Ongoing Projects

VGOS Squared (FWF, P31625)

Project-Leader: Johannes Böhm (January 2019 – 31 December 2022)

Proposal Abstract:

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.

European Space Agency: ESA-EOP

Project Partners: TU München (Lead), BKG, GFZ, TU Wien

FWF: T697

Project-Leader: Hana Krasna (on maternity leave; 1 November 2014 – )

VLBI2Galileo (FWF, P33925)

Project Leader: Johannes Böhm

Project partners: ETH Zürich, TU München

Proposal Abstract:

Space geodetic techniques are essential for the determination of global geodetic reference frames, which are the basis for every kind on positioning and navigation on Earth and in space. These reference frames are the realization of Earth-fixed and space-fixed coordinate systems, including the Earth orientation in space for the transformation between the two systems. One important component of Earth orientation is the non-uniform rotation about its axis in about 24 hours. The determination of the corresponding Earth rotation angle with microsecond-accuracy is of utmost importance for exact positioning at the millimetre-level with Global Navigation Satellite Systems, such as the United States Global Positioning System (GPS) or Galileo of the European Union. While GPS or Galileo rely on measurements between Earth orbiting satellites and antennas on the ground, Very Long Baseline Interferometry (VLBI) uses globally distributed radio telescopes and the observation of quasars, which are extragalactic radio sources billions of light years away. The primary observable in VLBI is the difference in arrival time of the signal from the quasars at the radio telescopes. VLBI is the only technique of the determination of the Earth rotation angle, as the satellite techniques suffer from unmodelled errors in the satellite orbits. The goal of project VLBI2Galileo is the transfer of the advantages of VLBI to Galileo with a new type of observation.

The only geometric ties between the two space geodetic techniques stem from local measurements at co-location sites on the ground, as there is no link between the techniques in space. In this project, we investigate the benefit of VLBI transmitters on board Galileo satellites, i.e., we assume that Galileo satellites are emitting signals similar to quasars. These signals can then be observed with VLBI radio telescopes realizing ties between the techniques in space. In consequence, these observations will enable the direct observation of the Earth rotation angle by Galileo observations, which is currently not possible. Additionally, we investigate the application of specially designed time encoded signals for ranging between the Galileo satellites and the VLBI radio telescopes, which will particularly improve the radial component of the Galileo orbits.

We are going to use the Vienna VLBI and Satellite Software (VieVS) for extensive simulations to optimize the schedules (at what time should which VLBI radio telescopes observe which satellites and quasars) for the best possible determination of Galileo orbits and references frames. Finally, we will be able to provide information about the accuracies, which can be achieved with these new observation types.