Sea interferometry, also known as sea-cliff interferometry, is a form of
radio astronomy
Radio astronomy is a subfield of astronomy that studies Astronomical object, celestial objects using radio waves. It started in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming from the Milky Way. Subsequent observat ...
that uses
radio waves
Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths ...
reflected
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The ...
off the
sea
A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order section ...
to produce an
interference pattern
In physics, interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference. The resultant wave may have greater amplitude (constructive int ...
.
It is the radio wave analogue to
Lloyd's mirror
Lloyd's mirror is an optics experiment that was first described in 1834 by Humphrey Lloyd (physicist), Humphrey Lloyd in the ''Transactions of the Royal Irish Academy''. Its original goal was to provide further evidence for the Light#Wave theory, w ...
.
The technique was invented and exploited in
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
between 1945 and 1948.
[
]
Process
A radio detecting
antenna is placed on top of a cliff,
[ which detects ]radio propagation
Radio propagation is the behavior of radio waves as they travel, or are wave propagation, propagated, from one point to another in vacuum, or into various parts of the atmosphere.
As a form of electromagnetic radiation, like light waves, radio w ...
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 before reaching the detector where ''h'' and ''i'' are the height of the cliff and the inclination (or ]altitude angle
The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and ''azimuth''.
Therefore, the horizontal coordin ...
) of the incoming wavefront respectively.[ Since it consists of just one detector, there is no need for connecting cables or for ]preamplifier
A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier a ...
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.][
]
Data quality
The quality of data obtained by a sea interferometer is affected by a number of factors. Wind waves
In fluid dynamics, a wind wave, or wind-generated water wave, is a surface wave that occurs on the free surface of Body of water, bodies of water as a result of the wind blowing over the water's surface. The contact distance in the wind directi ...
on the water surface and variable atmospheric refraction
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of light ...
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 sunspot
Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...
s emit strong radio waves[
]
and that the source of radio wave emission from Cygnus A is small (less than 8 arcminute
A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...
s in diameter). The technique also discovered six new sources including Centaurus A.
References
{{reflist
Radio astronomy