Microwave radiometer
Encyclopedia
A microwave radiometer is a radiometer
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....

 that measures energy emitted at sub-millimetre-to-centimetre wavelengths (at frequencies of 1-1000 GHz
GHZ
GHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state — a quantum entanglement of three particles.# Galactic Habitable Zone — the region of a galaxy that is favorable to the formation of life....

) known as microwaves. Their primary application has been onboard spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

 measuring atmospheric and terrestrial radiation, and they are mostly used for meteorological or oceanographic remote-sensing. Their secondary application is also meteorological, as zenith-pointing surface instruments that view the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 in a region above the stationary instrument.

By understanding the physical processes associated with energy emission at these wavelengths, scientists can calculate a variety of surface and atmospheric parameters from these measurements, including air temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...

, salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

, soil moisture, sea ice
Sea ice
Sea ice is largely formed from seawater that freezes. Because the oceans consist of saltwater, this occurs below the freezing point of pure water, at about -1.8 °C ....

, precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

, the total amount of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 and the total amount of liquid water in the atmospheric column directly above or below the instrument.

The most common form of microwave radiometer was introduced by Robert Dicke in 1946. There is a book-length review of satellite radiometers.

Examples of microwave radiometers on meteorological satellites include the Special Sensor Microwave/Imager, Scanning Multichannel Microwave Radiometer
Scanning Multichannel Microwave Radiometer
The scanning multichannel microwave radiometer [pronounced simmer] was a five-frequency microwave radiometer flown on the Seasat and Nimbus 7 satellites. Both were launched in 1978, with the Seasat mission lasting less than six months until failure of the primary bus. The Nimbus 7 SMMR lasted from...

 and Microwave Sounding Unit
Microwave sounding unit
The Microwave sounding unit was the predecessor to the Advanced Microwave Sounding Unit.The MSU was first launched aboard the TIROS-N satellitein late 1978 and provided global coverage...

. The Microwave Imaging Radiometer with Aperture Synthesis
Microwave Imaging Radiometer with Aperture Synthesis
Microwave Imaging Radiometer with Aperture Synthesis is the major instrument on the Soil Moisture and Ocean Salinity satellite . MIRAS is a planar antenna composed of a central body and three telescoping, deployable arms, in total 69 receivers on the Unit...

 is an interferometer/imaging radiometer capable of resolving soil moisture and salinity over small regions of surface.
The Juno spacecraft
Juno (spacecraft)
Juno is a NASA New Frontiers mission to the planet Jupiter. Juno was launched from Cape Canaveral Air Force Station on August 5, 2011. The spacecraft is to be placed in a polar orbit to study the planet's composition, gravity field, magnetic field, and polar magnetosphere...

, launched in 2011, is planning to characterize the atmosphere of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

using a suite of microwave radiometers.

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