Vienna Goes VLBI Global Observing System
VGOS Squared (FWF, P31625)
Project-Leader: Johannes Böhm (January 2019 – 31 December 2022)
In geodetic Very Long Baseline Interferometry (VLBI), we use globally distributed VLBI radio telescopes to observe quasars billions of light years away. The difference in arrival times of the signals at the stations is the primary observable used in geodesy to estimate baseline vectors between radio telescopes, positions of quasars, and Earth orientation parameters. The observables are determined by time-tagging the signals with the use of atomic clocks, e-transferring the data, and cross-correlation at special computing facilities, called correlators. With these unique capabilities, VLBI plays a key and outstanding role in the determination of terrestrial and celestial reference frames as well as Earth orientation parameters. For example, VLBI is the only technique for the observation of Universal Time (UT1) which is related to the Earth rotation angle and thus of fundamental importance for any kind of positioning and navigation with the Global Navigation Satellite Systems (GNSS). VLBI is also critical for the accuracy of the scale of the terrestrial reference frame, which is realised by positions and velocities of globally distributed sites. The stability of the scale at an accuracy level of 0.1 mm/year is a prerequisite for the observation of small geodynamic quantities, such as sea level rise at about 3 mm/year.
The VLBI community is currently working on a tremendous improvement of the VLBI technique, called VLBI Global Observing System (VGOS), and based on new fast slewing radio telescopes and increased observation bandwidth resulting in astrometric and geodetic quantities of unprecedented accuracies. From a TU Wien (Vienna) point of view, we see the following two tasks as the main fields where we can not only contribute, but really bring VLBI and in particular VGOS activities to the next level. First, there is potential in improving the scheduling of VLBI sessions, i.e., in specifying which radio telescopes should observe which quasars at what time. In particular, we will investigate the application of tree-based schedules (“looking further ahead when scheduling") in combination with graph theory as well as innovative functions to describe the sky coverage at the stations. Second, correlation is and certainly will be the bottleneck in VGOS with a dramatically increased requirement in terms of bandwidth, number of processing cores, and storage. We will use dedicated storage and computing cores for correlation activities on the Vienna Scientific Cluster (VSC), a collaboration of several Austrian universities that provides supercomputer resources and corresponding services to their users. We will investigate the correlation and fringe-fitting of VGOS data along with the automation of the processes from correlation to analysis, thus serving as a role model for other universities to deal with correlation in future.
Independent Generation of Earth Orientation Parameters
European Space Agency: ESA-EOP
Project Partners: TU München (Lead), BKG, GFZ, TU Wien
Satellite Observations by Radio Telescopes for Superior Reference Frame Interconnections
SORTS: FWF I2204 and DFG (Axel Nothnagel)
Project-Leader at TU Wien: Johannes Böhm (1 January 2016 – 31 December 2019)
Satellite observations by radio telescopes have the potential to dramatically improve the frame ties between the International Celestial Reference Frame (ICRF) realized with Very Long Baseline Interferometry (VLBI) observations to extragalactic radio sources and satellite orbits realizing celestial frames dynamically. At the same time, the consistency of the International Terrestrial Reference Frame (ITRF) as a multi-technique solution from VLBI and satellite geodesy observations will benefit substantially, which otherwise solely relies on local tie measurements at co-located terrestrial observing sites. Accurate frame ties are fundamental for precise navigation in space and are a prerequisite for the observation of sea level rise and other global geodynamic processes.
In project SORTS, we cover all technical aspects which are necessary for the realization and analysis of VLBI-like observations to satellites dedicated to such space-tie projects. Now is the optimal time frame for such a project because proposals for satellites which carry a VLBI transponder, laser reflectors and a GNSS (Global Navigation Satellite Systems) receiver like GRASP or MicroGEM are close to acceptance. In project SORTS, we will investigate the application of noise versus modulated signals in terms of signal-to-noise ratio and projected precision of group and phase delays. Next, we will set up a fully realistic scheduling tool taking into account all station- and source-specific restrictions like slewing rates and integration times and we will close all gaps from scheduling to the actual observations of satellites with radio telescopes allowing for automated procedures. Particular emphasis will be put on joint schedules with alternate observations to quasars and satellites. To generate group and phase delays from the observations by radio telescopes to satellites at low Earth orbits, we will adapt all models in the correlation software. Finally, we will expand existing geodetic analysis software for the purpose of optimally combining quasar and satellite observations. With this software, we will use realistically simulated observations to determine orbital arcs from various types of radio telescope observations and we will run series of Monte-Carlo simulations to provide feedback to earlier steps in the processing chain like the scheduling and to assess the impact on the frames ties.
Project SORTS is a joint endeavor of the Vienna University of Technology and the University of Bonn with the strengths of both groups greatly complementing each other. Furthermore, in close co-operation with partners at the VLBI stations at Wettzell in Germany and Onsala in Sweden, we will be able to validate our developments with real observations already at a very early stage, for example with VLBI observations to GNSS satellites.