GPS satellite-to-satellite tracking for TOPEX/Poseidon precise orbit determination and gravity field model improvement

Abstract Satellite-to-satellite tracking (SST) between the high-altitude GPS satellites and a low-flying spacecraft, was realized for geodetic applications with the American/French altimeter mission TOPEX/Poseidon. This tracking scenario has already been tested by experimental GPS receivers with reduced performance on Landsat-4 and Landsat-5 some 10 years ago. Because of its capability to provide continuous data coverage, GPS space-based tracking for precise orbit restitution is superior to any ground-based tracking system. To investigate the accuracy potential offered by this observation system, the EPOS (Earth Parameter and Orbit System) software of GFZ Potsdam was extended to process GPS-SST data for precise TOPEX/Poseidon orbit determination and gravity field model improvement. The used observations are undifferenced pseudo-ranges and carrier phases as obtained from the on-board GPS receiver. In a two step approach the dynamic orbit determination method is applied with the precomputed ephemerides and clock parameters of the GPS satellites introduced as fixed parameters. The comparisons with precise TOPEX/Poseidon orbits computed by JPL in a different approach but using the same observations, and those computed by CNES/GRGS from DORIS-doppler and laser tracking data, reveal a homogeneous radial orbit consistency of about 3 cm and a total agreement of the adjusted satellite positions from different solutions within 12 cm. GPS-SST data covering one TOPEX/Poseidon orbit repeat cycle (10 days) are then incorporated into the normal equation system of the GRIM4 global gravity field model. As predictable from the very small observation residuals in the orbit adjustments, the contribution of this data set to the overall quality of the GRIM4 model is perceivable but not very significant, due to the relatively large altitude of the satellite.