Sea interferometry
Encyclopedia
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. It is the radio wave analogue to Lloyd's mirror
. The technique was invented and exploited in Australia
between 1945 and 1948.
A radio detecting aerial is placed on top of a cliff, which detects waves coming directly from the source and 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 before reaching the detector where h and i are the height of the cliff and the inclination of the incoming wavefront respectively. It acts as a second aerial twice the height of the cliff below the first.
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 preamplifier
s. 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.
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. These difficulties can be overcome by observing for extended periods, and calibrating the instrument on sources of known position.
Among the discoveries made using sea interferometry are that sunspot
s emit strong radio waves 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
.
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
that uses radio waves reflected off the sea
Sea
A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...
to produce an interference pattern. It is the radio wave analogue to Lloyd's mirror
Lloyd's mirror
Lloyd's mirror is a classic optics experiment and was first described in 1834 and again in 1837 by Humphry Lloyd in the proceedings of the Royal Irish Academy of Science. In the experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come...
. The technique was invented and exploited in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
between 1945 and 1948.
A radio detecting aerial is placed on top of a cliff, which detects waves coming directly from the source and 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 before reaching the detector where h and i are the height of the cliff and the inclination of the incoming wavefront respectively. It acts as a second aerial twice the height of the cliff below the first.
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 preamplifier
Preamplifier
A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...
s. 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.
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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's surface must be taken into account. These difficulties can be overcome by observing for extended periods, and calibrating the instrument on sources of known position.
Among the discoveries made using sea interferometry are that sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
s emit strong radio waves and that the source of radio wave emission from Cygnus A
Cygnus A
Cygnus A is one of the most famous radio galaxies, and among the strongest radio sources in the sky.It was discovered by Grote Reber in 1939. In 1951, Cygnus A, along with Cassiopeia A, and Puppis A were the first "radio stars" identified with an optical source, of these, Cygnus A became the first...
is small (less than 8 arcminutes in diameter). The technique also discovered six new sources including Centaurus A
Centaurus A
Centaurus A is a prominent galaxy in the constellation of Centaurus. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance...
.