archiving and retrieval is a necessity in order to provide near real-time access to data over an extended period of time. The IGS has found, however, that some data flow paths require optimization in order to prevent the flow of redundant data to data centers, as well as scheduling of data deliveries to avoid congestion over electronic networks. The IGS would also like to encourage the stations and operational data centers to upload the data to regional and global data centers even faster than the current 36 hour average. This schedule would permit the analysis centers to produce more rapid orbit products.

REFERENCES

Beutler, G. “The 1992 IGS Test Campaign, Epoch ‘92, and the IGS Pilot Service: An Overview” in Proceedings of the 1993 IGS Workshop. Druckerei der Universitat Bern. 1993.

Blewitt, G., Y. Bock, and J. Kouba. “Constructing the IGS Polyhedron by Distributed Processing” in Proceedings of the IGS Workshop on the Densification of the ITRF through Regional GPS Networks. JPL . 1995.

Gurtner, W. “RINEX: The Receiver Independent Exchange Format” in GPS World, v. 5, no. 7. July 1994.

Gurtner, W. and R. Neilan. “Network Operations, Standards and Data Flow Issues” in Proceedings of the IGS Workshop on the Densification of the ITRF through Regional GPS Networks. JPL. 1995.

Liu, R., et. al.. “Introducing the Central Bureau Information System of the International GPS Service for Geodynamics” in International GPS Service for Geodynamics Resource Information. January 1995.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Noll, C. E. “Data Archiving and Distribution for the Crustal Dynamics Project: The CDDIS” in Contributions of Space Geodesy to Geodynamics: Technology. AGU Geodynamics Series, Vol. 25. 1993.

Remondi, B. W. “Extending the National Geodetic Survey Standard Orbit Formats” in NOAA Technical Report NOS 133 NGS 46. 1989.

Smith, D. E. and M. Baltuck. “Introduction” in Contributions of Space Geodesy to Geodynamics: Crustal Dynamics. AGU Geodynamics Series, Vol. 23. 1993.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
Central Bureau Information System of the International GPS Service for Geodynamics

James Zumberg

Jet Propulsion Laboratory

The Central Bureau Information System (CBIS) of the International GPS Service for Geodynamics (IGS; see companion article by Beutler) provides public access to products of the IGS, and also provides a means of electronic messaging among IGS participants. The system, developed in late 1993 by W Gurtner of the Astronomical Institute, University of Berne, operates at the Jet Propulsion Laboratory where it is maintained by R Liu and M Urban. The system is completely automated.

The CBIS is a client on the Internet's World Wide Web, allowing access with Mosaic, Netscape, and Lynx interfaces at the address http://igscb.jpl.nasa.gov/ . At the top level of the CBIS one views the following:

The IGS accomplishes its mission through the following components:

  • a network of about 50 globally distributed permanent Tracking Stations;

  • several Operational Centers operating parts of the global network ;

  • global and several regional Data Centers;

  • currently 7 Analysis Centers and several Associate Analysis Centers ;

  • a Central Bureau located at the Jet Propulsion Laboratory and ;

  • a Governing Board.

The charter of the IGS is defined in the Terms of Reference.

The Central Bureau operates an Information System (CBIS) to provide all necessary information to both IGS contributors and the public organizations and individuals who use the IGS products such as orbits and tracking data.

Categories in italics can be selected for further information. By sending a message to igscb@igscb.jpl.nasa.gov , you can request that your Internet e-mail address be added to one or more of the mail and report distribution lists, and receive information on how to send your own messages to the system.

Anonymous ftp access is also available at igscb.jpl.nasa.gov (ip address 128.149.70.171). Once connected, cd igscb will take you to the area shown in detail in Table 1. Of particular interest are:

  • log files for each station, which identify receiver and antenna type, and history;

  • Analysis Center information, including techniques and strategies;

  • Data Center information, including access instructions;

  • GPS file format information for Rinex (data) and sp3 (precise orbits);

  • IGS precise orbits.

The CBIS is undergoing revision to make navigation within it simpler and more intuitive.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

TABLE 1 Directory Structure of Files in the Central Bureau Information System

IGSCB.DIR

NEWS.TXT

README.TXT

TREE.TXT

complete file list

new features/changes

CBIS general info

directory structure info

center

README.CEN

center info

Analysis Center descriptions

Data Center descriptions

Operational Center descriptions

analysis

‘center'.acn

data

‘center'.dcn

oper

‘center'.ocn

data

format

rinex2.txt

sp3.txt

RINEX format specifications

SP3 orbit format specifications

data center holdings

data center holdings by year

data availability by month

data availability by year

IGS data network diagram

holding

‘center'.syn

‘center'.s‘yy'

glob‘mmyy'.syn

glob‘yyyy'.syn

network

igs.net

general

glonass

constell.glo

nagu‘yyyy'.mes

nagu‘yyyy'.sub

GLONASS constellation status

NAGU messages by year

NAGU subject index by year

NANU GPS constellation status

EUREF Information System info

NANU messages by year

NANU subject index by year

catalog of GPS-related info sources

ZIMM current tracking status

AGU symposia/meetings

IAG symposia/meetings

gps

constell.gps

euref.txt

nanu‘yyyy'.mes

nanu‘yyyy'.sub

sources.txt

status.zim

org

meetings.agu

meetings.iag

mail

address

cddis.adr (.Z)

directory.txt

dose.adr

igsmail.adr

igsreport.adr

scign.adr

IGSMESS.INDEX

igsmess.‘nnn'

IGSREPORT.INDEX

igsreport.‘nnn'

CDDIS SGP address catalog

IGS Colleague Directory text

DOSE Mail distribution list

IGS Mail distribution list

IGS Report distribution list

SCIGN Mail distribution list

IGS Mail message index

IGS Mail messages

IGS Report index

IGS Reports

DOSE Mail archive

SCIGN Mail archive

igsmail

igsreport

regional

DOSE

SCIGN

product

‘wwww'

igs‘wwww'7.erp

igs‘wwww'[0-6].sp3

igs‘wwww'7.sum

IGS earth rotation parameters

IGS combined daily orbits

IGS weekly product summary

analysis center product holdings

IERS earth orientation

IERS earth rotation parameters

holding

‘center'.prd

iers

bulletinb.‘nn'

eop90c04.‘yy'

resource

g_board.igs

resource‘nn'.ps

terms.igs

IGS Governing Board

IGS Resource Information (PostScript)

IGS Terms of Reference

software

cbis

dos, unix, vms

CBIS browsing/ftp program

compression/decompression programs

quality check program for GPS data

compress

dos, vms

qc

aux, dos, unix, vms

station

coord

igsmap.ps

itrf‘yy'.ssc

map of IGS tracking stations (PostScript)

ITRF92 station coordinates

station log form (blank)

antenna diagrams

receiver/antenna table

station logs

old station logs

local tie changes/updates

local tie file

general

BLNKFORM.LOG

antenna.gra

rcvr ant.tab

log

‘site'‘mmyy'.log

oldlog

‘site'‘mmyy'.log

tie

localtie.chg

localtie.tab

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
GPS Operations and Data Handling at JPL

Garth Franklin, Byron Iijima, Peter Kroeger, Ulf Lindqwister, Thomas Lockhart, Anne Mikolajcik, Mark Smith

Jet Propulsion Laboratory

INTRODUCTION

JPL/NASA has been installing and operating permanent GPS stations for 5+ years, starting with the deployment of the 6-station TOPEX/POSEIDON ground tracking network. This permanent Network was installed during the early 1990s in support of the Topex oceanographic mission in collaboration between JPL/NASA, CNES, CEE, and ISAS. Since then JPL/NASA has installed an additional 30+ stations globally in support of the IGS and the GPS Global tracking Network, and another 20+ stations for various regional and local Networks (for example, the SCIGN array in Southern California) and projects (for example, the permanent DOSE site at Mammoth Lakes). We are currently operating 55+ permanent GPS stations for global, regional, and local Networks and projects. Current plans call for implementing another 20-25 sites in the next 2-3 years.

DATA HANDLING

JPL/NASA uploads data via regular telephone lines, Internet, and NASCOM (direct NASA communications lines from the three DSN stations) in 24-hour or 1-hour file segments. All routine data uploading and handling operations at the JPL/NASA data center have been automated. The data transfers start immediately after UTC midnight, and under ideal conditions all the data is obtained within 12 hours. In practice, 95+% of the data is collected automatically every day, with the remaining data uploaded the next day by the automated upload system or manually. All global stations that are part of the IGS Network are forwarded daily to the CDDIS Global Data Center at the Goddard Space Flight Center.

The data is uploaded automatically via telephone lines or direct serial connections using Microphone Pro scripts running on Macintosh computers. The networked Macintoshes at JPL use Telebit T2500 Trailblazer modems to dial up stations with standard telephone connections. Three parallel lines are currently in use to dial 35+ stations. The data files are usually uploaded in CONAN binary format to reduce data transmission time and save costs. Remote Macintoshes, which are connected to the Internet, use direct serial connections to the TurboRogue receiver to upload data from 20 stations. The resulting files are stored on the Macintoshes until a workstation at JPL completes a successful FTP transfer from the Macintoshes to the local workstation (after which the file is removed from the Macintosh).

The data collection and handling computer at JPL is a DEC 3000/500 Alpha workstation which transfers the files from the Macintoshes and then imports them into the GPS Network Operating System (GNOS) which inventories, validates, formats, and distributes the data. GNOS is controlled by an INGRES database that stores information about each data set collected. This information is used to determine the status of the site which is displayed in an easily understood format to the Network Operator.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Site

Location

Current Communications

Transfer to JPL (bps)

Planned Communications

Offloads per day current (planned)

gold

Goldstone, CA

Internet

N/A

 

1 (24)

madr

Madrid, Spain

Internet

N/A

 

1 (24)

tidb

Canberra, Australia

Internet

N/A

 

1 (24)

cro1

St. Croix, U.S. Virgin Islands

Internet

11470

 

1 (24)

eisl

Easter Island

Internet

6000

 

1 (24)

gol2

Goldstone, CA

Internet

64000

 

24

jplm

Pasadena, CA

Internet

983000

 

1 (24)

mcm4

McMurdo,

Antarctica

47500

 

24

sant

Santiago, Chile

Internet

7450

 

1 (24)

nlib

North Liberty, IA

Internet

6917

 

1 (24)

pie1

Pie Town, New Mexico

Internet

6091

 

1 (24)

cice

Ensenada, Mexico

Internet -> Radio Modem

26200

 

1 (24)

iisc

Bangalore, India

Internet -> Short Haul Modem

6300

 

1 (24)

fai2

Fairbanks, AK

Internet -> Switch

40960

 

24

fair

Fairbanks, AK

Internet -> Switch

40960

 

1 (24)

kokb

Kokee, HI

Internet -> Switch

16380

 

24

tid2

Canberra, Australia

Internet -> Switch

54000

 

24

auck

Auckland, New Zealand

Internet -> Telephone

50790 / 6960

 

1

chat

Chatsworth, New Zealand

Internet -> Telephone

50790 / 5557

 

1

moin

Limon, Costa Rica

Internet -> Telephone

44530 / 5234

 

1 (24)

braz

Brasilia, Brazil

Telephone

N/A

Internet -> Telephone

1 (24)

gode

Goddard, MD

Telephone

6418

Internet

1 (24)

guam

Guam

Telephone

3781

Internet -> Telephone

1 (24)

mdo1

McDonald, TX

Telephone

6187

Internet

1 (24)

quin

Sacramento, CA

Telephone

6446

Internet

1 (24)

sey1

Mahe, Seychelles

Telephone

2535

DSN Phone lines

1 (24)

thu1

Thule, Greenland

Telephone

2703

Internet -> Telephone

1 (24)

usud

Usuda, Japan

Telephone

5259

Internet

1 (24)

yar1

Yaragadee, Australia

Telephone

7824

Internet

1

bogt

Bogota, Columbia

Telephone (shared)

N/A

Internet

1 (24)

shao

Shanghai, China

Telephone (shared)

2123

Internet -> Telephone

1 (24)

areq

Arequipa, Peru

Telephone -> Fax Switch

2356

Internet

1 (24)

kwaj

Kwajalein Island

Telephone

N/A

Internet Mail

1 (24)

gal1

Galapagos Island

Mail

N/A

Internet

1

SCIGN

Southern California

Telephone

7000

 

1

FIGURE 1 JPL operated permanent GPS reference stations.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 2 GPS data flow for JPL operated sites.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

JPL Data Arshive

Anonymous FTP access:

machine: bodhi.jpl.nasa.gov (128.149.70.66)

Username: anonymous

Password: Your E-Mail Address (for statistical purposes)

Tree Structure:

FIGURE 3 JPL data archive.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
National Geodetic Survey Continuously Operating Reference System (CORS)

William. Strange and Neil Weston

National Geodetic Survey, National Ocean Service, NOAA

INTRODUCTION

The National Geodetic Survey (NGS) Continuously Operating Reference System (CORS) makes available for multiple use GPS observational data from stations operated by a number of different organizations to meet their specific needs. Special emphasis is placed on providing pseudorange and carrier phase data from stations established to broadcast real time correctors in support of navigation. A major objective of the NGS CORS is to provide surveying, mapping, and other positioning users easy access to the National Spatial Reference System (NSRS). The CORS is also intended to support GPS applications in areas such as crustal deformation, atmospheric water vapor determination, and ionospheric studies. CORS interaction with the Earth and Atmospheric Sciences is two directional. GPS stations established to support navigation, surveying, and other positioning activities can provide science users data to support their analyses. The science users can support upgrading of the CORS stations and the feedback from their analyses can allow application users of GPS to increase the ease and reduce the cost of performing their positioning.

THE CORS NETWORK

A basic premise of the CORS is that NGS will focus on making available data from GPS reference stations established by other groups rather than establishing additional stations. The CORS stations can be divided into four groups as a function of the operating agency objectives. These four groups are:

  • Real Time Navigation Stations

  • Positioning Support Stations

  • Atmospheric Analysis Stations

  • Geodynamic Research Stations

Currently data is being provided from 70 CORS stations (see Figure 1) these stations are divided among the four groups as described below.

Real Time Navigation Stations

Several Federal agencies have established or will establish GPS reference stations to support real time navigation. The differential GPS (DGPS) network furthest along in development is that of the United States Coast Guard (USCG) which was declared initially operational on 30 January 1996. By agreement between the USCG and NGS the data from this network is made available to the CORS for distribution to non-navigation users. The CORS is currently receiving data from 39 of the 46 USCG stations. Data from the remainder of these stations will become available over the next few months. The USCG is also supporting the U. S. Army Corps of Engineers (USACE) in establishing similar stations along navigable rivers. Currently CORS receives data from three USACE stations. Data from at least six additional stations are expected over the next six months.

The USCG and USACE stations have two sets of receivers and antennas at each site (Ashtech ZXII receivers). With few exceptions the antennas are mounted on 10 to 30 ft. Rohn towers located 20 to 50 meters apart. Antenna height is controlled by the requirement to track satellites to a 7.5 degree elevation angle. CORS takes data continuously from only one system with the other system sampled intermittently and serving as a backup if the primary system goes down. The potential exists to sample both systems continuously. Data are transmitted to the CORS Central Data Facility (CDF) using the AT&T X.25 telephone packet service. Raw GPS data is taken directly from the receiver to a Packet Assembler/Disassembler (PAD), placed in packets, and transmitted to the CDF within milliseconds after it is observed. The receivers can sample at a 0.5 sec sample rate, but transmission line capacity limits CORS access to a maximum sample rate of five seconds. These stations are now sampled at 30 sec rate because of cost considerations.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 1 CORS network as of April 1996.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

The possibility exists that USCG type DGPS stations may be extended nationwide. During 1996 tests are planned to evaluate the use of such stations for positive train control for railroads. Studies are also underway to evaluate the use of such stations to support real time automobile positioning. It is estimated that about 40 additional stations would be required to cover the rest of the U.S.

Another real time navigation program that is well underway is the Federal Aviation Administration (FAA) program to provide GPS air navigation. This program has two components -- a Wide Area Augmentation System (WAAS) and a Local Area Differential GPS (LADGPS) System. The Contract to develop the WAAS has been awarded. This contract specifications reflects the requirements to provide data to CORS. The approximately 26 WAAS stations will have multiple systems and will provide data real time to the CORS. Data from these stations is expected to become available to CORS over the next two to three years.

Positioning Support Stations

Numerous GPS reference stations are being established by Federal, state and local governmental agencies to support surveying, mapping, engineering and Geographical Information System (GIS) requirements. A number of stations of this type which employ dual frequency receivers with full wavelength L2 are being incorporated into the CORS. Thirteen stations of this type are now providing data. Ten of these stations are operated by the Texas Department of Transportation. These stations have Trimble SSE receivers with a single system at each site. The antennas are mounted either on buildings or on towers located above monumented points. Data from all stations is aggregated in Austin, Texas in raw Trimble format and is downloaded daily over INTERNET.

The other three positioning support stations include a station operated by the Riverside Water and Flood Control District at Blythe, California. This station has an Ashtech ZXII receiver. Data from this station is downloaded by the Scripps Institute of Oceanography (SIO). The data is downloaded from SIO to CORS daily over INTERNET. Another station is operated by the Harris-Galveston Flood Control District in Houston, Texas to support local subsidence monitoring. The antenna is mounted on top of a compaction meter rod which is anchored below the compacting layer. This station has a Trimble SSE receiver. Data is downloaded daily over INTERNET. The final station of this type is the station established by NGS at Gaithersburg, Maryland. This is the only station now providing data at a five second sampling rate. Data is downloaded over INTERNET hourly. The station has a Trimble SSE receiver.

Atmospheric Analysis Stations

The NOAA Forecast Systems Laboratory (FSL) is establishing stations to investigate the use of GPS to monitor precipitable water vapor in support of weather forecasting and climatology. Seven of these stations are currently providing data to CORS. These stations also provide data from surface meteorological sensors. The data from these stations is downloaded to FSL in Boulder at 30 minute intervals and converted to the RINEX format. At present the data is downloaded to the CORS CDF once daily over INTERNET. Within the next few months it is planned to begin downloading the data hourly over INTERNET.

Additional stations of this type are planed by FSL over the next year. As this technology is developed to support operational weather forecasting several hundred stations of this type can be expected to be established across the United States. All of these stations are occupied By Trimble 4000 SSE receivers.

Geodynamic Stations

Nine stations now providing data to CORS are stations established to support reference system establishment, orbit determination, and crustal motion monitoring. Four of these stations are operated by NGS. The data from these stations is obtained once daily directly from the stations over INTERNET. The other five are NASA supported stations operated by the Jet Propulsion Laboratory (JPL). The data from these stations is downloaded daily over INTERNET from the CDDIS data center located at NASA Goddard Space Flight Center. The receivers at these sites are some version of Rogue/TurboRogue receivers.

THE CENTRAL DATA FACILITY

The CORS Central Data Facility (CDF) performs five primary functions: acquisition; formatting; quality control; dissemination; and archiving.

Equipment Compliment

The CORS CDF is in the process of being upgraded and will be operating in the upgraded mode shortly. This

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

discussion describes the upgraded facility. Figure 2 shows the hardware configuration. A data acquisition computer (HP745) will take in data from all stations, both over a 56K bps X.25 telephone line and over the INTERNET. This computer will format the data into a standard RINEX, version 2, format (if it is not already in RINEX), place the data in files in a compressed format, run the data through the UNAVCO Quality Control program and transfer the data to a data distribution computer (HP 715/33). The data distribution computer keeps the data for 31 days for direct INTERNET access. INTERNET access can be obtained using the World Wide Web (WWW), through the NGS home page either by way of the NOAA home page or by typing in the Uniform Resource Locator (URL) which is http://www.ngs.noaa.gov when using a browsing tool such as Mosaic. The CORS data can also be accessed using the ftp command as follows:

ftp proton.ngs.noaa.gov

Login: anonymous

Password: your complete E-mail address

cd cors

Data from the USCG and USACE stations and the NGS Gaithersburg station are placed in hourly files, with the data available within minutes after the end of each hour. Data for other stations are placed in daily files and are available the following day. After 31 days data is migrated to a CD ROM mastering machine which operates in a UNIX environment and produces CD ROM masters. The number of days of data on each CD ROM is determined automatically and changes with time as more stations are brought on line. Data requests for data more than 31 days old are filled in one of two ways. If a small amount of data is needed the CD ROM is mounted and the data copied to the CORS computer in a special file for INTERNET access by the user. If a large amount of data is required the relevant CD ROMs are copied.

Because data from the USCG and USACE stations are not stored on site, any disruption at the CDF could result in permanent loss of data from a large number of stations. To minimize this possibility a parallel computer setup is being implemented as shown in Figure 2. With this parallel setup the data files transferred from the primary acquisition computer to the primary distribution computer are immediately sent to the secondary distribution computer. Thus, all files are maintained on both distribution computers and users can be automatically transferred to the secondary distribution computer if the primary computer goes down. The secondary acquisition computer continually queries the primary acquisition computer to determine if it is functioning. If it finds the primary acquisition computer has failed to function, the secondary computer will automatically take over the data acquisition function.

File Structure

The information contained in the CORS directory is placed in a README file, which contains general information on CORS and on using the data and program subdirectories of the CORS directory. There are five sub-directories containing data, utility programs, and information (Figure 3). These are:

  • rinex files containing the observational data in RINEX, version 2, format;

  • coord files containing the ITRF and NAD 83 coordinates of station antenna L1 phase centers;

  • station_log files containing site information for the stations similar to that contained in the IGS log files;

  • utilities programs that can be used to manipulate the RINEX files;

  • itrf files with information about the ITRF reference system.

Under the rinex subdirectory the sub-directory structure is as indicated in figure 3. The actual RINEX data files have the following data structure:

[ SSSS ][ DDD ][ H ].[ YY ][ T ] ;

where SSSS is a four character site antenna identifier, DDD is the day of the year, H for stations with hourly files (this is a letter to identify the UTC hour of the day), YY is the year, and T is the file type.

The four character site identifier is coordinated with IGS to prevent duplication. Many of the CORS sites have more than one receiver and antenna present. In such situations the last character of the identifier is a number which identifies the antenna being referenced. The letter assigned to H represents the UTC hour of the data in the file according to the convention: a = hour beginning at 00 UTC; b = hour beginning at 01 UTC; etc. The file type identifier, T, can be one of four characters:

o - observational data file;

n - navigation file;

s - summary file;

m - meteorological data file.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

CORS Data Collection and Distribution

(Proposed Configuration, Phase 1.1)

FIGURE 2 CORS data collection and distribution.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

FIGURE 3 Directory structure.

The utility programs available in the utilities subdirectory are DOS based. There are also versions that work with Silicon Graphics, Sun Microsystem, and Hewlett Packard environments. The programs available are:

  • gzip386.exe - program to compress and uncompress files;

  • join24pc.exe - program to join RINEX observation or navigation hourly files;

  • cato.exe - program to join RINEX observation files;

  • decimate.exe - program to eliminate data to produce a reduced data rate (e.g. to go from a 5 sec data set to a 30 sec data set).

Note: program join24 will not join files across midnight.

Operational Considerations

Surveying as well as scientific users of GPS are increasingly interested in centimeter (or better) accuracy in the vertical coordinate. To support this accuracy it is necessary to model antenna phase center variations to allow mixing of antenna types. To make such models available to surveying users they must be incorporated into commercial software. NOAA has underway an extensive program to develop and test antenna phase center models. A test range has been established at the NGS facility at Corbin, Virginia and a continuing program of determination of antenna phase center models based on field measurements is underway. Evaluation of the models based on testing at Corbin and at other sites in the Washington, D.C., area where there are precise ground connections between CORS sites and adjacent ground monuments is also underway.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×

Mitigation of multipath is important to a wide variety of users of CORS data. It would be optimum to have all CORS stations equipped with antennas having maximum resistance to multipath and to have these antennas mounted in such a way as to minimize multipath. However, it is unrealistic to expect that operators of stations of the CORS network will fund new antennas that are not required for their application. Scientific users of CORS data should consider funding the implementation of choke ring antennas at existing sites as an inexpensive way of gaining additional stations for use in scientific applications. In so far as mounting of antennas are concerned there are often operational constraints. For example, the USCG has placed its stations, whenever possible, at existing facilities where there is access to support and broadcast antennas. The USCG antennas are placed on 3 to 10 meters above the ground to allow tracking down to 7.5 degrees above the horizon. NGS is working with the USCG to determine the magnitude of multipath at the USCG sites, to determine the effectiveness of improved antenna types in decreasing multipath and to develop multipath models on a sit specific basis.

Antenna stability is of great interest to scientific users of CORS data. Many scientific users of CORS data would like antenna stability at the millimeter level. Practical considerations may make this difficult if not impossible to achieve at many sites. The large distance above the ground required by the USCG for their antenna locations would make millimeter stability extremely expensive, if not impossible. Certainly the scientific community could be expected to fund the incremental cost of achieving millimeter stability. At the USCG sites there are ways of evaluating antenna stability. Because there are two antennas at each site the differential position between the two antennas can be monitored and used as a measure of antenna stability. Also, NGS has established two ground monuments at the USCG sites and positioned them relative to the CORS antennas. This provides another means of monitoring antenna stability.

Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
×
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Page 95
Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Page 96
Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Page 97
Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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Suggested Citation:"7 Networks and Data Sources." National Research Council. 1997. The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications. Washington, DC: The National Academies Press. doi: 10.17226/9254.
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