Very-long-baseline interferometry: Difference between revisions

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'''Very-long-baseline interferometry''' ('''VLBI''') is a type of [[astronomical interferometer|astronomical interferometry]] used in [[radio astronomy]]. In VLBI a signal from an [[astronomical radio source]], such as a [[quasar]], is collected at multiple radio telescopes on Earth. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.

Data received at each antenna in the array isinclude paired with timing information,arrival usuallytimes from a local [[atomic clock]], andsuch thenas storeda for[[hydrogen later analysis on magnetic tape or hard diskmaser]]. At thata later time, the data isare correlated with data from other antennas similarlythat recorded the same radio signal, to produce the resulting image. The resolution achievable using interferometry is proportional to the observing frequency and the distance between the antennas farthest apart in the array. The VLBI technique enables thisthe distance between telescopes to be much greater than that possible with conventional [[interferometry]], which requires antennas to be physically connected by [[coaxial cable]], [[waveguide]], [[optical fiber]], or other type of [[transmission line]]. The greater telescope separations are possible in VLBI due to the development of the [[closure phase]] imaging technique by [[Roger Clifton Jennison|Roger Jennison]] in the 1950s, allowing VLBI to produce images with superior resolution.

VLBI is most well known for imaging distant cosmic radio sources, spacecraft tracking, and for applications in [[astrometry]]. However, since the VLBI technique measures the time differences between the arrival of radio waves at separate antennas, it can also be used "in reverse" to perform earth rotation studies, map movements of [[tectonic plate]]s very precisely (within millimetres), and perform other types of [[geodesy]]. Using VLBI in this manner requires large numbers of time difference measurements from distant sources (such as [[quasar]]s) observed with a global network of antennas over a period of time.