This page discusses
a few of the applications for synthetic aperture radar. These applications
increase almost daily as new technologies and innovative ideas are developed.
While SAR is often used because of its all-weather, day-or-night capability,
it also finds application because it renders a different view of a "target,"
with synthetic aperture radar being at a much lower electromagnetic
frequency than optical sensors.
Resolutions are for original images prior to downsampling for world
wide web viewing.)
Reconnaissance, Surveillance, and
for synthetic aperture radar are for reconnaissance, surveillance, and
targeting. These applications are driven by the military's need for
all-weather, day-and-night imaging sensors. SAR can provide sufficiently
high resolution to distinguish terrain features and to recognize and
identify selected man made targets. (Example: SAR
image of M-47 Tanks, 1-ft resolution) and an optical
photo of the same tanks)
Treaty Verification and Nonproliferation.
The ability to
monitor other nations for treaty compliance and for the nonproliferation
of nuclear, chemical, and biological weapons is increasingly critical.
Often, monitoring is possible only at specific times, when overflights
are allowed, or it is necessary to maintain a monitoring capability
in inclement weather or at night, to ensure an adversary is not using
these conditions to hide an activity. SAR provides the all-weather capability
and complements information available from other airborne sensors, such
as optical or thermal-infrared sensors.
Interferometry (3-D SAR).
synthetic aperture radar (IFSAR) data can be acquired using two antennas
on one aircraft or by flying two slightly offset passes of an aircraft
with a single antenna. (Example:
Interferometric SAR image created by two imaging passes [two synthetic
apertures]) Interferometric SAR can be used to generate very accurate
surface profile maps of the terrain.
Sandia has developed
new mathematical techniques for relating the radar reflection from the
terrain surface to the time delay between radar signals received at
the two antenna locations. The techniques are directed at removing ambiguities
in estimates of surface heights and are referred to as 2-D least squares
Navigation and Guidance.
radar provides the capability for all-weather, autonomous navigation
and guidance. By forming SAR reflectivity images of the terrain and
then "correlating" the SAR image with a stored reference (obtained from
optical photography or a previous SAR image), a navigation update can
be obtained. Position accuracies of less than a SAR resolution cell
can be obtained. SAR may also be used to guidance applications by pointing
or "squinting" the antenna beam in the direction of motion of the airborne
platform. In this manner, the SAR may image a target and guide a munition
with high precision.
Foliage and Ground Penetration.
radars offer the capability for penetrating materials which are optically
opaque, and thus not visible by optical or IR techniques. Low-frequency
SARs may be used under certain conditions to penetrate foliage and even
soil. This provides the capability for imaging targets normally hidden
by trees, brush, and other ground cover. To obtain adequate foliage
and soil penetration, SARs must operate at relatively low frequencies
(10's of MHz to 1 GHz).
have shown that SAR may provide a limited capability for imaging selected
underground targets, such as utility lines, arms caches, bunkers, mines,
etc. Depth of penetration varies with soil conditions (moisture content,
conductivity, etc.) and target size, but individual measurements have
shown the capability for detecting 55-gallon drums and power lines at
depths of several meters. In dry sand, penetration depths of 10's of
meters are possible.
Moving Target Indication.
The motion of a
ground-based moving target such as a car, truck, or military vehicle,
causes the radar signature of the moving target to shift outside of
the normal ground return of a radar image. Sandia has developed techniques
to automatically detect ground-based moving targets and to extract other
target information such as location, speed, size, and Radar Cross Section
(RCS) from these target signatures. Please view our MTI / CCD imagery library.
A technique known
as coherent change detection offers the capability for detecting changes
between imaging passes. (Example: Coherent
Change Detection image of vehicle tracks and an optical
photo of the same area) To detect whether or not a change has occurred,
two images are taken of the same scene, but at different times. These
images are then geometrically registered so that the same target pixels
in each image align. After the images are registered, they are cross
correlated pixel by pixel. Where a change has not occurred between the
imaging passes, the pixels remain correlated, whereas if a change has
occurred, the pixels are uncorrelated. Of course, targets that are not
fixed or rigid, such as trees blowing in the wind, will naturally decorrelate
and show as having "changed." While this technique is useful for detecting
change, it does not measure direction or the magnitude of change.
radar is used for a wide variety of environmental applications, such
as monitoring crop characteristics, deforestation, ice flows, and oil
spills. (Example: SAR image
of a naturally occurring oil seepage) Oil spills can often be detected
in SAR imagery because the oil changes the backscatter characteristics
of the ocean. Radar backscatter from the ocean results primarily from
capillary waves through what is known as Bragg scattering (constructive
interference from the capillary waves being close to the same wavelength
as the SAR). The presence of oil dampens the capillary waves, thereby
decreasing the radar backscatter. Thus, oil slicks appear dark in SAR
images relative to oil-free areas.
Top of page