Sea interferometry
Sea interferometry, also known as Sea-cliff interferometry, is a form of radio astronomy that uses radio waves reflected off the sea to produce an interference pattern.[1] It is the radio wave analogue to Lloyd's mirror.[2] The technique was invented and exploited in Australia between 1945 and 1948.[3]
A radio detecting aerial is placed on top of a cliff,[2] which detects electromagnetic waves coming directly from the source and waves reflected off the water surface.[1] The two sets of waves are then combined to form an interference pattern such as that produced by two separate aerials.[1] The reflected wavefront travels an additional distance 2h sin(i) before reaching the detector where h and i are the height of the cliff and the inclination of the incoming wavefront respectively.[4] It acts as a second aerial twice the height of the cliff below the first.[4]
Sea interferometers are drift instruments, that is, they are fixed and their pointing direction changes with the rotation of the Earth.[5] The interference patterns for a sea interferometer commence sharply as soon as the source rises above the horizon, instead of fading in gradually as for a normal interferometer.[2] Since it consists of just one detector, there is no need for connecting cables or for preamplifiers.[4] A sea interferometer also has double the sensitivity of a pair of detectors set up to the same separation.[4] Sea interferometry greatly increases the resolving power of the instrument.[2]
The quality of data obtained by a sea interferometer is affected by a number of factors. Waves on the water surface and variable refraction adversely affect the signal, and the curvature of the Earth's surface must be taken into account.[2] These difficulties can be overcome by observing for extended periods, and calibrating the instrument on sources of known position.[2]
Among the discoveries made using sea interferometry are that sunspots emit strong radio waves[6] and that the source of radio wave emission from Cygnus A is small (less than 8 arcminutes in diameter). The technique also discovered six new sources including Centaurus A.[7]
References
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