Astrogeodesy by VLBI Global Observing System

ongoing

Very Long Baseline Interferometry (VLBI) is a powerful technique in astronomy to generate the most detailed radio images of exotic astronomical objects, mostly Active Galactic Nuclei (AGNs); in geodesy, it enables both societal and scientific applications through its unique and essential role in defining the reference frames that underpin GNSS and its many applications. Some scientific applications of geodesy, such as monitoring sea level rise, require high levels of precision and accuracy. The next generation VLBI system, the VLBI Global Observing System (VGOS), is being developed with the ambitious goal of achieving, on global scales, the accuracy of 1 mm in station position and 0.1 mm/yr in station velocity. For a long time it was thought that the radio propagation medium was the dominant error source and hence considerable effort was invested towards its reduction. However, in more recent years, partly thanks to the pioneering work of the PI, it has become clear that the true potential of geodetic VLBI can never be reached without understanding and correcting for the distortions caused by the angular structure of the AGNs (i.e. distribution of the radio emission on the sky). The Astrogeodesy project will (1) study, characterize, monitor, and interpret the frequency- and time-dependent structure of the AGNs based on VGOS observations and (2) develop approaches to model them as point-like fiducial anchors on the sky. The aim is to close the gap between current capabilities and the target goal of 1 mm accuracy by combining the three disciplines of geodesy, astrometry, and radio astronomy. At the same time, this combination will in return enable VGOS to be a powerful tool providing data to enhance these three fields. About 30 VGOS antennas have been or are being built across the world, and regular VGOS observations are carried out with the completed antennas. It is timely to develop a new data calibration and analysis concept for VGOS through this project.