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.
Sal EE, Hörsalsvägen 11, Chalmers
Opponent: PD Dr.-Ing Axel Nothnagel, Institut für Geodäsie und Geoinformation, Universität Bonn, Germany