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Geodetic and astrometric VLBI

Very Long Baseline Interferometry (VLBI) is a highly accurate technique, used since the 1970s in geodesy that has delivered groundbreaking scientific discoveries. This technique allows scientists to measure global distances with millimeter accuracy; thus, VLBI contributes significantly to determining the global terrestrial reference frame, named International Terrestrial Reference Frame (ITRF). It is the only one of the space geodetic techniques referenced to the kinematically non-rotating frame and the only one that provides all Earth Orientation Parameters (EOP), i.e., polar motion, universal time, and celestial pole offsets.

In the VLBI technique, several radio telescopes spread worldwide observe the same object in the sky. Figure 1 illustrates the concept. The received radio signals are processed, digitally recorded together with the time obtained from exact atomic clocks (hydrogen maser), and sent to a special computing device called a correlator. By comparing the measurements in a process called ‘cross-correlation’ the time difference between the arrivals of the signal at each two stations is determined. With this information the distance between the stations can then be derived with an accuracy of a few millimeters. In astrometry the same method is used to obtain the directions of the radio sources with an average precision of about 40 microarcseconds (about ten billionths of a degree). This corresponds to measuring the exact location of a tennis ball on the Moon as seen from the Earth.

The positions of the VLBI stations contribute significantly to the realization of the global terrestrial reference frame (ITRF). The stable reference points provided by VLBI are ideal for surveying the Earth to determine changes with high precision. These changes are based on phenomena such as global sea level rise, tidal effects, and tectonic plate motions.

In the analysis of VLBI measurements numerous disturbances have to be considered. For example the atmosphere “slows” the radio waves down, especially in the lowest layer the --- troposphere, about ten kilometers high, where our weather takes place. But it is not just an effect that has to be corrected when we are analyzing our data; VLBI also allows to determine meteorological parameters from its observations. The tropospheric information obtained by VLBI over long periods provides important insight about the Earth's climate. The rotational fluctuations of the Earth itself must also be considered: the changes in the location of the Earth's poles (polar motion) and the slight variations in the length of a day. The results obtained from VLBI measurements are very revealing. Figure 2 shows how the stations in Westford (United States) and Wettzell (Germany), separated by nearly 6 000 kilometers, move apart by nearly two centimeters per year due to continental drift. The decreasing scatter in the measurements toward more recent years allows geodesists to track the progress in improvements made to the VLBI technique. Long-term climate and environmental evolutions are also easily observed due to the high accuracy of the VLBI results.

Besides natural radio sources in the universe such as quasars, radio galaxies (an example of which is shown in Figure 3), and BL Lac objects, artificial radio sources such as satellites and space probes can be observed with the same instrumentation. In the process of extending the geodetic VLBI technique for measurements of spacecraft, not only is it necessary to determine the instrumental requirements, but especially the theoretical models may need to get adapted or extended. The method of differential VLBI (D-VLBI) is of particular interest for such applications.

In addition to aiding the navigation of space probes and the determination of spacecraft ephemerides, VLBI observations can be used to link different reference frames and different geodetic observation techniques. The VLBI group at the GFZ examines these applications, where the connection between the reference frames of VLBI and GNSS (Global Navigation Satellite Systems) are of particular interest (Figure 4).

VLBI Group Members

Members (in alphabetical order):

  • Georg Beyerle, Development of software for geodetic/astrometric VLBI data analysis
  • Sujata Dhar(Ph.D. Student), Earth Orientation Parameter by VLBI and GNSS, Simulations, Combinations of space geodetic techniques
  • Robert Heinkelmann, Head of the VLBI group, GFZ IVS Analysis Center Representative, Celestial and Terrestrial Reference Frames, Earth rotation analysis and validation, tropospheric and ionospheric effects
  • Chaiyaporn Kitpracha (Ph.D. student), Tropospheric effects in VLBI and GNSS observables and tropospheric ties
  • Harald Schuh, Director of Dept. 1 "Geodesy" of GFZ, past President of the International Association of Geodesy (IAG), past Chair of the International VLBI Service for Geodesy and Astrometry (IVS)
  • Minghui Xu, Comparison of radio and optical celestial reference frames, reference frame determination and global solutions (AGORA)

Former Members (in alphabetical order):
James Anderson
Kyriakos Balidakis
Santiago Belda
Susanne Glaser
Suxia Gong
Okky Syahputra Jenie

Maria Karbon
Li Liu

Susanne Lunz
Nicat Mammadaliyev
Sadegh Modiri
Pakize Küreç Nehbit
Tobias Nilsson
Shrishail Raut
Benedikt Soja
Jungang Wang

For more details, go to this link: pdf file

Current Research

PORT: Potsdam Open-source Radio interferometry Tool

PORT (Potsdam Open-source Radio interferometry Tool) is a software package dedicated to the analysis of modern and historical very long baseline interferometry observations (VLBI). PORT is being developed at GFZ Potsdam and is utilized to perform the GFZ VLBI Analysis Center responsibilities, as well as state-of-the-art research in the field of space geodesy, geophysics, and astrometry. PORT can estimate quantities such as station positions, quasar positions, Earth orientation parameters, atmospheric delay parameters, either employing weighted least-squares or a Kalman filter and smoother. PORT is designed to process VGOS observations, as well as traditional S-/X-band and single-band observations, regardless of data length; PORT processes both Intensive and multi-hour/day sessions. Developing PORT, special care has been taken to deal with issues such as the source structure. PORT abides by the latest analysis standards suggested by the IERS (International Earth Rotation and Reference Systems Service) and the IVS (International VLBI Service for Geodesy and Astrometry), and is ITRF2020-ready.

PORT is written mainly in MATLAB, with some Python modules, and is maintained on a Git server. VLBI data analysis employing PORT may be carried by scripting or interactively via a graphical user interface.

Upon completion, PORT will be able to schedule, simulate, and analyze VLBI observations, as well as to utilize VLBI observations to derive consistent terrestrial and celestial reference frames.

For more information about PORT, please visit https://git.gfz-potsdam.de/vlbi-data-analysis/port

If you need additional information, please contact Georg Beyerle.

References and further reading

[1] Heinkelmann R.: VLBI geodesy: observations, analysis, and results. In: Geodetic sciences – observations, modeling and applications. S. Jin (ed.), InTech open, ISBN 980-953-307-595-7, 2013

[2] Schuh H. & Behrend D.: VLBI: A fascinating technique for geodesy and astrometry. J Geodyn 61, DOI 10.1016/j.jog.2012.07.007, 68—80, 2012

[3] Schuh H. & Böhm J.: Very Long Baseline Interferometry for Geodesy and Astrometry. In: Sciences of Geodesy – II, Innovations and Future Developments, G. Xu (ed.), DOI 10.1007/978-3-642-28000-9, Springer Berlin Heidelberg, 339—376, 2013

[4] Schuh et al.: The Potsdam Open Source Radio Interferometry Tool (PORT), Publications of the Astronomical Society of the Pacific, 133:104503 (13pp), 2021, https://doi.org/10.1088/1538-3873/ac299c

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