The technique of very long baseline interferometry (VLBI) is the observation of a celestial object simultaneously with a set of radio telescopes, which may be located in places widely separated. The radiation from the object is received at slightly different times (delay) at each telescope, because of their different position on the Earth. 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 the observation.
In observation campaigns for astronomical purposes, angular resolutions in the sky of the order of milli-arcsecond (which would distinguish a giant planet around a nearby star) are obtained.
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.
The International VLBI Service for Geodesy and Astrometry (IVS) is an international collaboration of organizations which operate or support Very Long Baseline Interferometry (VLBI) components. IVS provides a service which supports geodetic and astrometric work on reference systems, Earth science research, and operational activities. The IVS groups geodesy and astrometry together because they use the same observations and the same analysis gives both types of results. IVS is established under the International Association of Geodesy (IAG).
Some of the scientific results derived from VLBI include:
· Motion of the Earth's tectonic plates
· Regional deformation and local uplift or subsidence.
· Definition of the celestial reference frame
· Variations in the Earth's orientation and length of day
· Maintenance of the terrestrial reference frame
· Measurement of gravitational forces of the Sun and Moon on the Earth and the deep structure of the Earth
· Improvement of atmospheric models