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A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves Scattering 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 (σ0, "sigma zero" or "sigma naught") of a surface. They are often mounted on weather satellites to find wind speed and direction, and are used in industries to analyze the roughness of surfaces.
These devices are found in automatic weather stations 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 fog for instance). This is not always the case (e.g. patchy fog).
The primary application of spaceborne scatterometry has been measurements of near-surface over the ocean. Such instruments are known as wind scatterometers. By combining sigma-0 measurements from different azimuth angles, the near-surface wind Euclidean vector over the ocean's surface can be determined using a Geophysics 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.
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.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. Scatterometer backscatter data are applied to the study of vegetation, Water content, polar ice, tracking Antarctic icebergsK.M. Stuart and D.G. Long, Tracking large tabular icebergs using the SeaWinds Ku-band microwave scatterometer, Deep-Sea Research Part II, , Vol. 58, pp. 1285–1300, 2011. and global change.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. 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, ESA, and NASDA. The first operational wind scatterometer was known as the SEASAT Scatterometer (SASS) and was launched in 1978.W.L. Grantham, et al., The SeaSat-A Satellite Scatterometer, IEEE Journal of Oceanic Engineering, Vol. OE-2, pp 200–206, 1977. It was a fan-beam system operating at Ku-band (14 GHz). In 1991 ESA launched the European Remote-Sensing Satellite ERS-1 Advanced Microwave Instrument (AMI) scatterometer,E. Attema, The Active Microwave Instrument Onboard the ERS-1 Satellite, Proceedings of the IEEE, 79, 6, pp. 791–799, 1991. followed by the ERS-2 AMI scatterometer in 1995. Both AMI fan-beam systems operated at C-band (5.6 GHz). In 1996 NASA launched the NASA Scatterometer (NSCAT), on board the NASDA ADEOS I satellite, a Ku-band fan-beam system.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. 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. ESA and EUMETSAT launched the first C-band ASCAT in 2006 onboard Metop-A.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. The Cyclone Global Navigation Satellite System (CYGNSS), launched in 2016, is a constellation of eight small satellites utilizing a Bistatic radar approach by analyzing the reflection from the Earth's surface of Global Positioning System (GPS) signals, rather than using an onboard radar transmitter.
A work, published by the 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.
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