A celestial reference frame derived from observations with the Very Long Baseline Interferometry Global Observing System
Authors: Krásná H. , Jacobs C. S. , Schartner M. , Charlot P. 
Published: November 2024

Aims. We computed a celestial reference frame (CRF) from Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) data after five years of regular observations carried out under the umbrella of the International VLBI Service for Geodesy and Astrometry. In this paper we evaluate its strengths and weaknesses, document the source selection and scheduling strategies for the individual sessions, and investigate the effect of using this new VGOS CRF in the analysis of individual geodetic VLBI sessions where the radio source positions are fixed to their a priori coordinates.
Methods. We estimated the VIE2023-VG CRF in a global adjustment of 155 multi-baseline 24-hour VGOS sessions until 2024.0. We carried out several comparisons with the third version of the International Celestial Reference Frame (ICRF3) adopted by the International Astronomical Union in 2018, and with VIE2023sx CRF which includes VLBI S/X data until 2024.0. Furthermore, we studied the effect of more frequent estimations of tropospheric parameters (30,10, and 5 min for zenith wet delay) on the estimated CRF in the current VGOS network. We evaluated the VIE2023-VG CRF in the geodetic analysis of VGOS sessions by computing the baseline lengths and station positions and statistics on the Earth orientation parameters estimated in the single-session analysis where the source positions were fixed to either the VIE2023-VG CRF or to ICRF3-SX.
Results. The current VIE2023-VG CRF is built with 1.39 million VGOS group delays and includes 418 radio sources, where 172 sources (41%) are introduced in only four research and development sessions alone. We show that the VIE2023-VG CRF has excellent source position precision. The median formal error from the least-squares adjustment is 30 μas for right ascension (scaled by cosine of declination) and 47 μas for declination. In terms of systematic distortions versus ICRF3-SX, the largest terms in the vector spherical harmonics up to the degree and order two, reach in absolute values around 60 μas, caused by correlations between the individual terms. Because of the lack of observations in the southern hemisphere, a constraint for a zero slope in declination difference with respect to ICRF3-SX is imposed in the global adjustment. Therefore, VGOS should prioritize the development of southern stations in order to limit the need for such constraints on the frame. Further we show that fixing the a priori CRF to VIE2023-VG CRF instead of the ICRF3-SX in the single-session analysis improves the weighted root mean square of the baseline length by up to 3 mm, especially for the extremely long baselines (>12 000 km), with a weighted mean difference between the baseline length time series up to 2 mm. Therefore, in order to meet the ambitious goal of 1 mm accuracy for a terrestrial reference frame set by the Global Geodetic Observing System, the development of a VGOS-only CRF is required for use in the geodetic analysis of the new generation VGOS data.