Scatterometer

Source: Wikipedia, the free encyclopedia.

A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves

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.

Optical

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 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).

Radar

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. 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

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.

icebergs[3] and global change.[4]
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

Metop-A.[8] The Cyclone Global Navigation Satellite System (CYGNSS), launched in 2016, is a constellation of eight small satellites utilizing a 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 anisotropic (direction dependent) long distance dispersion by wind to explain the strong floristic affinities between landmasses.

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.

Semiconductor and precision manufacturing

Scatterometers are widely used in metrology for roughness of polished and lapped surfaces in semiconductor and precision machining industries.[9] They provide a fast and non-contact alternative to traditional stylus methods for topography assessment.[10][11] Scatterometers are compatible with vacuum environments, are not sensitive to vibration, and can be readily integrated with surface processing and other metrology tools.[12][13]

Uses

Illustration of the ISS-RapidScat location on the International Space Station

Examples of use on Earth observation satellites or installed instruments, and dates of operation:[14]

  • 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 CYGNSS constellation (launched 2016)

References

  1. ^ .
  2. ^ 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.
  3. , Vol. 58, pp. 1285–1300, 2011.
  4. ^ 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.
  5. ^ W.L. Grantham, et al., The SeaSat-A Satellite Scatterometer, IEEE Journal of Oceanic Engineering, Vol. OE-2, pp 200–206, 1977.
  6. ^ E. Attema, The Active Microwave Instrument Onboard the ERS-1 Satellite, Proceedings of the IEEE, 79, 6, pp. 791–799, 1991.
  7. ^ 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.
  8. ^ 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.
  9. ^ John C. Stover. SPIE Optical Engineering Press, 1995 – Science – 321 pages.
  10. ^ Myer, G, et al (1988) "Novel Optical Approach to Atomic Force Microscopy", Applied Physics Letters, 53, 1045–1047
  11. ^ Baumeister, Theodore, et al. (1967) Standard Handbook for Mechanical Engineers. McGraw-Hill, LCCN 16-12915
  12. ^ John M. Guerra. "A Practical Total Integrated Scatterometer", Proc. SPIE 1009, Surface Measurement and Characterization, 146 (March 21, 1989)
  13. ^ "Roughness via Scatterometry". ZebraOptical. Retrieved 30 December 2016.
  14. ^ "Scatterometry & Ocean Vector Winds: Satellite Studies". Florida State University. Retrieved 30 December 2016.

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