Precise Orbit Determination for Low-Earth Orbiting Satellites using GPS Data:Recent Advances

摘要

In this paper, we describe some recent advances in GPS-based precise orbit determination for low-Earth orbiting satellites. We will focus mainly on the Topex/Poseidon (T/P) satellite, which was launched in 1992. The T/P satellite carries a 6-channel Motorola Monarch GPS demonstration receiver (GPSDR), which is capable of collecting dual frequency (L1/L2) data when the GPS anti-spoofing (AS) function is inactive. Data from the first two post-launch years have been used to routinely compute T/P orbits with a radial accuracy at the 2 cm RMS level. Since the routine activation of GPS AS, in 1994, the GPSDR has collected GPS data mainly at the LI frequency. Although the corresponding single frequency orbits (which have been routinely produced with radial accuracies at the 4-6 cm RMS level) are less accurate than the dual frequency orbits, they are available on a next-day basis and have been used to support a variety of emerging operational oceanographic applications. Most notable in this last category was the monitoring of the 1997-98 El Nino event. In the last year, several new modifications to the solution strategy have been investigated. With these enhancements, our results indicate that the next-day T/P GPS (AS) orbits can be computed with a radial accuracy of better than 3 cm RMS. Such orbit quality is unprecedented for single frequency data, and approaches the quality of the official precise orbit ephemeris (computed using data from Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)). We also discuss the Geosat Follow-On (GFO) mission, an altimetric satellite launched in February, 1998. The GFO satellite carries advanced 8-channel codeless TurboRogue GPS receivers, capable of collecting dual frequency (L1/L2) GPS data in the presence of AS. These data are expected to support 2-3 cm radial RMS accuracy for the GFO orbit, independent of AS status.