Interferometry: Very long baseline interferometry
 
In principle, and practice, very large distances between two or more radio telescopes can be used to obtain high resolution in measuring a radio source’s coordinates without the stations having direct or real time links. This is done by the principle of Very Long Baseline Interferometry (VLBI). Baselines between the radio telescopes have been extended in some arrays to thousands of kilometres by employing simultaneously several radio astronomy observatories.
With very accurate timekeeping methods available, the radio telescopes’ signals are recorded individually but synchronously with accurate time markers added as the sky moves across each of the apertures. By adding the recorded signals together at some future convenient time, keeping their recorded times accurately matched, an interference pattern is produced, corresponding to the one that would have been produced if the signals had been combined at the actual time of the observations.
The Australia Telescope National Facility (ATNF) uses eight radio antennas located at three observatories in New South Wales. It has a baseline of 300 km. Like MERLIN, the antennas can be used individually or in various combinations and combines the principles of both VLBI and aperture synthesis.
The Very Long Baseline Array (VLBA) operated by the US National Radio Astronomy Observatory with headquarters at Socorro, New Mexico, consists of an array of ten radio telescopes situated with all but two in the continental USA, the others being on Hawaii and the Virgin Islands. With a maximum baseline of 8000 km, the VLBA can achieve a resolution of 0.001 arc sec, some 50 times better than the Hubble Space Telescope. In principle, putting a radio telescope in a spacecraft or on the Moon to be used in conjunction with one or more situated on Earth should give baselines of hundreds of thousands of kilometres with resolutions orders of magnitude better than those obtained by the VLBA.