Combination of GPS and DORIS Data for Troposphere and Local tie Studies

Combining satellite geodetic data at the measurement level is a big challenge as several types of systematic errors can arise and could forbid any common processing scheme or at least bias the derived geodetic results. However, the global assimilation of these different data could help us increase the accuracy of common parameters or allow us to derive more frequent determination of EOP parameters. In a first attempt of a real multi-technique combination, we have processed GPS and DORIS ground data during the CONT-02 campaign (October 2002). GPS orbits and clocks were held fixed while all DORIS satellites parameters and stations coordinates were estimated in a free-network adjustment. In second step, we have added GPS data from the TOPEX/Poseidon mission, making full-use of one these rare collocations in space. At ground collocated sites a unique tropospheric parameter was estimated as a randomwalk process plus an unknown bias. Stations coordinates were estimated as well an unknown local tie vector. All DORIS orbits (including TOPEX) were estimated simultaneously. The impact of such GPS+DORIS combination was investigated at collocated sites. In particular, derived tropospheric solutions were compared internally during daily overlap periods and externally with VLBI results when available. Observed biases and potential accuracy improvements are discussed. Local ties vectors were also estimated and compared to current ITRF determinations to investigate the use of such technique to derive "pseudo-local ties" at all collocated sites.