region and requires focusing. The focusing is accomplished by applying a phase correction to the synthetic aperture. ISAR applications ISAR is used in maritime surveillance for the classification of ships and other objects. In these applications the motion of the object due to wave action often plays a greater role than object rotation. 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. Images sometimes produce an uncanny similarity to a visual profile with the interesting effect that as the object rocks towards or away from the receiver the alternating doppler returns cause the profile to cycle between upright and inverted. 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. Research has been done also with land based ISAR. The difficulty in using this capability is that the object motion is far less in magnitude and usually less periodic than in the maritime case. Perhaps the most visually striking and scientifically compelling application of ISAR is in the deep space imaging of asteroids. A particularly beautiful example of this is the so-called "dog's bone" 216 Kleopatra asteroid, which lies roughly 20% further away from the earth than the sun. The asteroid is only 60 miles wide at its midpoint. Yet the imagery is crisp and looks like an optical image. This has been cited as akin to using a Los Angeles telescope the size of the human eye's lens to image a car in New York. Of course, the trick here is that the asteroid is presented among a very sparse background, allowing for substantial disambiguation. In February 2013, Indra Sistemas, a Spanish technology company, announced the first passive ISAR. A passive radar is characterised by not emitting any form of radiation, i.e., it uses the signals present in the environment. In this case, the radar uses digital terrestrial television signals as the non-cooperative source of illumination in the environment. Errors in ISAR Errors in the ISAR imaging process generally result in defocusing and geometry errors in the image. ISAR transform errors include: Unknown target or antenna motion: Unmodeled motion will cause the target image to defocus and be at an incorrect location. This error