First Results on VHF-band SAR Imaging using Circular Tracks

Abstract:
Low frequency synthetic-aperture radar (SAR) in combination with change detection techniques can be used to search for stationary ground targets. In particular, the method has shown a great potential when vehicle-size objects are obscured by foliage. However, when only one single SAR image can be expected for the analysis another approach must be taken to improve the target-to-clutter ratio. One possibility is to increase the aperture angle during the radar data acquisition to make sure that the strong cardinal responses of a typical vehicle are registered. This implies an angle of at least 180deg to include the strongest (broadside) response when the target orientations are unknown. Tests on this have been conducted with the airborne VHF SAR system CARABAS-II using the maximum angle 360deg, i.e. a full circle. Several terrain vehicles were positioned in a forest parcel located within the area circled by the platform. A first look on this data set indicates a significantly brighter response from the hidden targets compared to the corresponding images generated from the linear tracks made during the same flight mission. The time domain signal processing for a circular track becomes more complex and detailed knowledge of the imaging geometry is necessary, i.e. both accurate navigation data as well as a dense digital elevation model (DEM) of the illuminated ground segment.