It is analogous to conventional SAR, except that ISAR technology uses the movement of the target rather than the emitter to create the synthetic aperture.
ISAR images are often produced by rotating the target and processing the resultant Doppler histories of the scattering centers.
The rotation will result in the generation of cross range dependent Doppler frequencies which can be sorted spatially by a Fourier transform.
If the target is rotated through large angles, the Doppler frequency history of a scatterer becomes non-linear, following a sine wave trajectory.
For instance a feature which extends far over the surface of a ship such as a mast will provide a high sinusoidal response which is clearly identifiable in a two dimensional image.
ISAR for maritime surveillance was pioneered by Texas Instruments in collaboration with the Naval Research Laboratory and became an important capability of the P-3 Orion and the S-3B Viking US Navy aircraft.
This has been cited as akin to using a telescope in Los Angeles that is the size of the human eye's lens to image a car in New York.
In this case, the radar uses digital terrestrial television signals as the non-cooperative source of illumination in the environment.