INTERGEO Agenda - english

Reference frames play a fundamental role not only in geodesy. The quantification of geophysical processes on Earth, as well as the precise determination of satellite positions in space, and consequently applications such as GNSS positioning, require the existence of a stable and highly accurate coordinate system. The terrestrial reference system is realized through the International Terrestrial Reference Frame (ITRF) defined by the IAG, while the ICRF3 represents the International Celestial Reference Frame as the realization of the inertial, space-fixed reference system. One of the four geodetic space techniques contributing to the realization of these reference systems is Very Long Baseline Interferometry (VLBI). VLBI occupies a unique position, as it is the only technique capable of determining the complete set of Earth Orientation Parameters (EOP) required to link the terrestrial and celestial reference frames. Since 2017, the first telescopes of the new VLBI generation, the VLBI Global Observing System (VGOS), have been in operational use. Owing to its broadband observations, VGOS achieves higher precision than conventional legacy VLBI and is intended to replace it in the long term. Based on the currently available data volume, an observation period of approximately eight years, and a network of 17 operational VGOS telescopes, an independent reference frame can already be derived. However, its long-term stability still remains inferior to that of the legacy VLBI network, which is based on more than 45 years of observations and a substantially larger number of stations. Against this background, a combination at normal equation level was developed for the joint determination of a TRF and CRF from legacy S/X VLBI and VGOS sessions processed at DGFI-TUM. Several methods exist for connecting the legacy VLBI and VGOS networks, all of which were applied in the combination to establish a strong linkage between both systems. We present the results of the analyses of legacy VLBI and VGOS data, their combination, and comparisons of the derived TRF, CRF, and EOP series with external solutions. Overall, the combination performed well and shows good agreement with external reference frames and parameter series.