PRECISE CENTER-NATIONAL-DETUDES-SPATIALES ORBITS FOR TOPEX POSEIDON -IS REACHING 2-CM STILL A CHALLENGE/

To achieve maximum benefit from the altimetric data collected by the F rench-American TOPEX/POSEIDON spacecraft, radial orbit accuracy of 10 cm or better is required. This unprecedented requirement led the Frenc h Space Agency Centre National d'Etudes Spatiales (CNES) to develop a new high-accuracy tracking system, Doppler orbitography and radioposit ioning integrated by satellite (DORIS), and a new precision orbit prod uction facility, the Service d'Orbitographie DORIS. A global effort pr oduced new models and new orbit determination strategies. The result o f these efforts has been assessed after 1 year of operation. The origi nal goal has clearly been met, and the TOPEX/POSEIDON orbits produced by NASA and CNES agree to better than the 5 cm RMS level in the radial direction. At this level of accuracy, traditional techniques cannot c orrectly describe the actual orbit error, and some new procedures are proposed.