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Very Long Baseline Interferometry, or VLBI, is the technique by which radio telescopes separated by up to thousands of kilometres are used at the same time for astronomical or geodetic observations. The signals are received and amplified at the participating antennas, are digitized and sent to a correlator, either by storing them on tape or disk for later shipment, or, more recently, by sending them over network links. The correlator cross-correlates and Fourier transforms the signals from each pair of antennas, and the result of this process can be used to determine the brightness distribution of the sky at radio frequencies. The angular resolution and positional accuracies achieved in these observations are as high as a fraction of a milli-arcsecond.

The radio regime was the first waveband accessible to astronomers after they had studied the skies only at optical wavelengths for hundreds of years. In the first half of the 20th century, rapid progress in high-frequency technology facilitated access to radio wavelengths. Owing to the (then) relatively long wavelengths, radio observations even with large single telescopes had poor angular resolution of no better than 10 arcmin, but after World War II the first interferometers emerged and increased the resolution to less than 1 arcmin. Astronomers were puzzled that even at the highest resolution achieved with radio-linked interferometers (about 0.1 arcsec), some radio sources appeared point-like. This fuelled the desire to use even longer baselines.

Going from directly-linked interferometers to independent elements required recording the data in some way and processing them later, but once the technical hurdles were overcome, the angular resolution of these observations increased dramatically: from 1967 to 1969, the longest baselines used in VLBI observations increased from 845 km to 10 592 km, yielding an angular resolution of the order of 1 mas. Thirty years after the first systematic radio astronomical observations had been carried out the resolution had increased more than a million fold. For more detailed reviews on radio interferometry and VLBI we refer the reader to chapter one in Thompson et al. (2001) and Kellermann and Moran (2001).

Though many important discoveries had been made early on, VLBI observations remain indispensable in many fields of astrophysical research. They will much benefit from improvements in computer technology, in particular from faster (and cheaper) network links.

This article gives a brief overview of how radio interferometry works, of the calibration and data processing steps required to make an image, and the various flavours and niches which can be explored. It then reviews recent progress in the research of various astrophysical phenomena which has been made possible with VLBI observations.

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