Scatterometer

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

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Optical

Airport scatterometer (or diffusometer). Dohlednostmereni.jpg
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 Scattometer principle.GIF
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 spaceborne scatterometry has been measurements of near-surface winds over the ocean. [1] Such instruments are known as wind scatterometers. By combining sigma-0 measurements from different azimuth angles, the near-surface wind vector over the ocean's surface can be determined using a 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.

A snapshot of Typhoon Soulik while at Category 4 intensity captured by Eumetsat's ASCAT (Advanced Scatterometer) instrument on board the Metop-A satellite Soulik 2013 July 10 Scatterometer Ascending Pass.png
A snapshot of Typhoon Soulik while at Category 4 intensity captured by Eumetsat's ASCAT (Advanced Scatterometer) instrument on board the Metop-A satellite

Scatterometer wind measurements are used for air-sea interaction, climate studies and are particularly useful for monitoring hurricanes. [2] Scatterometer backscatter data are applied to the study of vegetation, soil moisture, polar ice, tracking Antarctic 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 NASA, ESA, and NASDA. The first operational wind scatterometer was known as the Seasat Scatterometer (SASS) and was launched in 1978. [5] 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, [6] 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, [1] a Ku-band fan-beam system. [7] 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. [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 Illustration of ISS-RapidScat on ISS (20160909).jpg
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]

Related Research Articles

<span class="mw-page-title-main">European Organisation for the Exploitation of Meteorological Satellites</span> European intergovernmental organisation

The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) is an intergovernmental organisation created through an international convention agreed by a current total of 30 European Member States.

<span class="mw-page-title-main">European Remote-Sensing Satellite</span> European Space Agency Earth-observing satellite program

European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2.

<span class="mw-page-title-main">Space-based radar</span> Use of radar systems mounted on satellites

Space-based radar or spaceborne radar is a radar operating in outer space; orbiting radar is a radar in orbit and Earth orbiting radar is a radar in geocentric orbit. A number of Earth-observing satellites, such as RADARSAT, have employed synthetic aperture radar (SAR) to obtain terrain and land-cover information about the Earth.

<span class="mw-page-title-main">TOPEX/Poseidon</span> Satellite mission to map ocean surface topography

TOPEX/Poseidon was a joint satellite altimeter mission between NASA, the U.S. space agency; and CNES, the French space agency, to map ocean surface topography. Launched on August 10, 1992, it was the first major oceanographic research satellite. TOPEX/Poseidon helped revolutionize oceanography by providing data previously impossible to obtain. Oceanographer Walter Munk described TOPEX/Poseidon as "the most successful ocean experiment of all time." A malfunction ended normal satellite operations in January 2006.

<span class="mw-page-title-main">Seasat</span>

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 satellite monitoring of oceanographic phenomena and to help determine the requirements for an operational ocean remote sensing satellite system. Specific objectives were to collect data on sea-surface winds, sea-surface temperatures, wave heights, internal waves, atmospheric water, sea ice features and ocean topography. Seasat was managed by NASA's Jet Propulsion Laboratory and was launched on 27 June 1978 into a nearly circular 800 km (500 mi) orbit with an inclination of 108°. Seasat operated until 10 October 1978 (UTC), when a massive short circuit in the Agena-D bus electrical system ended the mission.

<span class="mw-page-title-main">ADM-Aeolus</span>

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<span class="mw-page-title-main">QuikSCAT</span> Earth observation satellite

The NASA QuikSCAT was an Earth observation satellite carrying the SeaWinds scatterometer. Its primary mission was to measure the surface wind speed and direct over the ice-free global oceans via its effect on water waves. Observations from QuikSCAT had a wide array of applications, and contributed to climatological studies, weather forecasting, meteorology, oceanographic research, marine safety, commercial fishing, tracking large icebergs, and studies of land and sea ice, among others. This SeaWinds scatterometer is referred to as the QuikSCAT scatterometer to distinguish it from the nearly identical SeaWinds scatterometer flown on the ADEOS-2 satellite.

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<span class="mw-page-title-main">Wave radar</span> Technology for measuring surface waves on water

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<span class="mw-page-title-main">Sentinel-3</span> Earth observation satellite series

Sentinel-3 is an Earth observation heavy satellite series developed by the European Space Agency as part of the Copernicus Programme. It currently consists of 2 satellites: Sentinel-3A and Sentinel-3B. After initial commissioning, each satellite was handed over to EUMETSAT for the routine operations phase of the mission. Two recurrent satellites— Sentinel-3C and Sentinel-3D— will follow in approximately 2024 and 2028 respectively to ensure continuity of the Sentinel-3 mission.

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<span class="mw-page-title-main">Oceansat-2</span> Indian Earth observation satellite

Oceansat-2 is the second Indian satellite built primarily for ocean applications. It was a part of the Indian Remote Sensing Programme satellite series. Oceansat-2 is an Indian satellite designed to provide service continuity for operational users of the Ocean Colour Monitor (OCM) instrument on Oceansat-1. It will also enhance the potential of applications in other areas. The OceanSat-2 mission was approved by the government of India on 16 July 2005.

Frank Wentz is the CEO and director of Remote Sensing Systems, a company he founded in 1974, which specializes in satellite microwave remote sensing research. Together with Carl Mears, he is best known for developing a satellite temperature record from MSU and AMSU. Intercomparison of this record with the earlier UAH satellite temperature record, developed by John Christy and Roy Spencer, revealed deficiencies in the earlier work; specifically, the warming trend in the RSS version is larger than the University of Alabama in Huntsville (UAH) one. From 1978 to 1982, Wentz was a member of NASA's SeaSat Experiment Team involved in the development of physically based retrieval methods for microwave scatterometers and radiometers. He has also investigated the effect of climate change on satellite-derived evaporation, precipitation and surface wind values. His findings are different from most climate change model predictions.

<span class="mw-page-title-main">ADEOS II</span> Japanese Earth observation satellite

ADEOS II was an Earth observation satellite (EOS) launched by NASDA, with contributions from NASA and CNES, in December 2002. and it was the successor to the 1996 mission ADEOS I. The mission ended in October 2003 after the satellite's solar panels failed.

<span class="mw-page-title-main">ADEOS I</span> Japanese Earth observation satellite

ADEOS I was an Earth observation satellite launched by NASDA in 1996. The mission's Japanese name, Midori means "green". The mission ended in July 1997 after the satellite sustained structural damage to the solar panel. Its successor, ADEOS II, was launched in 2002. Like the first mission, it ended after less than a year, also following solar panel malfunctions.

ScatSat-1 was a satellite providing weather forecasting, cyclone prediction, and tracking services to India. It has been developed by ISRO Satellite Centre, Bangalore whereas its payload was developed by Space Applications Centre, Ahmedabad. The satellite carries a Ku-band scatterometer similar to the Oceansat-2 which became dysfunctional after its life span of four-and-a-half years. India was dependent on NASA's ISS-RapidScat for prediction of cyclone forecasting and weather prediction. The data generated by this mini-satellite are used by National Aeronautics and Space Administration (NASA), European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and National Oceanic and Atmospheric Administration (NOAA).

<span class="mw-page-title-main">ISS-RapidScat</span>

ISS-RapidScat was an instrument mounted to the International Space Station'sColumbus module that measured wind speeds. It was launched aboard SpaceX CRS-4 in September 2014 and operated until August 2016. ISS-RapidScat was a scatterometer designed to support weather forecasting by bouncing microwaves off the ocean's surface to measure wind speed via wind waves. It featured a 75 cm (30 in) rotating radar dish that operated at 13.4 GHz. It could collect data between 51.6 degrees north and south latitude, with a swath 900 km wide (560 mi).

<span class="mw-page-title-main">Sentinel-6 Michael Freilich</span> Earth observation satellite

The Sentinel-6 Michael Freilich (S6MF) is a radar altimeter satellite developed in partnership between several European and American organizations. It is part of the Jason satellite series and is named after Michael Freilich. S6MF includes synthetic-aperture radar altimetry techniques to improve ocean topography measurements, in addition to rivers and lakes. The spacecraft entered service in mid 2021 and is expected to operate for 5.5 years.

Oceansat is a series of earth observation satellites built, launched, and operated by Indian Space Research Organisation, and dedicated to oceanography and atmospheric studies. Oceansat satellites facilitate a range of applications including documenting chlorophyll concentration, phytoplankton blooms, atmospheric aerosols and particulate matter as well as marine weather forecast to predict cyclones.

Michael H. Freilich was an American oceanographer who served as director of the NASA Earth Science division from 2006–2019.

References

  1. 1 2 F. Naderi; M. H. Freilich & D. G. Long (June 1991). "Spaceborne Radar Measurement of Wind Velocity Over the Ocean—An Overview of the NSCAT Scatterometer System". Proceedings of the IEEE. 79 (6): 850–866. doi:10.1109/5.90163.
  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. ↑ 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.
  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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