Assessment of parameters describing the signal delay in the neutral atmosphere derived from VGOS sessions

Abstract

The VLBI Global Observing System (VGOS) is the next generation VLBI system for geodetic and astrometric VLBI. It has been designed by the International VLBI Service for Geodesy and Astrometry (IVS) to improve the accuracy and precision of the estimated geodetic parameters by one order of magnitude compared to the so-called legacy S/X VLBI system. During the VGOS design phase, small-scale and rapid variations in the signal propagation delay caused by the neutral atmosphere were identified as one of the major limiting error sources in terms of accuracy of geodetic VLBI. Performing as many observations as possible per time unit to cover the local sky at the stations as uniformly as possible, has been developed as a strategy to address this topic. The VGOS idea is to achieve this goal by employing fast-slewing radio telescopes ,of typically 12–13 m diameter, that are equipped with broad-band receiving devices of reasonably high sensitivity and digital backends with high sampling capability. Compared to standard S/X legacy VLBI sessions, at least a factor of two in the number of observations per station is currently achieved within operational VGOS sessions (VO). Dedicated VGOS Research and Development (R&D) sessions (VR) achieve an even larger number of observations through minimizing the scan lengths.VGOS is still in its build-up phase and by 2022 the VGOS operational network has reached 10 internationally distributed stations. Among those is the Onsala Space Observatory which is operationally active with its VGOS twin telescopes since 2019. We analyse VGOS sessions of both VO- and VR-series and assess the current ability of VGOS to sense small-scale, rapid variations in the signal propagation delay caused by the neutral atmosphere. We compare the VGOS-derived results to corresponding results from simultaneous observation with co-located instrumentation at VGOS sites, i.e. receiving equipment for Global Navigation Satellite System (GNSS) observations. For the Onsala station we compare also to the results derived from the ground-based microwave radiometer.