Contributions of GPS and VLBI for understanding station motions

The occurrence of non-secular signals in site positions is well known. Since a few years, a main objective of research has been the separation of apparent motions from real geophysical motions and deformations. We investigated eight weekly Global Positioning System (GPS), one daily solution from Scripps Orbit and Permanent Array Center (SOPAC), and two Very Long Baseline Interferometry (VLBI) station coordinate time series and the corresponding baselines. One major hurdle in our investigation was the handling of constraints imposed on the GPS solutions. Seasonal signals in the height component with amplitudes of up to 9 mm for a few sites on the Asian continent were recovered. For baselines longer than 5000 km, the precision of the GPS technique is superior to the VLBI technique from the point of view of repeatability. The improvement on VLBI baseline repeatability after applying the atmospheric pressure loading time series provided by the Goddard Space Flight Center (GSFC) VLBI group reaches 3.3% (median value). Coordinate time series or baseline lengths derived from GPS and VLBI Analysis Centers (ACs) solutions have not yet achieved an unambiguously assessment of sub-cm agreement.