GRAVITY MODEL IMPROVEMENT USING THE DORIS TRACKING SYSTEM ON THE SPOT-2 SATELLITE

A high-precision radiometric satellite tracking system, DORIS, has rec ently been developed by the French space agency, Centre National d'Etu des Spatiales (CNES). DORIS was designed to provide tracking support f or missions such as the joint United States/French TOPEX/Poseidon. As part of the flight testing process, a DORIS package was flown on the F rench SPOT 2 satellite. A substantial quantity of geodetic quality tra cking data was obtained on SPOT 2 from an extensive international DORI S tracking network. These data were analyzed to assess their accuracy and to evaluate the gravitational modeling enhancements provided by th ese data in combination with the GEM-T3 gravitational model. These obs ervations have noise levels of 0.4 to 0.5 mm/s, with few residual syst ematic effects. Although the SPOT 2 satellite experiences high atmosph eric drag forces, the precision and global coverage of the DORIS track ing data have enabled more extensive orbit parameterization to mitigat e these effects. As a result, the SPOT 2 orbital errors have been redu ced to an estimated radial accuracy in the 10-20 cm RMS range, The add ition of these data, which encompass many regions heretofore lacking i n precision satellite tracking, has significantly improved GEM-T3 and allowed greatly improved orbit accuracies for Sun-synchronous satellit es like SPOT 2 (such as ERS 1 and EOS). Comparison of the ensuing grav ity model with other contemporary fields (GRIM-4C2, TEG2B, and OSU91A) provides a means to assess the current state of knowledge of the Eart h's gravity field. Thus, the DORIS experiment on SPOT 2 has provided a strong basis For evaluating this new orbit tracking technology and ha s demonstrated the important contribution of the DORIS network to the success of the TOPEX/Poseidon mission.