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.

(Note: Resolutions are for original images prior to downsampling for world wide web viewing.)

Reconnaissance, Surveillance, and Targeting.

Many applications 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).

Interferometric 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 phase unwrapping.

Navigation and Guidance.

Synthetic aperture 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.

Synthetic aperture 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).

Recent studies 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.

Change Detection.

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.

Environmental Monitoring.

Synthetic aperture 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.

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