Comparison of tropospheric gradients determined by VLBI and GPS

Abstract

The estimation of geodetic parameters from GPS and VLBI requires the modeling of tropospheric refraction in order to account for the path delays of radio signals being emitted by extragalactic radio sources and satellites, respectively. Precise models consider the deviation from the azimuthal symmetry by introducing horizontal tropospheric gradients. While estimation of gradients had been included in the GPS software package GIPSY several years ago it was implemented in the BERNESE GPS software package (version 4.2) and in the VLBI software OCCAM (modified version 4.0) quite recently. We present a comparison of tropospheric gradient estimates derived with the three software packages mentioned above. The parameters have been estimated for the fundamental station Wettzell (Germany) for four VLBI experiments in the first half of 1999 (Euro48, Euro50, Iris 136, Iris 139) and for simultaneous GPS observations. The gradients obtained by the different software packages show a similar behaviour although offsets between the solutions can be as large as two millimeters. Astonishingly there is no better agreement between the two gradient estimates derived by GPS (BERNESE compared with GIPSY) than between gradients from the different techniques (OCCAM compared with GIPSY or BERNESE). This is probably due to the application of different estimation strategies by the GPS solutions.