3

Remote Sensing of the Atmosphere

SYNOPSIS OF PROCEEDINGS

The papers presented in this session describe several exciting opportunities for atmospheric remote sensing using GPS. These applications are enabled either by observing the GPS signal after it has passed through the Earth's atmosphere using a ground-based receiver or observing the signal after it has passed through the limb of the Earth's atmosphere using a receiver on board a satellite positioned in low-earth orbit. Both cases rely on data received from GPS networks to enhance and refine the data collected by individual ground-based or space-based receivers. Properties of the atmosphere that can be measured include precipitable water vapor; integrated water vapor and total electron content along GPS signal paths; atmospheric refractivity; temperature, water vapor, atmospheric pressure, and ionospheric electron density; and scintillation of the ionosphere.

Using GPS to observe atmospheric refractivity was the subject of several presentations. Ronald McPherson discussed the potential use of GPS-based refractivity data for weather detection and prediction. He explained that until recently, data assimilation systems used for numerical weather prediction models required separate profiles of atmospheric temperature and water vapor. Refractivity, which is a combined measure of atmospheric pressure, temperature, and water vapor, was not directly useful. However, with new data assimilation techniques used by the National Center for Environmental Prediction, refractivity data may be used directly. Dr. McPherson also explained that the National Weather Service's observing system of the future will be user-driven, rather than instrument or advocate-driven. The challenge will be to develop new observing technologies that can fill existing gaps at lower cost than current observational technologies, such as radiosondes1 and polar and geostationary satellites. GPS-based observations of refractivity may prove to be a cost-effective option.

A group of authors led by Richard Anthes discussed how GPS limb sounding or occultation methods can be used to obtain very accurate profiles of atmospheric refractivity. The preliminary data presented were obtained from the GPS/Meteorology (GPS/MET) experiment, which was launched into low Earth orbit on board the Microlab-1 satellite in April 1995. Comparisons of the temperature component of this data to information obtained from radiosondes reveals a correlation as good as ± 1°C in some portions of the atmosphere. The ground-based GPS network needed to support a future constellation of space-based GPS/MET sensors was discussed in a paper presented by Michael Exner, along with a description of data processing and communications requirements.

Climatologists recognize that water vapor is an important greenhouse gas in the atmosphere that is related in complex ways to anthropogenically-caused global warming. Judith Curry and Peter Webster discussed two programs related to the global hydrological cycle and the modeling of the coupled earth climate system. These programs are the Global Energy and Water Cycle Experiment Water Vapor Program and the Atmospheric Radiation Measurement Program. Global measurements of precipitable water vapor and its variability are essential for both programs. According to the authors, GPS has great potential for contributing to the global measurement of precipitable water vapor in remote areas where there are no radiosondes, in moist equatorial oceanic regions where algorithms based on passive microwave observations do not presently match data derived from ship-launched radiosondes, and in very dry polar regions where passive microwave techniques cannot be used because of ice cover.

The measurement of precipitable water vapor is also important for regional and local hazardous weather detection and forecasting. One approach to designing a forecasting system utilizing GPS-derived data was discussed by Larry Cornman.

The GPS-based measurement of precipitable water vapor is accomplished by ground-based GPS receivers that estimate the signal delay caused by the atmosphere along the signal path from observed GPS satellites to the receiver. The component of this delay caused by precipitable water vapor can only be determined by the interferometric combination of measurements from two or more reference sites to eliminate other effects, such as SA, multipath, ionospheric propagation, and orbital positioning errors. Additional surface-based meteorological data are also required. The site, network, and data requirements were discussed by Thomas Runge and Fred Solheim.

1  

A radiosonde is a balloon-borne instrument used for the simultaneous measurement and transmission of meteorological data, such as atmospheric pressure, temperature, and humidity.



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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 3 Remote Sensing of the Atmosphere SYNOPSIS OF PROCEEDINGS The papers presented in this session describe several exciting opportunities for atmospheric remote sensing using GPS. These applications are enabled either by observing the GPS signal after it has passed through the Earth's atmosphere using a ground-based receiver or observing the signal after it has passed through the limb of the Earth's atmosphere using a receiver on board a satellite positioned in low-earth orbit. Both cases rely on data received from GPS networks to enhance and refine the data collected by individual ground-based or space-based receivers. Properties of the atmosphere that can be measured include precipitable water vapor; integrated water vapor and total electron content along GPS signal paths; atmospheric refractivity; temperature, water vapor, atmospheric pressure, and ionospheric electron density; and scintillation of the ionosphere. Using GPS to observe atmospheric refractivity was the subject of several presentations. Ronald McPherson discussed the potential use of GPS-based refractivity data for weather detection and prediction. He explained that until recently, data assimilation systems used for numerical weather prediction models required separate profiles of atmospheric temperature and water vapor. Refractivity, which is a combined measure of atmospheric pressure, temperature, and water vapor, was not directly useful. However, with new data assimilation techniques used by the National Center for Environmental Prediction, refractivity data may be used directly. Dr. McPherson also explained that the National Weather Service's observing system of the future will be user-driven, rather than instrument or advocate-driven. The challenge will be to develop new observing technologies that can fill existing gaps at lower cost than current observational technologies, such as radiosondes1 and polar and geostationary satellites. GPS-based observations of refractivity may prove to be a cost-effective option. A group of authors led by Richard Anthes discussed how GPS limb sounding or occultation methods can be used to obtain very accurate profiles of atmospheric refractivity. The preliminary data presented were obtained from the GPS/Meteorology (GPS/MET) experiment, which was launched into low Earth orbit on board the Microlab-1 satellite in April 1995. Comparisons of the temperature component of this data to information obtained from radiosondes reveals a correlation as good as ± 1°C in some portions of the atmosphere. The ground-based GPS network needed to support a future constellation of space-based GPS/MET sensors was discussed in a paper presented by Michael Exner, along with a description of data processing and communications requirements. Climatologists recognize that water vapor is an important greenhouse gas in the atmosphere that is related in complex ways to anthropogenically-caused global warming. Judith Curry and Peter Webster discussed two programs related to the global hydrological cycle and the modeling of the coupled earth climate system. These programs are the Global Energy and Water Cycle Experiment Water Vapor Program and the Atmospheric Radiation Measurement Program. Global measurements of precipitable water vapor and its variability are essential for both programs. According to the authors, GPS has great potential for contributing to the global measurement of precipitable water vapor in remote areas where there are no radiosondes, in moist equatorial oceanic regions where algorithms based on passive microwave observations do not presently match data derived from ship-launched radiosondes, and in very dry polar regions where passive microwave techniques cannot be used because of ice cover. The measurement of precipitable water vapor is also important for regional and local hazardous weather detection and forecasting. One approach to designing a forecasting system utilizing GPS-derived data was discussed by Larry Cornman. The GPS-based measurement of precipitable water vapor is accomplished by ground-based GPS receivers that estimate the signal delay caused by the atmosphere along the signal path from observed GPS satellites to the receiver. The component of this delay caused by precipitable water vapor can only be determined by the interferometric combination of measurements from two or more reference sites to eliminate other effects, such as SA, multipath, ionospheric propagation, and orbital positioning errors. Additional surface-based meteorological data are also required. The site, network, and data requirements were discussed by Thomas Runge and Fred Solheim. 1   A radiosonde is a balloon-borne instrument used for the simultaneous measurement and transmission of meteorological data, such as atmospheric pressure, temperature, and humidity.

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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 Measuring the scintillation, electron density, and total electron content of the ionosphere is important to both atmospheric research and forecasting the ionosphere's effect on electric power grids and space-based activities, such as communications, manned space flight, ballistic missile early warning, and, potentially, ballistic missile defense. David Anderson discussed global ionospheric specification and forecasting and the Parameterized Real-time Ionospheric Specification Model (PRISM), the primary tool for integrating data from a number of satellite-based and ground-based measurement systems. PRISM forecasts the ion and electron density profile of the ionosphere from 90 to 1600 kilometers altitude on a grid spacing of 2 degrees latitude and 5 degrees longitude. The auroral boundary and altitude of the high latitude ionospheric trough are also identified. The PRISM code is driven with near-real-time data from a network of ground-based GPS receivers as well as other sensors. Characterizing the properties of the ionosphere using GPS and other sensors will also improve the accuracy of GPS-based navigation. Next to SA, ionospheric delay is the greatest source of error for a single frequency GPS receiver.2 Therefore, the FAA's WAAS is being designed to broadcast ionospheric error corrections to its users. Yi-chung Chao described research at Stanford University to design an accurate model for estimating these corrections. . At the heart of the model is an interfrequency bias calibration algorithm that separates interfrequency bias from the actual ionospheric delay observed by dual-frequency reference station receivers that will be part of the WAAS network. WORKING GROUP DISCUSSIONS GPS-based remote sensing produces data about the atmosphere itself as well as data that can be used to increase the accuracy of positioning information used by geophysical researchers. Therefore, the remote sensing working group was comprised of both atmospheric researchers interested in numerical weather prediction, local weather forecasting, climate monitoring, and space weather (ionospheric scintillation, electron density, and total electron content); and geophysical researchers who study crustal deformations related to earthquakes and volcanic activity. The working group began by discussing opportunities for atmospheric scientists to apply GPS-derived data to their work, which ranges from short-range weather forecasting to research associated with water vapor distributions on global scales and time scales ranging from seasonal to decadal. A ground-based receiver with a collocated meteorological station, which adds perhaps $30,000 to $50,000 to the cost of the installation, can potentially provide continuous measurements of precipitable water vapor with accuracies on the order of 1 to 2 millimeters out of typically measured total values of 20 to 50 millimeters of equivalent water content. Continuous observations have the potential to improve short term, small scale weather forecasts and to monitor climate variables globally over land. As the resolution of North American numerical weather prediction models approaches 10 kilometers, higher spatial and temporal resolution in the initial data fields will be needed. GPS ground-based observations offer this potential. It also was noted that the global change and climate communities should be apprised, as soon as possible, of the potential opportunities GPS offers. The group agreed that GPS observations should be considered as one of the variables to be measured as part of the Global Climate Observing System. The group also discussed the need for global measurements for weather, space weather, and climate applications. International cooperation to make ground-based GPS observations acquired in countries outside the United States available to the global climate research and operational weather forecasting communities was suggested as a topic for future study. If the necessary cooperation cannot be fostered, the group decided that space-based approaches, such as GPS/MET, may offer a useful and cost-effective alternative to ground-based observations. However, both ground- and space-based GPS sensing systems must offer cost and performance advantages over existing approaches in order to play an important role in the weather and climate observing system of the future. Geophysical applications are moving to real-time operational needs, both in the case of ionospheric delay and signal delays caused by tropospheric water vapor. The time delay of a GPS signal due to water vapor is small compared to time delays caused by ionospheric propagation effects. However, water vapor can induce larger errors than those resulting from uncertainties in GPS orbits. Therefore, all sources of error in GPS measurements must be characterized. Corrections based on these errors must be computed quickly and must be accurate. Related data handling and distribution systems must improve accordingly in order to provide better spatial resolution for 2   Because the total electron content of the ionosphere has large diurnal variations and large variations over the 11-year solar cycle, signal delays for single frequency GPS receivers can vary widely. A typical SPS (Standard Positioning System) receiver has an algorithm that can remove about 50 percent of the ionospheric error, leading to an error ranging from less than 1 meter to 35 meters. However, 7 meters is often used as an average error value.

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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 the real-time monitoring of crustal deformations from earthquake and volcanic processes. The group was in general agreement on a number of other issues, which are summarized below. Data Accessing GPS data via the Internet is acceptable to users, as is archiving data on CD-ROM. Data from USCG and FAA networks (WAAS) via another agency or distribution system, such as the NOAA-CORS program, is acceptable. The formatting of data is not considered to be a significant issue. Real-time data are important for weather forecasting, but not for climate modeling. Site and Network Infrastructure In the opinion of the working group, meteorological applications require that surface meteorological stations be collocated with GPS reference stations with the following measuring capabilities: atmospheric pressure — 0.5 millibars; temperature — 0.5°C to 1.0°C; surface humidity — 5 to 10 percent. Infrastructure investments for reference sites, such as increased electrical power, improved cooling and air conditioning for equipment and instrumentation, and additional storage space, would be beneficial to all users. Dual-frequency receivers are preferable to L1-only receivers and should be used at reference stations whenever possible. The collocation of the FAA's WAAS reference stations with automatic surface observing system stations is beneficial to both navigation and weather forecasting and is worth considering whenever possible. Coordination Hundreds of GPS reference sites will probably be established in the U.S. in the next few years. Most users will benefit from federal interagency coordination to facilitate multiple uses of these sites. Standards and specifications for reference sites and networks will be difficult to establish universally because of differences in intended applications and their attendant requirements. Multiple uses may help an agency or group obtain Therefore, most users and providers can benefit the funding needed for it's application. from cooperation and collaboration with other users. Requirements The working group also discussed the numerical requirements for the four types of applications represented. These requirements are shown in Table 3-1.

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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 3-1 Remote Sensing and Geophysical Requirementsa   Numerical Weather Predictionb Space Weather Climatology Geophysicsc Receiver Sample Rate 30 seconds (ground) 30 seconds 30 seconds 30 seconds Predicted Orbit Accuracy <25 cm 30 cm 10 cm 50 cm (real time) 10 cm (long term) Orbit Access/Delivery Time Available at 00:00 universal coordinated time (UTC) each day 0.5 hours Not defined Real-time continuous delivery (real time) 1 week (long term) Required Data Product Total signal delay (ground-based) Bending angle (space-based) Total electron content Precipitable water vapor solution (30 minute intervals) Position differences, covariance, and variance over baselines <500 km Data Access/Delivery Time 0.5 hours for local models <5.0 hours for global models 0.5 to 1.0 hours 6 months 30 seconds (real time) Not defined (long term) Accuracy <1 cm 2 to 5 total electron content units 1 to 2 mm 5 mm (real time) 1 mm (long term) Spatial Distribution of Reference Stations 500 km globally <200 km locally (ground) <500 km (space) Global network with emphasis on low latitude sites Not defined 3 to 50 km a The quantitative and qualitative requirements listed in this table were determined by the remote sensing working group. They do not represent requirements defined by an internationally recognized standardization committee or a government agency. b Where they have been defined, requirements are listed for both ground-based and space-based GPS sensing systems. c Requirements for both real-time and long-term geophysical applications are provided. If only one entry is present, the requirements are the same for both.