Sea interferometry, also known as sea-cliff interferometry, is a form of

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...

that uses

radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...

reflected off the

sea The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...

to produce an

interference pattern In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

. It is the radio wave analogue to Lloyd's mirror. The technique was invented and exploited in

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

between 1945 and 1948.

Process

A radio detecting aerial is placed on top of a cliff, which detects

radio propagation Radio propagation is the behavior of radio waves as they travel, or are 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 waves are affect ...

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) of the incoming wavefront respectively. 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. 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 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, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...

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 ligh ...

adversely affect the signal, and the

curvature of Earth Spherical Earth or Earth's curvature refers to the approximation of figure of the Earth as a sphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Greek philosophers. ...

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 phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. S ...

s emit strong radio waves and that the source of radio wave emission from

Cygnus A Cygnus A ( 3C 405) is a radio galaxy, and one of the strongest radio sources in the sky. A concentrated radio source in Cygnus was discovered by Grote Reber in 1939. In 1946 Stanley Hey and his colleague James Phillips identified that the sour ...

is small (less than 8

arcminute A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...

s in diameter). The technique also discovered six new sources including Centaurus A.

References

{{reflist Radio astronomy