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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 2 Networks and Data Sources SYNOPSIS OF PROCEEDINGS Networks of reference stations using the positioning, navigation, and timing capabilities of the basic GPS are spreading across the globe. This session of the workshop provided an opportunity to assess the rapidly developing GPS network infrastructure by identifying planned and existing capabilities in the United States and abroad. Papers were presented describing a number of reference station networks, the applications they were designed for, and the data they generate. Some of these networks, such as the U.S. Coast Guard's DGPS Service, broadcast real-time differential corrections to users to improve navigational accuracy. Others, such as the Southern California Integrated GPS Network (SCIGN), collect and archive GPS-derived data, which is then post-processed and used for research, such as measuring tectonic plate movements and monitoring seismic activity. Loni Czekalski provided an overview of the Federal Aviation Administration 's (FAA) GPS augmentation systems, including the Wide-Area Augmentation System (WAAS) and the Local-Area Augmentation System (LAAS). Most of her presentation focused on the WAAS, which consists of ground-based GPS reference stations, called wide-area master stations or wide-area reference stations; ground earth stations for Earth-to-satellite communications; and geostationary satellites to broadcast differential corrections and integrity data to users within U.S. domestic airspace. Data collection capabilities and geodetic monumentation considerations, both important to potential geoscience applications, were also mentioned. Gene Hall described the U.S. Coast Guard's (USCG) DGPS Navigation Service, which included 51 GPS reference stations transmitting differential corrections as of March 1, 1996. His overview covered network architecture, the technical characteristics of reference stations, system performance, site locations, and current operational status. Several networks located within the United States used primarily for post-processed data applications were described during this session of the workshop. These include the SCIGN, the Bay Area regional deformation (BARD) network, and the precision GPS geodetic array (PGGA). All of these networks are located in California and are used primarily for seismic research and monitoring crustal deformation. A network of reference stations established in the central United States by the Forecast Systems Laboratories of the National Oceanic and Atmospheric Administration (NOAA) was also described. This network was established for the purpose of collecting data on precipitable water vapor. According to the authors, this data may contribute to improved weather forecasting, climate monitoring, and geodetic positioning. However, the limited size of the network makes it difficult to validate this claim. The authors advocate co-locating surface meteorological sensors with existing GPS reference stations as a cost-effective means of performing the necessary validation. Other nations are also establishing and operating GPS networks to support a variety of geoscience applications. The authors of “Large Permanent GPS Networks in Japan” described a combination of two networks called GRAPES and COSMOS that will eventually be expanded to nearly 1,000 stations by Japan's Geographic Survey Institute. The average spacing between reference stations for the combined network will be approximately 20 kilometers throughout Japan. Future use of this network for both geophysical and meteorological research is currently being considered. Several papers discussed the global GPS network operated by the International GPS Service for Geodynamics (IGS). This organization, formed under the auspices of the International Association of Geodesy, provides highly accurate GPS data and precise data products to the global geophysical research community in a timely manner. The IGS has integrated a number of globally distributed reference stations and networks, including some that were mentioned above, into a global network that included 97 reference stations as of February 1996.1 Each organization that owns and operates one or more of these reference stations or networks participates in the global network on a voluntary basis. An overview of this network and the evolution of the IGS was presented by Gerhard Beutler. He also described planned developments for the IGS, including expansion of the network to 250 globally distributed reference stations, distribution of 1 More than 140 reference stations are now part of the IGS network.
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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 precise GPS orbital data within 24 hours of the last observation, and the inclusion of precipitable water vapor measurement capability at many reference stations. Additional papers related to the IGS network described the flow and archiving of data and the distribution of data products that can be accessed via the Internet. Every day, data flows automatically from each reference station in the IGS network through global data centers to the service's data analysis centers. The data analysis centers produce IGS data products, which include daily precise GPS satellite orbits, satellite clock corrections, earth rotation parameters, and reference station positions. Public access to these products through the World Wide Web is enabled by the Central Bureau Information System. Although the papers discussed so far have focused on networks that are used for either navigation or geoscience applications, most GPS reference station networks have the potential to provide both real-time differential corrections and post-processed data to a wide variety of users. Within the United States, the NOAA-National Geodetic Survey's (NGS) Continuously Operating Reference System (CORS) program has been established to make this dual use possible. The CORS network, described in a paper presented by William Strange, integrates and collects data from reference stations operated by NASA, NOAA, the FAA, the USCG, the Army Corps of Engineers, and in some cases, even the IGS. The post-processed data generated from these different sources is integrated at the CORS Central Data Facility, and the resulting data products are made available to users on CD ROM and via the Internet. WORKING GROUP DISCUSSIONS The working group on networks, data sources, and static positioning applications brought key network operators and users together to discuss options for improving coordination and shared use of the nation's GPS reference station infrastructure. The group noted that the growth rate of GPS networks will require closer cooperation in the United States, similar to the cooperation at the international level that exists in the IGS. Frequently, a group of researchers or a government agency develops plans to install a permanent GPS reference station in a given location, not knowing that a station is already operating a short distance away that could meet their requirements with only minor modifications Although some scientific researchers may require autonomous control of a GPS reference station and its ancillary equipment, many users only need to know how to obtain data in a timely manner, and if necessary, how to request a configuration change, such as a higher sampling rate, from the operators of existing GPS networks. However, this requires sufficient knowledge of network operations on the part of users to prevent unreasonable requests from interfering with the primary function of a network or reference station. It also requires better dissemination of information on the part of network operators and controlling agencies. In the opinion of the working group, the creation of a coordinated “catalog” of GPS networks and their technical characteristics, as well as instructions for accessing related data and information systems, may be a useful way to disseminate information to potential users. Table 2-1, which includes information on each of the GPS networks represented by the working group, is an example of the type of information that could be included in a comprehensive catalogue. Posting a catalog on the Internet, along with links to each network's controlling organization, could also allow users in the scientific community to send feedback, suggestions, and appropriate requests directly to network operators.
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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 TABLE 2-1 Characteristics of GPS Reference Station Networks Represented and Discussed at the Workshopa Network Sponsor Number of Stations (current/planned) Primary Use Coverage Maritime DGPS Network U.S. Coast Guard (and Army Corp of Engineers) 51 Coastal/harbor/inland waterway navigation U.S. coastlines, harbors, and inland waterways CORS NOAA 71b National Spatial Reference System Continental U.S. and Alaska/Hawaii Wind Profiler/PWV Network NOAA-Forecast Systems Lab 7 Weather forecasting/climate monitoring Central U.S. WAAS/LAAS FAA planned WAAS - approximately 27 planned LAAS - unavailable Aircraft navigation/landing North America SCIGN NASA/NSF/USGS 50/250 Crustal deformation and earthquake monitoring Southern California BARD USGS/Bay area universities 10 Crustal deformation and earthquake monitoring San Francisco Bay area IGS Multiple agencies (U.S. and International) 140/250 Global spatial reference, geodetic, geophysical, and atmospheric research Sites around the globe GRAPES and COSMOS Geographic Survey Institute of Japan ~ 600/1,000 Spatial reference, geodetic and geophysical research in Japan Japan a This table is not meant to represent every permanent GPS reference station network. Only the networks discussed in the papers in Chapter 7 of this report are shown. Blank entries in the table represent a lack of available information. b The CORS network uses the USCG, Army Corps of Engineers, and NOAA-FSL Wind Profiler/PWV GPS stations for its primary domestic infrastructure and accesses some IGS stations. FAA WAAS and LAAS stations will be added to this infrastructure as they become operational. c The CORS network has the capability to operate at the 5-second rate although the 30-second sample rate is the default. Select stations can be operated at the higher rate by arrangement. d SCIGN stations have the capability to operate at a higher rate by arrangement. e Some stations in the IGS network have the capability to operate at the 1-second sample rate or higher (50 MHz) in support of the GPS/MET mission LEO satellite for atmospheric occultation experiment. Future LEO satellite missions will require similar ground support. f 120 of the stations in the combined GRAPES and COSMOS network will be equipped to operate at a 1-second sample rate.
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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 Data Availability (access time since last observation) Nominal Accuracy (primary requirement) Sample rate (seconds) Internet Contact Information Real time DGPS corrections <10m (2 drms) and 1 to 3m (2 drms) for inland sites 5–30 www.navcen.uscg.mil 0.5 hour or less 5–30c www.ngs.noaa.gov/CORS/cors-data.html 1 hour 30 www-dd.fsl.noaa.gov/ipw_web/gps_ipw.html minutes to 1 hour (CORS) WAAS - 7.6 m (2 drms) LAAS - 0.6 m (vertical) ? www.faa.gov/and/and500/AND510/AND_510.HTM 1–30d milhouse.jpl.nasa.gov/ 30 quake.wr.usgs.gov/QUAKES/geodetic/bard/ 6 hours 1–30e igscb.jpl.nasa.gov/ 1–30f www.gsi-mc.go.jp/geodetic/gps/gps.html
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