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 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]
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]
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]
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 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]
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]
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]