Advancing quasar and satellite observations for VGOS

Licentiatavhandling, 2026

Very Long Baseline Interferometry (VLBI) is a space geodetic technique that contributes to the determination of the Earth Orientation Parameters (EOP) that are needed for the transformation between Earth fixed and space fixed reference frames. The VLBI Global Observing System (VGOS) is the newest development in geodetic VLBI with the aim of improving the determination of geodetic products to support the Global Geodetic Observing System's (GGOS) millimeter-level goals for reference frames to address Earth system challenges, such as climate change related sea level rise. However, so far the performance of VGOS is not yet at the millimeter-level due to mismodeling of error sources and (geo)physical phenomena.

One of the error sources that limits the ability to reach the VGOS performance goals is radio source structure; therefore, research is ongoing to better understand its impact and find ways to mitigate its effect. A proposed solution, as presented in this work based on real experiment data, is mitigating the source structure contribution by observing quasars only under certain favorable conditions, which can be guaranteed in 1-hour long VLBI intensive sessions that primarily determine the Earth rotation angle.

On a higher level, to obtain GGOS's millimeter-level goals, the determination of intra-technique biases between the 4 main space geodetic techniques is essential, since they impact the technique combination in the determination of the International Terrestrial Reference Frame. The Genesis mission of the European Space Agency aims at determining these intra-technique biases by launching a satellite that is observable by all 4 main space geodetic techniques. For this mission VGOS will be used to observe the Genesis satellite. Although some satellites have been observed with VGOS, many aspects remain to be tested and investigated. In this work, simulation studies of VGOS observations of Genesis are presented, related to precise orbit determination, satellite observation cadences, and simulation validations.

This research thus focuses on advancing both quasar and satellite observations for VGOS, by investigating real experiments that avoid source structure, and by assessing simulation studies in which satellites are being observed.