Integrated processing of GPS and VLBI at the observation level

The combination of different space geodetic techniques is essential for providing reliable and precise geodetic reference frames and earth orientation parameters (EOP), where applying ties, that is, constraints on common parameters between different techniques, is critical. A combination at the observation level allows all ties to be properly applied and ensures results with the consistency, precision, and reliability that can hardly be achieved through combination at the parameter or normal equation level. In this study, we realize integrated GPS and VLBI processing at the observation level during five VLBI CONT campaigns (CONT05 to CONT17). We investigate the impact of applying different types of ties in the integrated solution on both station coordinates and the EOP, including global ties (i.e., EOP), station coordinate ties, and tropospheric ties. We demonstrate that the large number of GPS observations plays a dominant role in the integrated solution and yields more precise estimates of polar motion and station coordinate, while the comparatively smaller number of VLBI observations provides the full set of EOP, especially UT1-UTC and CPO. The VLBI network becomes more stable when the ties are applied, with station coordinate repeatability improved by up to 20% horizontally and 30% vertically. Compared to the IERS EOP 14 C04 product, the EOP precisions of the integrated solution are approximately 40 μas for polar motion, 10 μs for UT1-UTC, and about 45 μas for celestial pole offsets (CPO). The integrated processing improves the agreement of CPO components with respect to IERS EOP 14 C04 by up to 10%. Inspecting day-boundary discontinuities, the integrated processing improves the y-pole component by 10% to 15% and UT1-UTC by around 20%. This study constitutes a solid basis for the consistent realization of reference frames and EOP at the observation level.