very-long-baseline interferometry

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very-long-baseline interferometry (VLBI) The basic principle of very-long-baseline interferometry (VLBI) is that the difference in time between the arrival of a radio signal at a pair of radio telescopes is a function of the length and orientation of the line joining the two. Because the radio sources used as targets for VLBI are very far away (and thus independent of the Earth) and move very little relative to each other, VLBI is a good means of acquiring information about variations in the Earth's rotation rate and orientation in space as well as the positions of the radio telescopes. The S- and X-band (2–8 GHz) microwave signals that are received are very weak, and large antennae (10–100 m) are normally required, but data from smaller (4–5 m) transportable antennae can be combined with data from the larger ones. Although radio interferometry has been possible since the 1930s, the main advance in the field came in the 1960s when the development of precise atomic clocks removed the need for physical communication between the telescopes. The latter introduced additional delays into the signal path and limited the length of the baseline. Nowadays, the incoming signal is mixed (heterodyned) with a pure harmonic signal from a stable local oscillator (usually a hydrogen maser which can provide frequency stability of one part in 10¹⁴) and the resulting beat frequency is time-tagged and recorded for later analysis. This process is carried out simultaneously for many frequency bands spanning the range of interest.

The analysis of VLBI observations is complicated by several factors. First, the incoming signal is delayed (and to some extent refracted) by the Earth's ionosphere and troposphere (the ionized outer layer and the lower layer of the Earth's atmosphere respectively). Ionospheric delay is dispersive; that is, it affects different frequencies of a signal in a systematically different way. The effect can be corrected by using measurements at more than one frequency. Tropospheric delay is harder to correct, because it is non-dispersive. The ‘dry’ component (neglecting the presence of atmospheric water vapour) varies mostly with local surface pressure and temperature and can be modelled. However, the ‘wet’ component, which depends on water vapour content along the signal path, cannot easily be estimated, although it is smallest for observations at high angles of elevation. Secondly, the raw measurements are subject to a phase ambiguity. Because the incoming oscillation repeats each cycle, there will be an unknown integral number of cycles which must be added to the phase-delay measurement. Unless this integer can be estimated (which would require a very good pre-existing knowledge of the length of the baseline), this difficulty must be overcome by using the group delay, the change of phase delay with frequency, which has no such ambiguity but is less precisely determined. Thirdly, errors in the local oscillators and clocks must be considered. Finally, because radio telescopes are large structures, relating the actual point of measurement to a ground marker may be difficult, and the point of measurement may vary with frequency and will shift as the telescope deforms in response to orientation, wind, and temperature changes. Despite these difficulties, VLBI measurements of position can achieve accuracies of a few centimetres, which is sufficient for measuring very small changes in tides, Earth rotation rates and wobble, as well as plate-tectonic movements over several years.

Peter Clarke

very long baseline interferometry (VLBI) A technique in radio interferometry where telescopes, often many thousands of kilometres apart, may be operated as an interferometer to achieve angular resolutions better than 0ʺ.001. Each telescope works independently, recording the signals together with timing information on arrays of hard disks. Later the data are replayed and the signals combined. In the new technique of e‐VLBI, the data are transferred in real time to the central processor over the Internet. A side-benefit of VLBI work is the location of the observatories to accuracies of a few centimetres, useful in the study of continental drift. Examples of VLBI arrays are the European VLBI Network (EVN), the Very Long Baseline Array, and the Australia Telescope. The advent of space-borne radio telescopes will extend VLBI baselines to hundreds of thousands of kilometres.

very-long-baseline interferometry (VLBI) A technique for determining, with precisions of centimetres, the distances between different radiotelescopes, using the phase difference between radio signals detected by them that come from very distant radio sources (quasars). Some 135 different sources are routinely used.