Assessment of performance of first VGOS operational sessions scheduled with source-centric approach

Abstract

The new Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) started regular observations in 2019. In our previous studies, based on the analysis of then available VGOS data, the high potential of the data for the geodetic and astrometric products has been shown. At the same time, possibilities for improvement have been identified. One obvious deficiency of the current VGOS network has been the lack of antennas in the southern hemisphere. Joining of the HARTVGS (South Africa) and YARRA12M (Australia) the VGOS network in the second half of 2024 doubled the number of the southern hemisphere antennas to four. In this paper, we present the improvement in densifying the VGOS celestial reference frame (CRF) in the southern region. Furthermore, our comparison of VGOS-CRF (VIE2024) with ICRF3-SX revealed that 21% sources have normalized angular separation higher than 5 sigma. Assuming that one of the reasons is the different contribution of the source structure delay, sessions allowing for a simultaneous imaging of sources are needed. Therefore, since July 2025 the new source-centric scheduling approach for the operational VGOS sessions (VS) has been tested by the International VLBI Service for Geodesy and Astrometry. In this contribution, we assess the performance of the available VS sessions and evaluate the precision and accuracy of the resulting geodetic parameters with respect to parameters from the established operational VGOS sessions (VO).