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VLBIAbbrev. for very long baseline interferometry. A method of using widely separated radio telescopes as parts of an interferometer without connecting them directly. The very long baselines achieved give the interferometer a high angular resolution and allow the study of fine structure in very distant radio sources such as quasars.
Broad-bandwidth signals from each telescope are recorded on magnetic tape, together with timing information from high-precision clocks – usually based on hydrogen masers. The tapes are then transported to a central site where they are replayed to a VLBI correlator which processes them to produce an image (see aperture synthesis).
Telescopes at many observatories are routinely diverted from their normal programmes to make VLBI observations, and standard broad-bandwidth recording systems such as the ‘Mark III’ are used. Dedicated networks also exist (see VLBA) whose sole purpose is VLBI. Sometimes telescopes from around the world are combined to give intercontinental VLBI with baselines of more than 10 000 km. Baselines greater than the diameter of the Earth can be achieved by using a telescope mounted on an orbiting satellite, as is suggested for VSOP and RadioAstron.
VLBI can also be used to measure certain geodynamical phenomena, such as the movements of tectonic plates, polar wobble, and the rise and fall of Earth tides.