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.