EXPECTED ORBIT DETERMINATION PERFORMANCE FOR THE TOPEX POSEIDON MISSION

The TOPEX/Poseidon (T/P) mission, launched during the summer of 1992, has the requirement that the radial component of its orbit must be com puted to an accuracy of 13 cm root-mean-square (rms) or better, allowi ng measurements of the sea surface height to be computed to similar ac curacy when the satellite height is differenced with the altimeter mea surements. This will be done by combining precise satellite tracking m easurements with precise models of the forces acting on the satellite. The Space Geodesy Branch at Goddard Space Flight Center (GSFC), as pa rt of the T/P precision orbit determination (POD) Team, has the respon sibility within NASA for the T/P precise orbit computations. The prela unch activities of the T/P POD Team have been mainly directed towards developing improved models of the static and time-varying gravitationa l forces acting on T/P and precise models for the non-conservative for ces perturbing the orbit of T/P such as atmospheric drag, solar and Ea rth radiation pressure, and thermal imbalances. The radial orbit error budget for T/P allows 10 cm rms error due to gravity field mismodelin g, 3 cm due to solid Earth and ocean tides, 6 cm due to radiative forc es, and 3 cm due to atmospheric drag. A prelaunch assessment of the cu rrent modeling accuracies for these forces indicates that the radial o rbit error requirements can be achieved with the current models, and c an probably be surpassed once T/P tracking data are used to fine tune the models. Provided that the performance of the T/P spacecraft is nom inal, the precise orbits computed by the T/P POD Team should be accura te to 13 cm or better radially.