POSITIONING RESULTS WITH DORIS ON SPOT2 AFTER 1ST YEAR OF MISSION

A new space system named DORIS has been developed by the Centre National d'Etudes Spatiales in France with the collaboration of the Institut Geographique National for precise orbit determination and positioning. It is a Doppler-based system with a worldwide ground network of transmitting beacons and an onboard receiver. The SPOT2 satellite launched in January 1990 carries DORIS as an experimental instrument. In this study, we present positioning results obtained from the analysis of the first 10 months of DORIS data through a semi-dynamical approach (i.e. the satellite orbit is improved simultaneously with the beacon positions). External calibration with satellite laser ranging (SLR) and very long baseline interferometry (VLBI) positions indicates that DORIS accuracy from a few months of data is presently 15-20 cm rms in absolute and relative (baseline length > 1000 km) positioning. For. very short baselines (< 100 km), external comparison with ground-based geodetic measurements indicates a few cm accuracy. Month to month repeatability of both absolute and relative (baseline length > 1000 km) positioning results is in the range 10-20 cm rms. For the shortest baselines (< 100 km), the month to month repeatability is better than 5 cm in cartesian coordinates. This repeatability is only approximately 2 cm in the north and east components whereas it is 12 cm in the vertical component, suggesting that tropospheric errors are important. It should be possible to improve these results in the future by reducing the residual orbit error, especially the atmospheric drag perturbation which was particularly troublesome in 1990, a year having very large solar activity and by improving modeling of the tropospheric correction.