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Instrument Description: Spaceborne Scatterometers

Sensor or Instrument Description

Spaceborne scatterometers are satellite instruments that were originally designed to map wind speed and direction over the oceans, but they also measure various land and ice variables. Scatterometers actively transmit electromagnetic pulses to the Earth's surface and measure the backscatter response, or the power of the return pulse scattered back to the antenna. Researchers can derive various geophysical variables from the backscatter response; wind speed and direction are not measured directly, but are inferred from backscatter responses. Applications include studies of tropospheric dynamics and air-sea momentum fluxes (Naderi, Freilich, and Long 1991).

Scatterometers measure surface properties with relatively coarse spatial resolutions (comparable to those from passive microwave radiometers) since they must average pulses received over a wide area to accurately measure the return amplitude. While this does not allow detailed analyses of surfaces, it does have the advantage of covering a larger portion of the earth on a more frequent basis than synthetic aperture radar (SAR). This coverage is desirable for monitoring synoptic-scale phenomena such as global ocean winds and snow cover, continental ice sheets, and polar sea ice extent.

To date, four spaceborne scatterometers have been deployed. All were originally designed to measure ocean winds:

	SASS: Seasat-A Satellite Scatterometer System SASS was launched on board NASA's Seasat-A satellite on 27 June 1978. The mission lasted until 10 October 1978 when a satellite power failure terminated the mission. However, the mission did provide a baseline for studies of global change.
	ESCAT: ERS Scatterometer ESCAT is the name given to the Active Microwave Instrument (AMI) on board the European Space Agency's (ESA) Earth Remote Sensing (ERS)-1 and -2 satellites when it is in scatterometer mode. ERS-1 launched on 17 July 1991 and ERS-2 launched on 21 April 1995.
	NSCAT: NASA Scatterometer The NSCAT instrument flew on the Japanese Aerospace Exploration (JAXA) Advanced Earth Observing Satellite-I (ADEOS-I) that was launched 16 August 1996. On 30 June 1997 a power failure terminated the mission. NSCAT's primary science objective was to measure surface wind speed and direction over the global oceans, with a requirement to provide coverage every two days under all weather and cloud conditions.
	SeaWinds The SeaWinds scatterometer flies on NASA's Quick Scatterometer (QuikSCAT) satellite and was launched on 19 June 1999. A similar scatterometer, SeaWinds-II, flew on JAXA's ADEOS-II satellite from 14 December 2002 to 24 October 2003 when a power failure terminated the mission. SeaWind's primary science objective is to acquire high-resolution, continuous, all-weather measurements of near-surface vector winds over the ice-free global oceans (Kramer 1994).

Table 1 lists the operating characteristics of these scatterometers.

The advanced scatterometer (ASCAT) on the European Space Agency's meteorological operational (MetOp) platforms will be the follow-on to European wind scatterometers. The first mission (MetOp-1) is planned for 2005, MetOp-2 is planned for 2007, and MetOp is planned for 2012. The MetOp orbit is planned to be a sun-synchronous, polar, 29-day, repeat-cycle orbit with an ascending node at 21:30 local time and a minimum altitude of 822 km. The ASCAT mission was primarily designed to provide operational global ocean wind vectors (Figa-Saldaña et al. 2002).

Key Variables

Scatterometers measure geophysical variables related to the earth's water cycle, including: precipitation rate, cloud water, water vapor, sea surface winds, sea surface temperature, sea ice concentration, snow water equivalent, and soil moisture. Scatterometers are unique in their ability to determine wind direction over water (COAPS 2004).

Principles of Operation

A scatterometer is an incoherent surface-based radar that measures reflectivity over a set of different incident angles. Scatterometers average the detected returns from a sequence of pulses, a process known as post-detection integration (Henderson and Lewis 1998). Averaging ensures more accurate measurements of the backscattering coefficient, since single return pulses are typically noisy. Because pulses from multiple ground targets interfere with one another, the return signal is distorted: a process referred to as fading. Averaging together a sufficient number of return pulses has the effect of canceling out the noise caused by fading, often achieving ± 0.10 to 0.15 dB accuracy. The downside to averaging together multiple return pulses is a significant reduction in spatial resolution: 25 to 50 km, compared to 1 to 10 km with SAR.

The intensity of the backscattered signal depends on the roughness and the dielectric properties of the target. Changes in wind velocity cause changes in ocean surface roughness, modifying the radar cross section of the ocean and the magnitude of the backscattered power. Multiple collated measurements acquired from several directions can be used to solve wind speed and direction simultaneously (Kramer 1994).

For ice and snow, the backscatter is influenced by surface roughness (including orientation of the surface scatterers), liquid water content, snow grain size, brine concentration in sea ice, and density. Scatterometers can be calibrated to less than a few tenths of a decibel (dB); thus, seasonal and interannual differences of only 1 to 2 dB can be accurately monitored using scatterometry data.

Visit the following for further details on scatterometer designs:

• Physical Oceanography Distributed Active Archive Center (PO.DAAC)
• European Space Agency (ESA)

Sensor or Instrument Measurement Geometry

Scatterometers use different techniques to collect measurements at multiple incident and azimuth angles. The SASS, ESCAT, and NSCAT instruments used a fan-beam approach, where the scatterometer fans out, or points, several fixed-angle beams of radar pulses at the ground simultaneously. Another approach called the pencil-beam method, which is used by SeaWinds, is to rotate a single beam of pulses at multiple angles. The viewing geometry of each scatterometer is illustrated below:

SASS viewing geometry. Image courtesy of Long, Hardin, and Whiting (1993).

ESCAT viewing geometry. Image courtesy of Long and Drinkwater (2000).

NSCAT viewing geometry. Image courtesy of Long and Drinkwater (2000).

SeaWinds viewing geometry. Image courtesy of Spencer, Wu, and Long (2000).

Manufacturer of Sensor or Instrument

SASS: Lockheed and Ball Space Systems
ESCAT: European Space Agency
NSCAT and SeaWinds: NASA Jet Propulsion Laboratory (JPL)

Accuracy Budget

SASS, ESCAT, NSCAT, and SeaWinds measured wind speed with an accuracy of of ± 2 m/s and wind direction with an accuracy of ± 20°. See COAPS (2004) for a list of publications that address the accuracy and validation of NSCAT and SeaWinds measurements.

List of Acronyms

Table 2 lists acronyms and abbreviations used in this document.

References

See Scatterometer Data at NSIDC for a list of scatterometer publications. The following references were used in this document:

Kramer, H.J. 1994. Observation of the Earth and its environment. New York: Springer-Verlag.

Center for Ocean-Atmosphere Prediction Studies (COAPS). COAPS Scatterometry. 2004.
http://www.coaps.fsu.edu/scatterometry/.
Accessed December 2004.

Drinkwater, M.R., and C.C. Lin. 2000. Introduction to the special section on emerging scatterometer applications. IEEE Transactions on Geoscience and Remote Sensing 38(4): 1763-1764.

European Space Agency. "Scatterometer design." Earthnet Online. 2004.
http://earth.esa.int/ers/instruments/.
Accessed December 2004.

Figa-Saldaña, J., J.J.W. Wilson, E. Attema, R. Gelsthorpe, M.R. Drinkwater, and A. Stoffelen. 2002. The advanced scatterometer (ASCAT) on the meteorological operational (MetOp) platform: A follow on for European wind scatterometers. Canadian Journal of Remote Sensing 28(3): 404-412.

Henderson, F.M., and A.J. Lewis, eds. 1998. Principles & applications of imaging radar. Manual of Remote Sensing Third Edition, Volume 2. New York: John Wiley & Sons, Inc.

Long, D.G., and M.R. Drinkwater. 1999. Cryosphere applications of NSCAT data. IEEE Transactions on Geoscience and Remote Sensing 37(3): 1671-1684.

Long, D.G., M.R. Drinkwater, B. Holt, S. Saatchi, and C. Bertoia. 2001. Global ice and land climate studies using scatterometer image data. EOS, Transaction of the American Geophysical Union 82(43): 503. Also available online at http://www.agu.org/eos_elec/010126e.html. Paper and electronic versions are available in a PDF file (4 MB).

NASA Jet Propulsion Laboratory. "Missions - NSCAT." Winds: measuring ocean winds from space. 2004.
http://winds.jpl.nasa.gov/missions/nscat/index.cfm.
Accessed December 2004.

NASA Jet Propulsion Laboratory. "Missions - SeaWinds on QuikSCAT." Winds: measuring ocean winds from space. 2004.
http://winds.jpl.nasa.gov/missions/quikscat/index.cfm.
Accessed December 2004.

NASA Jet Propulsion Laboratory. "Missions - SeaWinds on ADEOS II." Winds: measuring ocean winds from space. 2004.
http://winds.jpl.nasa.gov/missions/seawinds/index.cfm.
Accessed December 2004.

Naderi, F., M.H. Freilich, and D.G. Long. 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. Invited paper.

Long, D.G., and M.R. Drinkwater. 2000. Azimuth variation in microwave scatterometer and radiometer data over Antarctica. IEEE Transactions on Geoscience and Remote Sensing 38(4): 1857-1870.

Long, D.G., P. Hardin, and P. Whiting. 1993. Resolution enhancement of spaceborne scatterometer data. IEEE Transactions on Geoscience and Remote Sensing 31(3): 700-715.

Spencer, M.W., C. Wu, and D.G. Long. 2000. Improved resolution backscatter measurements with the SeaWinds pencil-beam scatterometer. IEEE Transactions on Geoscience and Remote Sensing 38(1): 89-104.

Document Creation Date

January 2005

Document Revision Date

February 2008
May 2005

Document URL

http://nsidc.org/data/docs/daac/scatterometer_instrument.gd.html