Imaging Microwave Radars
Description
These instruments transmit at frequencies of
around 1 GHz to 10 GHz and measure the
backscattered signals to generate microwave
images of Earth’s surface at high spatial
resolutions (between 10 m and 100 m), with a
swath width of 100–500 km. Both synthetic
aperture radars and real aperture
side-looking imaging radar systems fall into
this category. The images produced have a
similar resolution to those from
high-resolution optical imagers, but radars
have the capability to ‘see’ through clouds,
providing data on an all-weather, day/night
basis.
SARs also have the ability to penetrate
vegetation and to sample surface roughness
and surface dielectric properties. They may
also be used to obtain polarisation
information. Although the operating
wavelength is generally fixed for a given
radar, radars operating at a variety of
wavelengths (typically L-, C- and X-band)
will be increasingly available during the
next decade.
The beam shape and direction of
new-generation SARs enable imagery to be
acquired more frequently from many points on
Earth. Multipolarised SARs (such as ASAR on
Envisat) enable land cover to be classified
more accurately and will soon provide
improved data on biophysical parameters such
as soil moisture and biomass.
A number of bistatic radar system concepts
are under study. A bistatic radar is a
system that operates with separated
transmitting and receiving antennas. A
number of large active radar missions are
foreseen for the coming decade, providing an
opportunity to fly relatively small
satellite missions with passive payloads in
formation with one of these missions in
order to gather the backscatter
information.
Applications
Although a variety of backscatter
measurements may be taken by imaging radars,
interpretation of these measurements is a
complex science that, in some respects, is
still developing. However, significant
advances have been made in a number of areas
and some SAR applications are now fully
operational.
Backscatter from the ocean can be used to
deduce surface waves, to detect and analyse
surface features such as ocean fronts,
eddies and oil slicks, and to detect and
track ships from their wakes. Operational
wave and sea ice forecasting is also an
important near-real time application of SAR
data.
Since land images may be used to infer
information on vegetation type and cover,
they are of use in forestry and agriculture.
The ability of SARs to penetrate cloud cover
makes them particularly valuable in
rainforest studies and resource monitoring
applications. The information obtained from
such images depends upon the characteristics
(e.g. wavelength) of the probing radiation.
Under certain conditions, for example, some
penetration of vegetation may be feasible.
Such imagery is often used in order to
complement visible/IR multi-spectral imagery
by, in effect, providing an additional
microwave channel.
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Current & planned instruments
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Advanced SAR
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SAR (RADARSAT-2)
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Arkon-2M SAR
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SAR (RCM)
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CSG SAR
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SAR (RISAT)
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C-band SAR
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SAR 2000
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COSI
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SAR-L
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Ku- and X-band radars (SCLP)
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SAR-X
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L-band SAR
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S-band SAR
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L-band RADAR (SMAP)
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Severjanin
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PALSAR-2 (ALOS-2)
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WSAR
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Paz SAR-X
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X-band SAR
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SAR
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One of the most important current applications
of imaging radars is in all-weather
measurements of snow and ice sheets, from
which information on topography and texture
may be inferred. Flood detection is another
proven capability of SAR.
A technique known as interferometry is used to
record the phase shift between two SAR images
recorded at slightly different times and
viewing angles. This provides accurate
information on the motion of surfaces and
targets such as sea ice and ice sheets, and
allows large-scale 3D topographical images to
be produced. Similar stereo images may be
produced using conventional SAR images taken
on adjacent orbits. Since differential SAR
interferometry can detect ground movements at
millimetre/sub-millimetre level, it is of
interest in the context of tectonic and
volcanic hazard studies, and in studies of
subsidence in urban areas.
ERS-2/Envisat interferogram of a
fast-moving (>1 m/h) glacier in
Greenland.
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