View source for Scatterometer

{{Short description|Meteorological instrumentation}}
A '''scatterometer''' or '''diffusionmeter''' is a scientific instrument to measure the return of a beam of light or radar waves [[Scattering|scattered]] by diffusion in a medium such as air. Diffusionmeters using visible light are found in airports or along roads to measure horizontal [[visibility]]. Radar scatterometers use radio or microwaves to determine the [[normalized radar cross section]] (σ<sup>0</sup>, "sigma zero" or "sigma naught") of a surface. They are often mounted on [[weather satellite]]s to find wind speed and direction, and are used in industries to analyze the roughness of surfaces.

== Optical ==
[[File:Dohlednostmereni.jpg|thumb|left|Airport scatterometer (or diffusometer).]]
Optical diffusionmeters are devices used in [[meteorology]] to find the optical range or the horizontal visibility. They consist of a light source, usually a [[laser]], and a receiver. Both are placed at a 35° angle downward, aimed at a common area. Lateral [[scattering]] by the air along the light beam is quantified as an [[attenuation coefficient]]. Any departure from the clear air extinction coefficient (e.g. in fog) is measured and is inversely proportional to the visibility (the greater the loss, the lower is the visibility).

These devices are found in [[automatic weather station]]s for general visibility, along airport runways for [[runway visual range]], or along roads for visual conditions. Their main drawback is that the measurement is done over the very small volume of air between the transmitter and the receiver. The visibility reported is therefore only representative of the general conditions around the instrument in generalized conditions ([[Synoptic scale meteorology|synoptic]] fog for instance). This is not always the case (e.g. patchy fog).
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== Radar==
{{split section|Radar scatterometer|date=May 2023}}
[[File:Scattometer principle.GIF|thumb|Radar scatterometer]]
A '''radar scatterometer''' operates by transmitting a pulse of [[microwave]] energy towards the [[Earth]]'s surface and measuring the reflected energy.  A separate measurement of the noise-only power is made and subtracted from the signal+noise measurement to determine the [[backscatter]] signal power.  [[radar cross section|Sigma-0]] (σ⁰) is computed from the signal power measurement using the distributed target radar equation.  Scatterometer instruments are very precisely calibrated in order to make accurate backscatter measurements.

The primary application of [[Kármán line|space]]borne scatterometry has been measurements of near-surface [[wind]]s over the [[ocean]].<ref name='Naderi1991'>{{cite journal |author1=F. Naderi |author2=M. H. Freilich |author3=D. G. Long |name-list-style=amp |title=Spaceborne Radar Measurement of Wind Velocity Over the Ocean—An Overview of the NSCAT Scatterometer System |journal=Proceedings of the IEEE |pages=850–866 |volume=79 |number=6 |date=June 1991 |doi=10.1109/5.90163|url=https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=1719&context=facpub |url-access=subscription }}</ref> Such instruments are known as wind scatterometers. By combining sigma-0 measurements from different [[azimuth]] angles, the near-surface wind [[Euclidean vector|vector]] over the ocean's surface can be determined using a [[Geophysics|geophysical]] model function (GMF) which relates wind and backscatter.  Over the ocean, the radar backscatter results from scattering from wind-generated capillary-gravity waves, which are generally in equilibrium with the near-surface wind over the ocean. The scattering mechanism is known as [[Bragg scattering]], which occurs from the waves that are in resonance with the microwaves.

The backscattered power depends on the wind speed and direction.  Viewed from different azimuth angles, the observed backscatter from these waves varies. These variations can be exploited to estimate the sea surface wind, i.e. its speed and direction. This estimate process is sometimes termed ''''wind retrieval'''' or ''''model function inversion''''. This is a non-linear inversion procedure based on an accurate knowledge of the GMF (in an [[empirical]] or semi-empirical form) that relates the scatterometer backscatter and the vector wind. Retrieval requires an angular diversity scatterometer measurements with the GMF, which is provided by the scatterometer making several backscatter measurements of the same spot on the ocean's surface from different azimuth angles.

[[File:Soulik 2013 July 10 Scatterometer Ascending Pass.png|thumb|A snapshot of [[Typhoon Soulik (2013)|Typhoon Soulik]] while at Category 4 intensity captured by Eumetsat's ASCAT (Advanced Scatterometer) instrument on board the [[Metop]]-A satellite|left]]Scatterometer wind measurements are used for air-sea interaction, climate studies and are particularly useful for monitoring [[Tropical cyclone|hurricane]]s.<ref>P.S. Chang, Z. Jelenak, J.M. Sienkiewicz, R. Knabb, M.J. Brennan, D.G. Long, and M. Freeberg. Operational Use and Impact of Satellite Remotely Sensed Ocean Surface Vector Winds in the Marine Warning and Forecasting Environment, ''Oceanography'', Vol. 22, No. 2, pp. 194–207, 2009.</ref> Scatterometer backscatter data are applied to the study of [[vegetation]], [[Water content|soil moisture]], [[Polar ice cap|polar ice]], tracking Antarctic [[icebergs]]<ref>K.M. Stuart and D.G. Long, Tracking large tabular icebergs using the SeaWinds Ku-band microwave scatterometer, ''Deep-Sea Research Part II'', {{doi|10.1016/j.dsr2.2010.11.004}}, Vol. 58, pp. 1285–1300, 2011.</ref> and [[global change]].<ref>D.G. Long, M.R. Drinkwater, B. Holt, S. Saatchi, and C. Bertoia. Global Ice and Land Climate Studies Using Scatterometer Image Data, ''EOS, Transactions of the American Geophysical Union'', Vol. 82, No. 43, pg. 503, 23 Oct. 2001.</ref> Scatterometer measurements have been used to measure winds over sand and snow dunes from space.  Non-terrestrial applications include study of Solar System moons using space probes. This is especially the case with the NASA/ESA Cassini mission to Saturn and its moons.

Several generations of wind scatterometers have been flown in space by [[NASA]], [[European Space Agency|ESA]], and [[National Space Development Agency of Japan|NASDA]].  The first operational wind scatterometer was known as the [[SEASAT|Seasat]] Scatterometer (SASS) and was launched in 1978.<ref>W.L. Grantham, et al., The SeaSat-A Satellite Scatterometer, ''IEEE Journal of Oceanic Engineering'', Vol. OE-2, pp 200–206, 1977.</ref> It was a fan-beam system operating at Ku-band (14&nbsp;GHz).  In 1991 ESA launched the [[European Remote-Sensing Satellite]] ERS-1 Advanced Microwave Instrument (AMI) scatterometer,<ref>E. Attema, The Active Microwave Instrument Onboard the ERS-1 Satellite, ''Proceedings of the IEEE'', 79, 6, pp. 791–799, 1991.</ref> followed by the ERS-2 AMI scatterometer in 1995.  Both AMI fan-beam systems operated at C-band (5.6&nbsp;GHz).  In 1996 NASA launched the ''NASA Scatterometer'' (NSCAT), on board the NASDA [[ADEOS I]] satellite,<ref name='Naderi1991' /> a Ku-band fan-beam system.<ref>W-Y Tsai, J.E. Graf, C. Winn, J.N. Huddleston, S. Dunbar, M.H. Freilich, F.J. Wentz, D.G. Long, and W.L. Jones. Postlaunch Sensor Verification and Calibration of the NASA Scatterometer, ''IEEE Transactions on Geoscience and Remote Sensing'', Vol. 37, No. 3, pp. 1517–1542, 1999.</ref> NASA launched the first scanning scatterometer, known as ''SeaWinds'', on [[QuikSCAT]] in 1999.  It operated at Ku-band. A second SeaWinds instrument was flown on the NASDA ADEOS-2 in 2002.  The [[Indian Space Research Organisation]] launched a Ku-band scatterometer on their Oceansat-2 platform in 2009.  [[European Space Agency|ESA]] and [[European Organisation for the Exploitation of Meteorological Satellites|EUMETSAT]] launched the first C-band ASCAT in 2006 onboard [[Metop]]-A.<ref>J. Figa-Saldaña, J.J.W. Wilson, E. Attema, R. Gelsthorpe, M.R. Drinkwater, and A. Stoffelen. The advanced scatterometer (ASCAT) on the meteorological operational (MetOp) platform: A follow on for European wind scatterometers, ''Canadian Journal of Remote Sensing'', Vol. 28, No. 3, June 2002.</ref> The [[Cyclone Global Navigation Satellite System]] (CYGNSS), launched in 2016, is a constellation of eight small satellites utilizing a [[Bistatic radar|bistatic]] approach by analyzing the reflection from the Earth's surface of [[Global Positioning System]] (GPS) signals, rather than using an onboard radar transmitter.

===Contribution to botany===
Scatterometers helped to prove the hypothesis, dating from mid-19th century, of the [[Anisotropy|anisotropic]] (direction dependent) long distance dispersion by wind to explain the strong [[Floristics|floristic]] affinities between landmasses.

A work, published by the journal ''[[Science (journal)|Science]]'' in May 2004 with the title "Wind as a Long-Distance Dispersal Vehicle in the Southern Hemisphere", used daily measurements of wind azimuth and speed taken by the '''SeaWinds scatterometer''' from 1999 to 2003. They found a stronger correlation of floristic similarities with wind connectivity than with geographic proximities, which supports the idea that wind is a dispersal vehicle for many organisms in the Southern Hemisphere.

===Semiconductor and precision manufacturing===

Scatterometers are widely used in metrology for roughness of polished and lapped surfaces in semiconductor and precision machining industries.<ref>John C. Stover. SPIE Optical Engineering Press, 1995 – Science – 321 pages.</ref> They provide a fast and non-contact alternative to traditional stylus methods for topography assessment.<ref>Myer, G, et al (1988) "Novel Optical Approach to Atomic Force Microscopy", Applied Physics Letters, 53, 1045–1047</ref><ref>Baumeister, Theodore, et al.
(1967) Standard Handbook for Mechanical Engineers. McGraw-Hill, LCCN 16-12915</ref> Scatterometers are compatible with vacuum environments, are not sensitive to vibration, and can be readily integrated with surface processing and other metrology tools.<ref>John M. Guerra. "A Practical Total Integrated Scatterometer", Proc. SPIE 1009, Surface Measurement and Characterization, 146 (March 21, 1989)</ref><ref>{{cite web |url= http://zebraoptical.com/Scatterometry.html |title= Roughness via Scatterometry |publisher= ZebraOptical |access-date=30 December 2016}}</ref>

==Uses==
[[File:Illustration of ISS-RapidScat on ISS (20160909).jpg|thumb|Illustration of the ISS-RapidScat location on the International Space Station]]
Examples of use on [[Earth observation satellite]]s or installed instruments, and dates of operation:<ref>{{cite web |url=https://coaps.fsu.edu/scatterometry/about/overview.php |title=Scatterometry & Ocean Vector Winds: Satellite Studies |publisher=Florida State University |access-date=30 December 2016}}</ref>
*NSCAT (NASA Scatterometer) instrument on [[ADEOS I]] (1996–97)
*SeaWinds instrument on [[QuikSCAT]] (2001–2009)
*OSCAT-2 instrument on [[SCATSAT-1]] (launched 2016)
*SCAT instrument on [[Oceansat-2]] (2009–2014)
*[[ISS-RapidScat]] on the International Space Station (2014–2016)
*ASCAT on [[MetOp]] satellites
*The [[Cyclone Global Navigation Satellite System|CYGNSS]] constellation (launched 2016)

==References==
{{Reflist|2}}

==External links==
*[https://web.archive.org/web/20110918033517/http://podaac.jpl.nasa.gov/OceanWind NASA/JPL Physical Oceanography website]
*[http://www.knmi.nl/scatterometer ESA Scatterometer site]
*[http://www.scp.byu.edu Scatterometer Climate Record Pathfinder]
*[http://manati.star.nesdis.noaa.gov NOAA Weather site] {{Webarchive|url=https://web.archive.org/web/20140811165841/http://manati.star.nesdis.noaa.gov/ |date=2014-08-11 }}

{{Jet Propulsion Laboratory}}

[[Category:Satellite meteorology]]
[[Category:Metrology]]