VLBI - Very Long Baseline Interferometry
The technique of VLBI is based on the observation of a celestial object simultaneously with a set of widely distributed radio telescopes, measuring the delay in the reception of the radiation from the selected object at each telescope to assess very precise variation in the relative position of the different telescopes. The resulting interference pattern (called fringes) allows this network of telescopes to behave like a single instrument, which equivalent size (and thus resolving power) is related to the distances between the telescopes participating in one observation.
In the field of space geodesy, precise observation of quasars allows to extract the positions on Earth of the radio telescopes involved in the observation. Observing plans organized globally enable tracking of variations of these positions, and therefore are a unique tool in the study of ground movements at small and large scales.
The celestial reference frame is defined by VLBI. The International Astronomical Union has adopted the 500 extragalactic radio sources (mostly quasars) used by VLBI as the defining objects of the celestial reference frame. Extragalactic objects form a true inertial reference frame because they are at such great distances that their motions across the sky are undetectable. Positions of stars in our galaxy are now tied to this reference frame, and this is the same reference frame used for measuring Earth orientation. VLBI is unique in its ability to make rapid, accurate measurements of the orientation of the terrestrial reference frame with respect to the celestial reference frame.
VLBI observations are sponsored by over 40 organizations located in 17 countries. VLBI determines with unequaled accuracy the terrestrial reference frame (antenna locations on the Earth), the celestial reference frame (quasar positions on the sky), and Earth's orientation in space.