Synthetic aperture radars differ from other radars by transmitting radar energy sideways along the flight path and reassembling the echoes using doppler interferometry to give the effect of a longer antenna than it's practical for a plane or spacecraft to carry. The distance flown creating the synthetic antenna is known as the synthetic aperture, and can be several hundred meters long, which yields finer resolution than possible from a smaller antenna. Sometimes called range-doppler interferometry.

High-res pictures through inclement weather are not all SAR is good for. The data can be used to produce 3-D views of natural and man-made terrain. It's also possible to tune the radars to detect subsurface features, which can be particularly useful in oceanography and petroleum exploration.

It takes advanced signal processing to continuously adjust the radar's range component for the continuously changing position of the aircraft. The azimuthal component is not really parallel to the flight path either but is actually radial to the instantaneous position of the antenna. This makes SARs an expensive tool. Gear this sophisticated only recently became available outside the military and space community.

There is a related technique called inverse synthetic aperture radar which depends on target motion to provide the required doppler shift. Look for it on an anti-aircraft installation near you soon.

Can anyone comment on possible maximum resolutions of SAR? Seems to me spacecraft ought to be able to make apertures that are *miles* long…

Update 19August01:

Richard Branson is testing an airship-deployed ultra-wide-band SAR for mine clearing. Developed by the UK Defence Evaluation and Research Agency, the UWB SAR penetrates vegetation and surface soil and can detect plastic mines as small as 10 cm, scanning the terrain at 100 m2 per second.

SAR and inverse SAR (or ISAR) are really the same concept and are often used interchangeably. For the pedantic engineer, SAR is typically defined where the antenna aperture moves, and the target structure is stationary, i.e. the SAR instrumentation is installed on an aircraft and SAR images of the ground are made. ISAR is when the antenna is stationary and the target moves, such as a flying aircraft, or missile, or a target is mounted on a pedestal in a radar range and rotated to obtain the SAR data.

Both techniques use essentially the same processing concepts in that they exploit the doppler phase history of the various scattering centers of the target as the relative angle between the aperture and the target changes. This angle is changed by moving either the target or the antenna. Typically the illuminating radar uses a frequency-stepped waveform to obtain multiple-frequency data. Using this multi-frequency data over multiple incident angles, the fast fourier transform is used to obtain the scattering intensities of the target scene in the relative down-range and cross-range directions.

SAR imagery is well-known for extremely high-resolution maps as radar energy can easily penetrate cloud cover (which is one way the planet Venus was mapped). SAR is also useful in automatic target recognition systems, where a database of SAR images is made of a class of targets, and discrimination features calculated for an unknown object and compared to the training set.

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