A microwave radiometer (MWR) is a

radiometer A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. Microwave radiometers operate in the microwave ...

that measures energy emitted at one millimeter-to-metre wavelengths (frequencies of 0.3–300

GHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

) known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermally-emitted

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...

. They are usually equipped with multiple receiving channels to derive the characteristic emission spectrum of planetary atmospheres, surfaces or extraterrestrial objects. Microwave radiometers are utilized in a variety of environmental and engineering applications, including

remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...

weather forecasting Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th cen ...

, climate monitoring,

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 ...

and

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 ...

studies. Using the microwave spectral range between 1 and 300 GHz provides complementary information to the visible and infrared spectral range. Most importantly, the atmosphere and also vegetation is semi-transparent in the microwave spectral range. This means its components like dry gases,

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...

, or hydrometeors interact with microwave radiation but overall even the cloudy

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

is not completely opaque in this frequency range.Microwave Remote Sensing—Active and Passive". By F. T. Ulaby. R. K. Moore and A. K. Fung. (Reading, Massachusetts: Addison-Wesley, 1981 and 1982.) Volume I: Microwave Remote Sensing Fundamentals and Radiometry. For

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...

and

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

monitoring, microwave radiometers are operated from space as well as from the ground.Thermal Microwave Radiation: Applications for Remote Sensing, C. Matzler, 2006, The Institution of Engineering and Technology, London, Chapter 1. As

instruments, they are designed to operate continuously and autonomously often in combination with other atmospheric remote sensors like for example cloud

radars Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

and

lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...

s. They allow the derivation of important meteorological quantities such as vertical

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...

profiles, columnar

quantity, and columnar liquid water path with a high temporal resolution on the order of minutes to seconds under nearly all

conditions. Microwave radiometers are also used for remote sensing of Earth's ocean and land surfaces, to derive ocean temperature and wind speed, ice characteristics, and soil and vegetation properties.

History

First developments of microwave radiometer were dedicated to the measurement of radiation of extraterrestrial origin in the 1930s and 1940s. The most common form of microwave radiometer was introduced by Robert Dicke in 1946 in the Radiation Laboratory of Massachusetts Institute of Technology to better determine the temperature of the microwave background radiation. This first radiometer worked at a wavelength 1.25 cm and was operated at the Massachusetts Institute of Technology. Dicke also first discovered weak atmospheric microwave absorption using three different radiometers (at wavelengths of 1.0, 1.25 and 1.5 cm). Soon after

satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...

were first used for observing the atmosphere, microwave radiometers became part of their instrumentation. In 1962 the Mariner-2 mission was launched by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

in to investigate the surface of

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

including a radiometer for

and

observations. In following years a wide variety of microwave radiometers were tested on

. The launch of the Scanning Multichannel Microwave Radiometer in 1978 became an important milestone in the history of radiometry. It was the first time a conically scanning radiometer was used in space; it was launched into space on board the NASA Nimbus satellite. The launch of this mission gave the opportunity to image the Earth at a constant angle of incidence that is important as surface emissivity is angle dependent. In the beginning of 1980, new multi-frequency, dual-polarization radiometric instruments were developed. Two spacecraft were launched which carried instruments of this type: Nimbus-7 and

Seasat Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global sa ...

. The Nimbus-7 mission results allowed to globally monitor the state of

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...

surface as well as surface covered by snow and

glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...

. Today, microwave instruments like the Advanced Microwave Sounding Unit (AMSU) and the Special Sensor Microwave Imager / Sounder (SSMIS) are widely used on different satellites. Ground-based radiometers for the determination of temperature profiles were first explored in the 1960s and have since improved in terms of reduced noise and the ability to run unattended 24/7 within worldwide observational networks. Review articles,Westwater, E. R., S. Crewell, C. Mätzler, and D. Cimini, 2006: Principles of Surface-based Microwave and Millimeter wave Radiometric Remote Sensing of the Troposphere, Quaderni Della Societa Italiana di Elettromagnetismo, 1(3), September 2005, 50–90. and a detailed online handbook are available. Mwspectrum

Principle of operation

Solids, liquids (e.g. the Earth's surface, ocean, sea ice, snow, vegetation) but also gases emit and absorb

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

radiation. Traditionally, the amount of radiation a microwave radiometer receives is expressed as the equivalent blackbody

also called brightness temperature. In the microwave range several atmospheric gases exhibit rotational lines. They provide specific

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which s ...

features shown at a figure on the right which allow to derive information about their abundance and vertical structure. Examples for such

features are the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

absorption complex (caused by magnetic dipole transitions) around 60 GHz which is used to derive temperature profiles or the

absorption line around 22.235 GHz (dipole rotational transition) which is used to observe the vertical profile of

. Other significant absorption lines are found at 118.75 GHz (oxygen absorption) and at 183.31 GHz (water vapor absorption, used for water vapor profiling under dry conditions or from satellites). Weak absorption features due to

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...

are also used for stratospheric ozone density and temperature profiling. Besides the distinct absorption features of molecular transition lines, there are also non-resonant contributions by hydrometeors (liquid drops and frozen particles). Liquid

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

emission increases with frequency, hence, measuring at two frequencies, typically one close to the water absorption line (22.235 GHz) and one in the nearby window region (typically 31 GHz) dominated by liquid absorption provides information on both the columnar amount of water vapor and the columnar amount of liquid water separately (two-channel radiometer). The so-called „water vapor continuum" arises from the contribution of far away water vapor lines. Larger rain drops as well as larger frozen hydrometeors (snow, graupel, hail) also scatter microwave radiation especially at higher frequencies (>90 GHz). These scattering effects can be used to distinguish between rain and cloud water content exploiting polarized measurements but also to constrain the columnar amount of snow and ice particles from space and from the ground.

Design

A microwave radiometer consists of an antenna system, microwave radio-frequency components (front-end) and a back-end for signal processing at intermediate frequencies. The atmospheric signal is very weak and the signal needs to be amplified by around 80 dB. Therefore,

heterodyne A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...

techniques are often used to convert the signal down to lower frequencies that allow the use of commercial amplifiers and signal processing. Increasingly low noise amplifiers are becoming available at higher frequencies, i.e. up to 100 GHz, making heterodyne techniques obsolete. Thermal stabilization is highly important to avoid receiver drifts. Usually ground-based radiometers are also equipped with environmental sensors (

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

) and GPS receivers (time and location reference). The antenna itself often measures through a window made of foam which is transparent in the microwave spectrum to keep the antenna clean of dust, liquid water and ice. Often, also a heated blower system is attached the radiometer which helps to keep the window free of liquid drops or dew (strong emitters in the MW) but also free of ice and

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughou ...

As seen from the figure above, after the

radiofrequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...

signal is received at the antenna it is downconverted to the intermediate frequency with the help of a stable local oscillator signal. After amplification with a Low Noise Amplifier and band pass filtering the signal can be detected in full power mode, by splitting or splitting it into multiple

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

bands with a spectrometer. For high-frequency calibrations a Dicke switch is used here. Sccalibra

Calibration

The calibration of microwave radiometer sets the basis for accurate measured brightness temperatures and therefore, for accurate retrieved atmospheric parameters as

profiles, integrated

and liquid water path. The simplest version of a calibration is a so-called "hot-cold" calibration using two reference blackbodies at known, but different, "hot" and "cold"

temperatures Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

, i.e. assuming a linear relation between input power and output

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

of the detector. Knowing the physical

of the references, their brightness temperatures can be calculated and directly related to detected voltages of the radiometer, hence, the linear relationship between brightness temperatures and voltages can be obtained. The

of the calibration targets should be chosen such that they span the full measurement range. Ground-based radiometers usually use an ambient

target as "hot" reference. As a cold target one can use either a liquid nitrogen cooled blackbody (77 K) or a

zenith The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...

clear sky TB that was obtained indirectly from radiative transfer theory. Satellites use a heated target as "hot" reference and the cosmic background radiation as "cold" reference. To increase the accuracy and stability of MWR calibrations further calibration targets, such as internal noise sources, or Dicke switches can be used. Time series for 14th April 2015

Retrieval of temperature and water vapor profiles

The retrieval of physical quantities using microwave radiometry (e.g.

profiles) is not straightforward and comprehensive retrieval algorithms (using inversion techniques like

optimal estimation In applied statistics, optimal estimation is a regularized matrix inverse method based on Bayes' theorem. It is used very commonly in the geosciences, particularly for atmospheric sounding. A matrix inverse problem looks like this: : \mathbf \vec ...

approach) have been developed. Temperature profiles are obtained by measuring along the oxygen

complex at 60 GHz. The emission at any altitude is proportional to the temperature and

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

. As oxygen is homogeneously distributed within the

and around the globe, the brightness temperature signals can be used to derive the temperature profile. Signals at the center of the absorption complex are dominated by the atmosphere closest to the radiometer (when ground-based). Moving into the window region, the signal is a superposition from close and far regions of the atmosphere. The combination of several channels contains therefore information about the vertical temperature distribution. A similar approach is used to derive vertical profiles of water vapor utilizing its absorption line at 22.235 GHz.

Satellite instrumentation

Microwave instruments are flown on several polar orbiting satellites for

Earth observation Earth observation (EO) is the gathering of information about the physical, chemical, and biological systems of the planet Earth. It can be performed via remote-sensing technologies (Earth observation satellites) or through direct-contact sensors ...

and operational

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...

as well as part of extraterrestrial missions. One distinguishes between '' imaging'' instruments that are used with

conical scanning Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used ...

for

of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

surface, e.g.

AMSR Aqua (EOS PM-1) is a NASA scientific research satellite in orbit around the Earth, studying the precipitation, evaporation, and cycling of water. It is the second major component of the Earth Observing System (EOS) preceded by Terra (launched 19 ...

, SSMI,

WINDSAT The Coriolis satellite is a Naval Research Laboratory (NRL) and Air Force Research Laboratory (AFRL) Earth and space observation satellite launched from Vandenberg Air Force Base, on 2003-01-06 at 14:19 GMT. Instruments WINDSAT ''WINDSAT' ...

, and '' sounding'' instruments that are operated in cross-track mode, e.g. AMSU/ MHS. The first type uses lower frequencies (1–100 GHz) in atmospheric windows to observe sea-surface

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

, soil moisture, sea-surface

wind speed In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer. Wind speed ...

over ocean,

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

and snow. The second type is used to measure along

lines to retrieve

and humidity profile. Furthermore, limb sounders, e.g., MLS, are used to retrieve trace gas profiles in the upper

. Other examples of microwave radiometers on meteorological satellites include the

Special Sensor Microwave/Imager The Special Sensor Microwave/Imager (SSM/I) is a seven-channel, four-frequency, linearly polarized passive microwave radiometer system.Hollinger, J.P. 1989: ''DMSP Special Sensor Microwave/Imager Calibration/Validation''. Final Report, Vol. I., Spac ...

, Scanning Multichannel Microwave Radiometer,

WindSat The Coriolis satellite is a Naval Research Laboratory (NRL) and Air Force Research Laboratory (AFRL) Earth and space observation satellite launched from Vandenberg Air Force Base, on 2003-01-06 at 14:19 GMT. Instruments WINDSAT ''WINDSAT' ...

Microwave Sounding Unit The microwave sounding unit (MSU) was the predecessor to the Advanced Microwave Sounding Unit (AMSU). The MSU was first launched aboard the TIROS-N satellite in late 1978 and provided global coverage (from Pole to Pole). It carries a 4-channel mic ...

and Microwave Humidity Sounder. The Microwave Imaging Radiometer with Aperture Synthesis is an interferometer/imaging radiometer capable of resolving soil moisture and salinity over small regions of surface.

Spaceprobe instruments

By the 2010s four microwave radiometers have been flown on interplanetary spacecraft. The first was

Mariner 2 Mariner 2 (Mariner-Venus 1962), an American space probe to Venus, was the first robotic space probe to conduct a successful planetary encounter. The first successful spacecraft in the NASA Mariner program, it was a simplified version of the B ...

, which used a microwave instrument to determine the high surface temperature of Venus was coming from the surface not higher up in the atmosphere. There are/were also radiometers on the Juno Jupiter probe, the

Rosetta Rosetta or Rashid (; ar, رشيد ' ; french: Rosette ; cop, ϯⲣⲁϣⲓⲧ ''ti-Rashit'', Ancient Greek: Βολβιτίνη ''Bolbitinē'') is a port city of the Nile Delta, east of Alexandria, in Egypt's Beheira governorate. The R ...

comet probe, and Cassini-Huygens."Science Overview".
''Jet Propulsion Laboratory.'' Retrieved: 21 December 2016. The Juno probe, launched in 2011, is characterizing the atmosphere of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

using a microwave radiometer suite. The Microwave Radiometer (MWR) instrument on Juno has several antennas observing in several different microwave wavelengths to penetrate the top cloud layer of the planet, and detect features, temperatures, and chemical abundances there.

Ground-based networks

MWRnet is a network established in 2009 of scientists working with ground-based microwave radiometers. MWRnet aims to facilitate the exchange of information in the MWR user community fostering the participation to coordinated international projects. In the long run, MWRnet’s mission aims at setting up operational software, quality control procedures, data formats, etc. similar to other successful networks such as EARLINET, AERONET, CWINDE.

References

External links

{{Commons category, Microwave radiometers
National Science Digital Library – MWR Quicklook Page

DOE Atmospheric Radiation Measurement MWR Instrument Description

Juno Radiometer (MWR)
Electromagnetic radiation meters Radiometry Microwave technology