GPS Orbit Determination at the National Geodetic Survey

M. S. Schenewerk

Abstract:	Re National Geodetic Survey (NGS) independently generates precise ephemerides for all available Global Positioning System (GPS) satellites. Beginning in 1991, these ephemerides have been produced from double-differenced phase observations solely from the Cooperative International GPS Network (CIGNET) tracking sites. The double-difference technique combines simultaneous observations of two satellites from two ground stations effectively eliminating satellite and ground receiver clock errors, and the Selective Availability (S/A) signal degradation currently in effect. CIGNET is a global GPS tracking network whose primary purpose is to provide data for orbit production. The CIGNET data we collected daily at NGS and are available to the public. Each ephemeris covers a single week and is available within one month after the data were taken. Verification is by baseline repeatability and direct comparison with other ephemerides. Typically, an ephemeris is accurate at a few parts in 10^7. This corresponds to a 10 meter error in the reported satellite positions. NGS is actively investigating methods to improve the accuracy of its orbits, the ultimate goal being one part in 10^8 or better. Th ephemerides are generally available to the public through the Coast Guard GPS Information Center or directly from NGS through the Geodetic Information Service. An overview of the techniques and software used in orbit generation will be given, the current status of CIGNET will be described, and a summary of the ephemeris verification results will be presented.
Published in:	Proceedings of the 23th Annual Precise Time and Time Interval Systems and Applications Meeting December 3 - 5, 1991 Pasadena, California
Pages:	49 - 58
Cite this article:	Schenewerk, M. S., "GPS Orbit Determination at the National Geodetic Survey," Proceedings of the 23th Annual Precise Time and Time Interval Systems and Applications Meeting, Pasadena, California, December 1991, pp. 49-58.
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