Integration of Local Ties to a Global Reference Frame

The Onsala Space Observatory (OSO) in Sweden is one of about 60 co-location stations hosting several permanently installed GNSS antennas and three radio telescopes used for geodetic Very Long Baseline Interferometry (VLBI), as well as a superconducting gravimeter, several tide gauges, and several microwave radiometers. The legacy 20 m radio telescope is used for astrometric and geodetic VLBI since 1979. Co-location stations play an important role during the combination process of the different space geodetic techniques to a global reference frame. Local survey at co-location stations provides the so-called local tie vectors which connect the different reference points of the space geodetic techniques in a common reference frame. In 2022 and 2023 measurements were carried out to determine local ties at OSO. Terrestrial observations from total stations and leveling instruments were combined with GNSS observations. One of the total stations was additionally equipped with the Total Station Astrogeodetic Control System (TSACS) developed by the National Geodetic Survey (NGS) at the National Oceanic and Atmospheric Administration in the United States of America. Based on the topical star catalog, the local azimuth and the deflections of the vertical (DOV) result from horizontal directions and vertical angles gathered from the time-referenced video stream of the instrument's built-in camera. For orienting a local network in the global frame and deriving local tie vectors, at least two different approaches are possible. The commonly used approach integrates GNSS baselines to the spatial network formed by the terrestrial observations. The more intricate approach introduces astrogeodetic observations, namely azimuths and deflections of the vertical, to the spatial network. For this measurement campaign, both approaches are harmonized and their excellent accordance is shown, so that all observations are combined in a common network adjustment without contradiction. Finally, local geoid information confirms the successful synthesis of all observations. This work describes selected measurement aspects and the seamless integration of a local network into a global frame.