very-long-baseline interferometry (VLBI)

A technique in radio astronomy that effectively creates a radio interferometer in which the component antennas are separated by very large distances, typically thousands of kilometres. The antennas are not connected electrically or through microwave links. Instead, video signals are registered on magnetic tape, together with very accurate timings, at each observing station. The tapes from each station are brought together later and played through a radio receiver to complete the analysis. The technique provides extremely accurate positions for radio sources, resolutions (but not maps) down to a few milliseconds of arc and the direct detection of continental drift. Antenna separations even greater than the Earth's diameter can be achieved by placing radio telescopes in orbit and using them in conjunction with ground-based telescopes. The launch of the Japanese satellite, HALCA (originally known as Muses-B), in February 1997 marked the first stage in the development of the international VSOP - the VLBI Space Observatory Programme. HALCA, an umbrella-shaped antenna 8 metres (26 feet) in diameter, was placed in an elliptical orbit, providing a baseline up to three times larger than the Earth.