Geodetic analysis for the Very Long Baseline Interferometry Global Observing System
Sammanfattning: Very Long Baseline Interferometry (VLBI) is an essential technique forspace-geodesy. It realizes the International Celestial Reference Frame(ICRF) and provides a link between the Earth- and space-fixed coordinatesystems by directly observing all Earth Orientation Parameters (EOP)simultaneously. In particular, it is the only technique available that candirectly measure UT1-UTC and nutation. This of special importance tosatellite-based techniques, which need regular input from VLBIobservations to account for drifts in their derived UT1-UTC estimates.Currently, daily UT1-UTC estimates from VLBI are provided by 1-hourIntensive sessions with three regular baseline configurations, whichprovide UT1-UTC with an appropriate accuracy of 20 μs. Increased UT1-UTC accuracy is given by bi-weekly 24-hour Rapid turnaround sessions forEOP determination, which employ a network of at least 8 stations.However, the typical delay for the results obtained from these sessions isclose to the specified upper limit of 15 days.The VLBI Global Observing System (VGOS) is the upcoming VLBIcomponent of the Global Geodetic Observing System (GGOS) of theInternational Association of Geodesy (IAG). It represents a completeredesign of the current VLBI system to meet the requirements for a systemcapable of observing phenomena with a magnitude of a few millimetres.For VGOS the main goals are a global accuracy of 1 mm for positionsand 1 mm/y for velocities and continuous monitoring of EOP and stationpositions. Major effort in hardware and software across the whole signalchain 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 beautomated to ensure reliable continuous operations. In this thesis theaspects of geodetic VLBI data analysis related to the transition to VGOSare investigated through two practical cases.The VGOS requirements necessitate upgrades in the stationhardware. In 2011 Onsala Space Observatory installed a digital backend(Digital Base-Band Converter (DBBC) system) alongside the operationalanalogue Mark IV system. The effect of this hardware change on the VLBIobservables and estimated geodeticparameters is investigated through analysing a series of sessions recordedin parallel on both the old and the new systems.Automated near-real time VLBI analysis is studied using the Intensivesessions on the Kokee—Wettzell baseline. The impacts in terms ofavailability of a priori data for the analysis are investigated to determinethe most crucial factors for high-accuracy UT1-UTC production.
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