Towards the tropospheric ties in the GPS, DORIS, and VLBI combination analysis during CONT14

In space geodetic data analysis, improving tropospheric delay modelling is motivated by the correlation between tropospheric zenith total delay (ZTD) and the station height. The gradients are correlated with the horizontal displacements. In the microwave techniques such as GPS, DORIS, and VLBI, the tropospheric delay effects are correlated over the collocation sites. This correlation allows for the estimation of common tropospheric parameters, often referred to as tropospheric ties. These tropospheric ties provide valuable complementary information for the computation of the terrestrial reference frame (TRF) and have been the subject of investigation in many studies. In this study, we investigate the effects of tropospheric ties on the daily TRF combination at the observation level using a batch least-squares estimation. The observations of GPS, DORIS, and VLBI were collected from 06 May 2014 to 20 May 2014 during the CONT14 campaign of VLBI. The tropospheric delay and gradient ties are computed using different numeric weather prediction (NWP) data sets provided by ECMWF and NCEP. We examined different levels of tropospheric ties 0.01, 5, and 10 mm for ZTD and 0.001, 0.5, and 1.0 mm for gradients in the combination of techniques. The results show that the combined solution with tropospheric ties derived from the four NWP data sets does not exhibit significant differences. For VLBI, the repeatability of station coordinates and network scale were found to be improved by around 20