Precision orbit determination for the GEOSAT exact repeat mission

Precise ephemerides have been determined for the Navy's Geodesy Satellite (Geosat) Exact Repeat Mission (ERM) using an improved gravity field model, PTGF-4A (Shum et al., 1989). The Geosat orbits were computed in a terrestrial reference system which is tied to the reference system defined by satellite laser ranging (SLR) to LAGEOS through a survey between the TRANET Doppier receiver and the SLR system located at Wettzell, Federal Republic of Germany. The remaining Doppler tracking station coordinates were estimated simultaneously with the geopotential in the PTGF-4A solution. In this analysis, three continuous 17-day Geosat orbits, which were computed using the 46-station TRANET data and global altimeter crossover data, have a crossover residual root-mean-square (rms) of 20 cm, indicating that the Geosat radial orbit error is of the order of 20 cm. The orbits computed based on data collected by a 7-station OPNET tracking network and crossover data have the same level of accuracy. Collinear sea surface height analysis using the improved orbits for two Geosat 17-day repeat cycles yields an rms height difference of 16 cm. Spectral analysis of the sea surface height difference indicates that the long-wavelength orbit error has been reduced significantly. Finally, the Geosat altimeter time tag bias, based on 54 days of the Geosat ERM altimeter crossover data, is 5±3 ms.

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