Impact of Local Ties in GNSS and VLBI Integrated Solution

In the multi-technique integrated solution, different space geodetic techniques are combined via common parameters and external observations, such as global ties (Earth Orientation Parameters, EOPs) and local ties (LTs), respectively. Local ties, which directly link the coordinates of co-location stations, are currently used in determining terrestrial reference frames (TRFs). We investigate the impact of global and local ties in Global Navigation Satellite Systems (GNSSs) and Very Long Baseline Interferometry (VLBI) integrated solutions in five VLBI continuous (CONT) campaigns. Compared to a baseline solution where GNSS and VLBI have separate minimal constraints implemented (no-net-rotation and no-net-translation, NNR+NNT), applying additional GTs and LTs improves the VLBI TRF and EOP estimates. As LTs are supposed to transfer the datum information between GNSS and VLBI, we further investigate the scenario where the VLBI datum is linked to GNSS via LT, without NNR+NNT applied to the VLBI network. We demonstrate that in this scenario the VLBI solution is deteriorated, especially the east-west component of station coordinates and UT1-UTC estimates, mainly due to the insufficient accuracy of the currently used LTs. We thus emphasize the importance of improving the accuracy of LT measurements and applying additional ties, for example, space ties and atmospheric ties.