Very Long Baseline Interferometry is a geometric technique. The Two-Earth-based antennas receive the sent out signal from the distant quasar. With this measurement it is possible to compute the baseline (the distance) between two or more Radiotelescopes. With a global network of antennas, VLBI determines the reference frame and the earth position of the antennas. It includes also the possibilities to determine the length of day, the earth's orientation parameters, the celestrial and the terrestrial reference frames and the motion of the tectonic plates.
International VLBI Service for Geodesy and Astronometry (IVS).