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]

Process

A radio detecting antenna is placed on top of a cliff,[2] which detects radio propagation coming directly from the source and radio 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 (or altitude angle) 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]

Data quality

The quality of data obtained by a sea interferometer is affected by a number of factors. Wind waves on the water surface and variable atmospheric refraction adversely affect the signal, and the curvature of Earth 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]

Discoveries

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
  1. "Radio Astronomy at Dover Heights: Sea interferometry". CSIRO. 2008-02-05. Retrieved 2010-03-18.
  2. Bolton, J. G.; Slee, O. B. (December 1953). "Galactic Radiation at Radio Frequencies V. The Sea Interferometer". Australian Journal of Physics. 6 (4): 420–433. Bibcode:1953AuJPh...6..420B. doi:10.1071/PH530420.
  3. W. T. III, Sullivan (1991). "Some highlights of Interferometry in early Radio Astronomy". Radio interferometry: Theory, techniques, and applications; Proceedings of the 131st IAU Colloquium, ASP Conference Series (ASP: San Francisco). p. 132. Bibcode:1991ASPC...19..132S.
  4. Goss, W. M.; McGee, Richard X. (2010). Under the Radar: The First Woman in Radio Astronomy: Ruby Payne-Scott. Springer-Verlag. pp. 97–99. doi:10.1007/978-3-642-03141-0. ISBN 978-3-642-03141-0.
  5. Heywood, John (1969). Radio Astronomy and How to build your own Telescope (2nd ed.). New York: Arc Books inc. p. 90.
  6. Hey, J. S. (1973). The Evolution of Radio Astronomy. Histories of Science Series. Old Woking, Surrey: Paul Elek Scientific Books. p. 40. ISBN 0-236-15453-2.
  7. Robertson, Peter (1992). Beyond southern skies: radio astronomy and the Parkes telescope. University of Cambridge. pp. 42, 43, 46. ISBN 0-521-41408-3.
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