Executive Summary

On March 11 and 12, 1996, the National Research Council held a workshop at the University Corporation for Atmospheric Research, Boulder, Colorado, to discuss plans and objectives for Global Positioning System (GPS) reference station1 networks and options for improving communication and coordination among GPS users in the Earth, oceanic, and atmospheric sciences and network operators.2 Approximately 75 individuals from government, academia, and industry who either use, manage, operate, or supply equipment for GPS reference station networks attended the workshop, which included the following items on its agenda:

  • introduction to civilian GPS policy and management within the federal government

  • presentations from public sector network operators and service providers and poster papers on networks, data formats, and access to GPS-derived data via the World Wide Web

  • presentations on site, network, and data requirements for GPS applications in the earth, oceanic, and atmospheric sciences

  • working groups on important issues facing GPS users, network operators, and policy makers

  • plenary session to discuss the issues raised by the working groups

SYNOPSIS OF PROCEEDINGS

A variety of public sector organizations in the United States and abroad have established or plan to establish networks of reference stations to observe and track the code and carrier phase of the GPS signal for the purpose of broadcasting real-time differential corrections to navigational users and/or to collect, process, and archive GPS-derived data.3 Papers presented at the workshop described a number of reference station networks, the applications for which they were designed, and the data they generate. Table ES-1 lists the networks that were discussed along with their primary use.

The rapidly developing combined infrastructure created by these networks can be used for a number of research applications in the earth, oceanic, and atmospheric sciences. Additional papers presented at the workshop focused on applications that use GPS to either remotely sense various properties of the atmosphere, including integrated water vapor and total electron content; dynamically position and navigate sensors on board oceangoing probes, aircraft, and satellites; or collect static positioning data to measure crustal deformation related to earthquake and volcano processes. Many of the presentations focused on the accuracy and real-time or post-processed data requirements of these applications. Others discussed sources of error that must be eliminated or accounted for in order to maximize the usefulness of GPS. The applications described at the workshop are summarized in Table ES-2.

1  

As a minimum, a GPS reference station consists of a GPS receiver, antenna, computer hardware, and software to analyze the incoming GPS signals.

2  

The exact wording of the statement of task can be found in Appendix A.

3  

The differential GPS method is based on knowledge of the highly accurate, geodetically surveyed location of a GPS reference station, which observes GPS signals in real time and compares their ranging information to the ranges expected to be observed at its fixed point. The differences between observed ranges and predicted ranges are used to compute corrections to GPS parameters, sources of error, and/or resultant_positions. Differential corrections can be broadcast to GPS users, who can apply the corrections to their received GPS signals or computed positions.



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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 Executive Summary On March 11 and 12, 1996, the National Research Council held a workshop at the University Corporation for Atmospheric Research, Boulder, Colorado, to discuss plans and objectives for Global Positioning System (GPS) reference station1 networks and options for improving communication and coordination among GPS users in the Earth, oceanic, and atmospheric sciences and network operators.2 Approximately 75 individuals from government, academia, and industry who either use, manage, operate, or supply equipment for GPS reference station networks attended the workshop, which included the following items on its agenda: introduction to civilian GPS policy and management within the federal government presentations from public sector network operators and service providers and poster papers on networks, data formats, and access to GPS-derived data via the World Wide Web presentations on site, network, and data requirements for GPS applications in the earth, oceanic, and atmospheric sciences working groups on important issues facing GPS users, network operators, and policy makers plenary session to discuss the issues raised by the working groups SYNOPSIS OF PROCEEDINGS A variety of public sector organizations in the United States and abroad have established or plan to establish networks of reference stations to observe and track the code and carrier phase of the GPS signal for the purpose of broadcasting real-time differential corrections to navigational users and/or to collect, process, and archive GPS-derived data.3 Papers presented at the workshop described a number of reference station networks, the applications for which they were designed, and the data they generate. Table ES-1 lists the networks that were discussed along with their primary use. The rapidly developing combined infrastructure created by these networks can be used for a number of research applications in the earth, oceanic, and atmospheric sciences. Additional papers presented at the workshop focused on applications that use GPS to either remotely sense various properties of the atmosphere, including integrated water vapor and total electron content; dynamically position and navigate sensors on board oceangoing probes, aircraft, and satellites; or collect static positioning data to measure crustal deformation related to earthquake and volcano processes. Many of the presentations focused on the accuracy and real-time or post-processed data requirements of these applications. Others discussed sources of error that must be eliminated or accounted for in order to maximize the usefulness of GPS. The applications described at the workshop are summarized in Table ES-2. 1   As a minimum, a GPS reference station consists of a GPS receiver, antenna, computer hardware, and software to analyze the incoming GPS signals. 2   The exact wording of the statement of task can be found in Appendix A. 3   The differential GPS method is based on knowledge of the highly accurate, geodetically surveyed location of a GPS reference station, which observes GPS signals in real time and compares their ranging information to the ranges expected to be observed at its fixed point. The differences between observed ranges and predicted ranges are used to compute corrections to GPS parameters, sources of error, and/or resultant_positions. Differential corrections can be broadcast to GPS users, who can apply the corrections to their received GPS signals or computed positions.

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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 ES-1 GPS Networks Represented Network Sponsor/Operator Primary Use Maritime Differential GPS Network U.S. Coast Guard and Army Corp of Engineers Coastal/harbor/inland waterway navigation Continuously Operating Reference System (CORS) National Oceanic and Atmospheric Administration (NOAA) National Spatial Reference System Wind Profiler/Water Vapor Network NOAA-Forecast Systems Laboratory Weather forecasting/climate monitoring Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS) Federal Aviation Administration (FAA) Aircraft navigation/approach/landing Southern California Integrated GPS Network (SCIGN) National Aeronautics and Space Administration (NASA), National Science Foundation (NSF), U.S. Geological Survey (USGS) Monitoring crustal deformation and earthquake processes Bay Area Regional Deformation (BARD) Network USGS and San Francisco Bay area universities Monitoring crustal deformation and earthquake processes International GPS Service for Geodynamics (IGS) Multiple U.S. and international agencies Global spatial reference, geodetic, geophysical, and atmospheric research GRAPES and COSMOS Geographic Survey Institute of Japan Japanese spatial reference, geodetic, geophysical research TABLE ES-2 GPS Applications in the Earth, Oceanic, and Atmospheric Sciences Atmospheric Remote Sensing Dynamic Positioning and Navigation Static Positioning Numerical weather prediction Aerogeophysics Crustal deformation related to earthquake and volcanic processes Hazardous weather detection and forecasting Satellite altimetry   Climatology Satellite orbit determination   Ionospheric research, forecasting, and modeling Physical oceanography   Hydrology Atmospheric wind velocity and boundary-layer vertical velocity     Autonomous air and land vehicle piloting, navigation, and control   OVERALL THEMES OF WORKING GROUP DISCUSSIONS Three working groups were formed at the workshop (GPS networks, data sources, and static positioning applications; dynamic positioning/navigation applications; and GPS-based remote sensing of the atmosphere) to discuss issues raised in the presentations related to sources of error, user requirements, network and data standardization, and user/operator coordination and communications.4 Each group presented the results of its deliberations at the final plenary session. The presentations and the resulting discussions have been summarized by the Steering Committee into five overall themes. 4   Although the workshop agenda included separate sessions on GPS networks and data sources and static positioning applications, the two subjects were discussed by the same working group. Separate portions of the discussions of this group can be found in Chapter 2 and Chapter 5.

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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 Synergism among Applications There is a synergism between the use of GPS for atmospheric remote sensing and its use for positioning and navigation in the geosciences. The same instrumentation and techniques used to collect atmospheric and meteorological data by observing atmospheric effects on the GPS signal can also be used to account for these effects when determining an accurate position solution. This synergism can be exploited by co-locating reference stations with weather stations and vice versa whenever possible. Optimizing Network and Reference Site Positioning Accuracy Further analysis is required to fully understand the relationship between GPS positioning accuracy, data sampling rates, data latency, and sources of error. Most networks cannot be optimized for all potential applications, which makes the relationship between accuracy, sampling rates, data latency, and sources of error very important Some sources of error, such as multipath and other interactions between antennas and the environment are poorly characterized for many existing network reference sites. Data Dissemination Real-time continuous dissemination of data derived from GPS observables is vital for dynamic positioning and navigation and is also useful for other applications, including weather forecasting and real-time earthquake monitoring. For other applications, such as climatology and long term geophysical studies, delayed access to data is acceptable. Network Coordination Hundreds, perhaps thousands of GPS reference sites comprising a variety of GPS networks will probably be established worldwide in the next several years. These sites and networks will probably be designed according to a number of different standards and specifications based on the requirements of their primary users. In order to maximize the value of these sites to multiple scientific and nonscientific users, however, some level of standardization and coordination will be necessary. This will require the continued involvement of nongovernmental scientific and technical organizations, intergovernmental organizations, and interagency working groups within the borders of the United States. Communications between Users and Network Operators 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 will require sufficient knowledge of network operations on the part of users to prevent unreasonable requests from interefering with the primary function of a network or reference station. It also requires adequate dissemination of information on the part of network operators and controlling agencies. 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 means of disseminating information to potential users. Posting this 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. Communications between network users and operators can also be fostered through forums like this workshop and the public meetings of the Civil GPS Service Interface Committee established by the U.S. Department of Transportation. Meetings within the scientific disciplines that use GPS could also be effectively used if network operators were encouraged to attend and participate when subjects related to GPS appear on the agenda.