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]

## Contents

* 1 Process
* 2 Data quality
* 3 Discoveries
* 4 References

## 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. ↑ 1.0 1.1 1.2 "Radio Astronomy at Dover Heights: Sea interferometry". CSIRO. 2008-02-05. https://www.atnf.csiro.au/outreach/about/history/doverheights/index.html#DoverInterferometry.
2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 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. doi:10.1071/PH530420. Bibcode: 1953AuJPh...6..420B.
3. ↑ Sullivan, W. T. III (1991). "Some highlights of Interferometry in early Radio Astronomy". in Cornwell, T. J.; Perley, R. A.. 19. San Francisco: Astronomical Society of the Pacific. p. 132. ISBN 0-937707-38-4. Bibcode: 1991ASPC...19..132S.
4. ↑ 4.0 4.1 4.2 4.3 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. https://books.google.com/books?id=YyTZrE04ap0C&pg=PA97.
5. ↑ Heywood, John (1969). Radio Astronomy and How to build your own Telescope (2nd ed.). New York: Arc Books inc. pp. 90.
6. ↑ Hey, J. S. (1973). The Evolution of Radio Astronomy. Histories of Science Series. Old Woking, Surrey: Paul Elek Scientific Books. pp. 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. https://books.google.com/books?id=QgQ-SFKIMdoC&pg=PA42.

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