GPS Orbit Determination at the National Geodetic Survey

摘要

The National Geodetic Survey (NGS) independently generate 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 observation of two satellites from two ground stations effectively eliminating satellite and ground receiver clock errors, and the Selective Availability (SIA) 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(exp 7). This corresponds to a 10 meter error in the reported satellite positions. NGS is activity investigating method to improve the accuracy of its orbits, the ultimate goal being one part in 10(exp 8) or better. The ephemeris 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.