Precise orbit computation for the Geosat Exact Repeat Mission

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Abstract
We have conducted an extensive investigation of orbit determination strategies for the Geosat Exact Repeat Mission (ERM). The goal of our studies is to establish optimum geodetic parameters and procedures for the computation of the most accurate Geosat orbits possible and to apply these procedures for the routine computation of Geosat orbits during the ERM for the following purposes: (1) to enhance the value of the Geosat oceanographic investigations by providing the user community with improved ephemerides, (2) to develop orbit determination techniques for the upcoming altimetric mission TOPEX/POSEIDON, and (3) to assess the radial orbit accuracy obtainable with recently developed gravity models. To this end, ephemerides for the entire first year of the ERM have been computed using the GEODYN II orbit program on the Cyber 205 supercomputer system at the Goddard Space Flight Center. The GEM‐T1 gravity solution was used in developing solutions in 25 separate 17‐day arcs. Estimated radial orbit errors have been reduced from a level of 3‐m root‐mean‐square (rms) with the operational orbits to about the 85‐cm level. Both the operational and precision ephemerides were produced from the same tracking data collected by the U.S. Navy's Operational Network, so most of this improvement is due to improved force and measurement modeling. Recently, a more accurate gravity field, GEM‐T2, has become available. Preliminary orbit tests performed with the GEM‐T2 gravity model, along with Geosat TRANET‐2 Doppler data that have recently been acquired, suggest that radial orbit accuracies of about 35 cm rms can be achieved for Geosat.