APPLICATION OF THE GEM-T2 GRAVITY-FIELD TO ALTIMETRIC SATELLITE ORBITCOMPUTATION

As part of a continuing effort to provide improved orbits for use with existing altimeter data, we have recomputed ephemerides for both the Seasat and Geosat Exact Repeat altimeter missions. The orbits were com puted in a consistent fashion, using the Goddard Earth Model T2 gravit y field along with available ground-based tracking data. Such an appro ach allows direct comparisons of sea level between the two altimeter s ystems. Evaluation of the resulting ephemerides indicates that root-me an-square accuracies of 30-50 cm have been achieved for the radial com ponent of the orbits for both satellites. An exception occurs for the last year of the Geosat Exact Repeat Mission, when the rms radial orbi t accuracy degrades to the 1-m level at times owing to the inability t o adequately model the drag force arising from the increased solar act ivity.