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Very Long Baseline Interferometry (VLBI)

VLBI is a special mode of radio interferometry, achieving the highest angular resolution on the sky of all astronomical observing techniques. Celestial sources are observed by radio telescopes all over the world. The data wre traditionally recorded on tapes with high-precision time tags, based on which they were combined post-observation in a central processing computer ("correlator"). Fringes, i.e. interference patterns, can be obtained between those telescopes and during those times when the source is observed simultaneously. It is quite easily imaginable that, due to the Earth's rotation, an eastern telescope will lose track of an celestial object earlier than a western telescope, because the source sets earlier. Conversely, the western telescope needs to wait longer until the source rises above the horizon (an effect that is negligible for telescopes with maximum baselines of up to a few hundred kilometers).

These days correlations between telescopes that are thousands of kilometers apart can be achieved in real-time, made possible by the advent of fast international computer links.

Above a (not quite complete) map of the locations of the VLBI observatories around the world. One can see concentrations of points in the USA, Europe and Australia, which host the major VLBI sub-networks listed below. These subnetworks have data processing centres, which are normally also their operations centres. Data processing requires a powerful correlator, i.e. a computer running software to combine the incoming data (on tape) from all participating telescopes.

For VLBI observing campaigns radio observatories around the world reserve the same timeslots everywhere and observers fan out to conduct coordinated observations (with identical observing frequencies, pointing directions on the sky and recording modes) at all stations, for later combination in the correlator computer. This is the most obvious example for astronomy being a truly global science.

While the long baselines give the VLBI network its capability to resolve minute details on the sky, it is the large single-dish radio telescopes that make it sensitive to faint emission from distant sources.