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).
References and further reading:
 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
 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
 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
Robert Heinkelmann, Head of the VLBI group
Tobias Nilsson, Head of the software development
James Anderson, Satellite observations, D-VLBI
Kyriakos Balidakis, Atmospheric effects, geophysical loading effects
Susanne Glaser (TUB), Simulations w.r.t. GGOS
Li Liu, Satellite observations
Sadegh Modiri, Copula-based correlation analyses of VLBI-determined parameters
Santiago Belda, FCN, EOP, TRF
For more details, go to this link: pdf file.