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Wind Scatterometry Background

Wind scatterometers are designed to measure the normalized radar cross-section of the ocean surface from which the near-surface wind can be estimated. It measures the normalized cross-section by comparing the ratio of the the power transmitted to the echo power received. The observed radar cross-section is a function of the surface roughness which is created primarily by wind-generated waves. Thus wind speed and direction over the ocean can be inferred from the radar backscatter returns.

An accurate measurement of ocean winds provides highly useful information for meteorologists and climatologists. Historically, the spatial sampling of winds over the ocean has been very sparse. Spaceborne scatterometers provide a source for global wind measurement not previously possible. Because scatterometers are microwave sensors, their signal is not affected by darkness or cloud cover.

The first scatterometer to fly was the Ku-band S-59 experiment on Skylab. The SeaSat scatterometer (SASS) was also Ku-band and flew for three months in 1978. It demonstrated the effectiveness of scatterometry for wind retrieval. The European Space Agency (ESA) launched the first European Remote Sensing (ERS-1) sattelite in 1990. It carried a C-band scatterometer. The second ERS-2 was launched in 1995. The Ku-band NASA Scatterometer (NSCAT) was launched in 1996 as a U.S. follow-on to SASS. It operated for nine months before the spacecraft power system failed. The Ku-band SeaWinds scatterometer was launched on QuikScat in the summer of 1999. Unlike the previous fan-beam scatterometers, SeaWinds is a dual-scanning pencil-beam scatterometer.

While providing significant advantages over traditional wind measurement methods, scatterometric wind retrieval is not without its problems. In the presence of rain, for example, wind estimation is degraded. Also, backscatter returns do not provide a unique estimate of the wind vector, which results in ambiguous estimates. Research in the MERS laboratory aims toward improving estimation performances in the face of such problems.

A good introduction to wind scatterometry is contained in the paper: F. Naderi, M. H. Freilich, and D. G. Long, "Spaceborne Radar Measurement of Wind Velocity Over the Ocean-An Overview of the NSCAT Scatterometer System", Proceedings of the IEEE, pp. 850-866, Vol. 79, No. 6, June 1991.

Seawinds on QuikScat

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