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.{{cite web
| date = 2008-02-05
| title = Radio Astronomy at Dover Heights: Sea interferometry
| publisher = CSIRO
| url = https://www.atnf.csiro.au/outreach/about/history/doverheights/index.html#DoverInterferometry
| access-date = 2024-03-24}} It is the radio wave analogue to Lloyd's mirror.{{cite journal
| last1 = Bolton
| first1 = J. G.
| author-link1 = John Gatenby Bolton
| last2 = Slee
| first2 = O. B.
| date = December 1953
| title = Galactic Radiation at Radio Frequencies V. The Sea Interferometer
| journal = Australian Journal of Physics
| volume = 6
| issue = 4
| pages = 420–433
| bibcode = 1953AuJPh...6..420B | doi = 10.1071/PH530420 | doi-access = free
}} The technique was invented and exploited in Australia between 1945 and 1948.
{{cite conference
| last = Sullivan
| first = W. T. III
| editor1-last = Cornwell | editor1-first = T. J.
| editor2-last= Perley | editor2-first = R. A.
| date = 1991
| title = Some highlights of Interferometry in early Radio Astronomy
| book-title = Radio interferometry: Theory, techniques, and applications; Proceedings of the 131st IAU Colloquium, ASP Conference Series
| publisher = Astronomical Society of the Pacific
| location = San Francisco
| volume = 19
| page = 132
| isbn = 0-937707-38-4
| bibcode = 1991ASPC...19..132S
}}
Process
A radio detecting antenna is placed on top of a cliff, which detects radio propagation coming directly from the source and radio waves reflected off the water surface. The two sets of waves are then combined to form an interference pattern such as that produced by two separate aerials. The reflected wavefront travels an additional distance {{math|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.{{Cite book
| last1 = Goss
| first1 = W. M.
| last2 = McGee
| first2 = Richard X.
| date = 2010
| title = Under the Radar: The First Woman in Radio Astronomy: Ruby Payne-Scott
| publisher = Springer-Verlag
| isbn = 978-3-642-03141-0
| doi = 10.1007/978-3-642-03141-0
| pages = 97–99
| url = https://books.google.com/books?id=YyTZrE04ap0C&pg=PA97
}} It acts as a second aerial twice the height of the cliff below the first.
Sea interferometers are drift instruments, that is, they are fixed and their pointing direction changes with the rotation of the Earth.{{cite book
| last = Heywood
| first = John
| date = 1969
| title = Radio Astronomy and How to build your own Telescope
| edition = 2nd
| publisher = Arc Books inc
| location = New York
| pages = 90}}
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. Since it consists of just one detector, there is no need for connecting cables or for preamplifiers. A sea interferometer also has double the sensitivity of a pair of detectors set up to the same separation. Sea interferometry greatly increases the resolving power of the instrument.
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. These difficulties can be overcome by observing for extended periods, and calibrating the instrument on sources of known position.
Discoveries
Among the discoveries made using sea interferometry are that sunspots emit strong radio waves
{{cite book
| last = Hey
| first = J. S.
| date = 1973
| title = The Evolution of Radio Astronomy
| series = Histories of Science Series
| publisher = Paul Elek Scientific Books
| location = Old Woking, Surrey
| isbn = 0-236-15453-2
| pages = 40
}}
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.{{Cite book
| last = Robertson
| first = Peter
| date = 1992
| title = Beyond southern skies: radio astronomy and the Parkes telescope
| publisher = University of Cambridge
| isbn = 0-521-41408-3
| pages = 42,43,46
| url = https://books.google.com/books?id=QgQ-SFKIMdoC&pg=PA42
}}
References
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