GEOMETRIC IMPROVEMENT OF THE SPOT SATELLITE ORBIT USING DORIS DOPPLER RESIDUAL DATA

After adjustment of a satellite orbit by classical dynamical methods, the residuals of tracking data from a Doppler satellite tracking network still contain information. They are used to further adjust geometrically the orbit, the station coordinates, and Doppler biases and drifts for each station. After extensive testing in simulation, the method is applied to 21 days of the DORIS data from the SPOT2 satellite. As this tracking network has an excellent coverage and a very low measurement noise, the orbit reproducibility in the tangential direction is reduced from 40-100 cm root-mean-squares (rms) with a classical dynamical orbit calculation to 7-32 cm rms for the geometrical orbit, based on the difference between 2-day orbit arcs overlapping over 1 day. The reproducibility in the radial direction is slightly improved to 6-30 cm rms on the same arcs, with the most significant effect on the 1/revolution frequency. The formal error is also mostly reduced on this frequency. The Doppler residuals are reduced from 2-13 to 0.7-1.4 mm/s. This is only 3 times the measurement noise level and allows identification of ionospheric (magnetic storms), tropospheric (large atmospheric water vapor), and geometric effects (low elevation angles) in the data. Five-day orbit arcs of this improved orbit are used to recalculate station positions: A reproducibility of 4-13 cm is achieved on short distances (50 km), which is better than what is obtained with 15-day arcs of the initial dynamical orbit. Furthermore, for nine stations collocated with laser satellite tracking stations, the rms difference between their position calculated with 15 days of orbit and their independently estimated geodetic values is reduced from 56 cm rms with the dynamical orbit to 33 cm rms with the geometric orbit: This demonstrates that the improved reproducibility corresponds to an improved accuracy.