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
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.