Geodetic analysis for the Very Long Baseline Interferometry Global Observing System

Licentiate thesis, 2016

Very Long Baseline Interferometry (VLBI) is an essential technique for space-geodesy. It realizes the International Celestial Reference Frame (ICRF) and provides a link between the Earth- and space-fixed coordinate systems by directly observing all Earth Orientation Parameters (EOP) simultaneously. In particular, it is the only technique available that can directly measure UT1-UTC and nutation. This of special importance to satellite-based techniques, which need regular input from VLBI observations to account for drifts in their derived UT1-UTC estimates. Currently, daily UT1-UTC estimates from VLBI are provided by 1-hour Intensive sessions with three regular baseline configurations, which provide UT1-UTC with an appropriate accuracy of 20 μs. Increased UT1- UTC accuracy is given by bi-weekly 24-hour Rapid turnaround sessions for EOP determination, which employ a network of at least 8 stations. However, the typical delay for the results obtained from these sessions is close to the specified upper limit of 15 days. The VLBI Global Observing System (VGOS) is the upcoming VLBI component of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG). It represents a complete redesign of the current VLBI system to meet the requirements for a system capable of observing phenomena with a magnitude of a few millimetres. For VGOS the main goals are a global accuracy of 1 mm for positions and 1 mm/y for velocities and continuous monitoring of EOP and station positions. Major effort in hardware and software across the whole signal chain are needed to accomplish these goals. This includes investments in, to name a few, new telescopes, front- and backends, recording systems, correlation, and data analysis. Most of the related systems need to be automated to ensure reliable continuous operations. In this thesis the aspects of geodetic VLBI data analysis related to the transition to VGOS are investigated through two practical cases. The VGOS requirements necessitate upgrades in the station hardware. In 2011 Onsala Space Observatory installed a digital backend (Digital Base-Band Converter (DBBC) system) alongside the operational analogue Mark IV system. The effect of this hardware change on the VLBI observables and estimated geodetic parameters is investigated through analysing a series of sessions recorded in parallel on both the old and the new systems. Automated near-real time VLBI analysis is studied using the Intensive sessions on the Kokee—Wettzell baseline. The impacts in terms of availability of a priori data for the analysis are investigated to determine the most crucial factors for high-accuracy UT1-UTC production.