VGOS Squared

Vienna Goes VLBI Global Observing System

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.

Summary for public relations work:

Project VGOS Squared dealt with the improvement of geodetic measurements of the Earth with quasars. Emissions from quasars, which are extragalactic radio sources billions of light years away, are observed with radio telescopes on the Earth surface to precisely determine the orientation of the Earth in space as well as the positions of the telescopes on Earth and the directions to the quasars on the sky. These results are indispensable for any kind of positioning and navigation on Earth and in space and for studies of the Earth system, for example, the observation of sea level rise. The technique is called Very Long Baseline Interferometry (VLBI) and it has been used for more than forty years in geodesy. In the last decade, the transition has been made from the legacy system to the so-called VLBI Global Observing System (VGOS) with smaller and thus faster radio telescopes observing more sources in shorter time intervals necessitating broader observation bandwidths between 2 and 14 GHz. In order to exploit the full potential of VGOS for improved accuracy in the geodetic measurements, research is necessary in various aspects. In project VGOS Squared, we focused on improved scheduling as well as on the derivation of the new VLBI observables, which are determined by cross-correlating the received signals at two or more radio telescopes.

VLBI scheduling refers to the generation of observation plans, specifying which radio telescopes observe which quasars at which time in order to derive the best possible geodetic parameters. We developed new scheduling strategies and optimization criteria and implemented all these ideas in a new scheduling software package, which is now widely used for the scheduling of VGOS and legacy VLBI sessions as well as research and development sessions. Another complication with VGOS is the different coverage of frequencies and linear polarizations received at the antennas, having an impact on the determination of the VLBI observables in the so-called correlation process. We set up the correlation routines on the Vienna Scientific Cluster 4 and successfully processed a series of VGOS sessions. We used this implementation to advance the algorithms and strategies, which are still under development. Moreover, a VLBI raw data simulator was developed playing an important role in the development of new concepts and realizations of VLBI applications. With this unique tool, new setups can be tested in simulation without actually carrying out the expensive observations. In summary, project VGOS Squared, in international cooperation, has contributed significantly to the further development of the VLBI Global Observing System.

Further reading:

  • Schartner M., Böhm J. (2019). VieSched++: A New VLBI Scheduling Software for Geodesy and Astrometry. Publications of the Astronomical Society of the Pacific, (1002), doi: 10.1088/1538-3873/ab1820
  • Schartner M., Böhm J. (2020). Optimizing schedules for the VLBI global observing system. Journal of geodesy, 94(1), pp. 12. doi: 10.1007/s00190-019-01340-z
  • Schartner M., Böhm J., Nothnagel A. (2020). Optimal antenna locations of the VLBI Global Observing System for the estimation of Earth orientation parameters. Earth, planets, and space : EPS, 72(1), pp. 87. doi: 10.1186/s40623-020-01214-1
  • Mikschi M., Böhm J., Schartner M. (2021). Unconstrained Estimation of VLBI Global Observing System Station Coordinates. Advances in Geosciences, doi: 10.5194/adgeo-55-23-2021
  • Gruber J., Nothnagel A., Böhm J. (2021). VieRDS: A Software to Simulate Raw Telescope Data for very Long Baseline Interferometry. Publications of the Astronomical Society of the Pacific, (1022), doi: 10.1088/1538-3873/abeca4