Scatterometers
Description
A scatterometer transmits radar pulses and
receives backscattered energy, the intensity
of which depends on the roughness and
dielectric properties of a particular
target. Scatterometers were originally
designed to measure oceanic surface winds,
where the amount of backscatter depends on
two factors – the size of the surface
ripples on the ocean, and their orientation
with respect to the propagation direction of
the pulse of radiation transmitted by the
scatterometer. The first is dependent on
wind stress, and hence wind speed at the
surface, while the second is related to wind
direction. As a result, measurements by
scatterometers may be used to derive both
wind speed and direction.
The main aim of these instruments is to
achieve high-accuracy measurements of wind
vectors (speed and direction), so resolution
is of secondary importance. (They generally
produce wind maps with a resolution of order
25–50 km). Because scatterometers operate at
microwave wavelengths, the measurements are
available irrespective of weather
conditions.
Spaceborne scatterometers have provided
continuous synoptic microwave coverage of
Earth for nearly two decades, starting with
the ERS series in 1991, NSCAT on ADEOS,
SeaWinds on QuikSCAT, and more recently
ASCAT on MetOp. The ERS and NSCAT
instruments employed a fan-beam
(multi-incidence) wind retrieval technique,
whereas QuikSCAT employed a conically
scanning (fixed incidence) technique.
Increases in swath width capability now mean
that a single instrument can provide around
90% coverage of global oceans on a daily
basis. The CEOS Ocean Surface Vector Winds
Virtual Constellation team is charged with
overseeing coordination to ensure adequate
continuity of scatterometer measurements by
different nations.
Applications
Information from scatterometers provides a
unique source of data on sea surface wind
speed and direction. This has important
applications in weather and wave
forecasting, the investigation of climate
models and elaboration of marine wind
climate. The assimilation of scatterometer
data into atmospheric forecasting models
greatly improves the description of cyclonic
features, which are so important in
predicting future weather patterns.
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Current & planned instruments
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Advanced Scatterometer
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Scatterometer (Meteor)
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Aquarius L-band Scatterometer
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Scatterometer (Oceansat)
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ASCAT
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SeaWinds
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SCAT
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WindRAD
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A large number of new, unforeseen, terrestrial
and sea-ice applications have emerged beyond
the original ocean winds mission of
scatterometers. These include: the measurement
of sea-ice extent and concentration; soil
moisture; snow accumulation; and regional
monitoring of ice shelves, rainforests and
deserts. The daily global coverage of
scatterometers in the polar regions and their
ability to discriminate sea ice, ice sheets
and icebergs, despite poor solar illumination
and frequent cloud cover, make them excellent
instruments for large-scale systematic
observations of polar ice.
ASCAT wind measurements south of Australia
on 19 March 2008. Superimposed is a 10 m
wind field forecast from the European Centre
for Medium-Range Weather Forecasts. Darker
colours correspond to areas of low surface
wind speeds. Colour tone changes correspond
to changes in the surface wind direction, as
the sensitivity of the measurement from the
different antenna beams varies with the
relative direction of the surface wind
vector with respect to the viewing direction
of each beam.
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