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Abstract:
The next-generation VLBI system, known as the VLBI Global Observing System (VGOS), is currently being deployed worldwide to replace the legacy S/X VLBI system. The latter has been operating for geodesy and astrometry during the last four decades and meanwhile suffers from aging instrumentation, thus needing replacement by VGOS. For a consistent continuation of the International Terrestrial Reference Frame (ITRF) in the VGOS era, the new VGOS antennas need to be tied to the legacy S/X VLBI antennas as accurately as possible. In this contribution we report the results of such position ties at eight geodetic sites with co-located legacy S/X VLBI and VGOS instrumentation, using phase delays that are extracted from global geodetic VLBI observations. An analysis of phase delay observables at X band from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hour VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. We also investigate the systematic errors in these short baseline observations, which allows us to seperate the purely instrumental effects, affecting both short-baseline and long-baseline observables and dominating the estimates of the short-baseline vectors, from other contributions due to geophysical/astrophysical effects.
