The Defense Advanced Research Projects Agency (DARPA) is developing a Video Synthetic Aperture Radar (ViSAR) system designed to provide a targeting capability for the AC-130 gunship in conditions where the current electro-optic systems will not perform. By using radar, the gunship's availability rises from 35% to 72%, as clouds currently obscure the EO/IR camera's view of the ground. Several technical issues must be addressed in the program in order to be successful. In order to achieve frame rates fast to track maneuvering targets, the radar must operate at frequencies over 170 which requires the development of new electronics. Secondly, as targets move in the FOV of a Synthetic Aperture Radar (SAR) their apparent position is translated in the generated imagery. Thirdly, as the imagery generated is range versus azimuth rather than elevation versus azimuth, tall objects appear to be "laid over" unless corrections are made for the true height of the object imaged. This paper will describe the DARPA program striving to overcome these issues and review the approaches be taken to achieve the imagery required for the close air support mission.
