Bistatic SAR for urban environments 2006-2008

The research project "Bistatic SAR for urban environments" has been conducted at FOI during the period 2006-2008. This report summarises the most important activities and results. The motivation of the project is that the Swedish Armed Forces have a limited capability to find, identify and position ground targets. The need for such a capability is expected to increase in the future, in particular in support of international operations. The objective of the project is to develop and evaluate airborne radar with a capability to detect and position concealed targets over wide areas and in difficult environments, such as urban environments and forested areas. SAR ("synthetic aperture radar") enables this capability for vehicle targets in the open independently of weather conditions and daylight. Past research at FOI has developed novel radar systems operating in the VHF and UHF bands, CARABAS-II and LORA, which also have this capability for concealed targets but require several overpasses since the detections are obtained using change detection. The present project has the aim to develop similar capabilities but without change detection and has focused on two cases: ? Remote mapping of the interior of buildings, i.e. floors, walls, ceilings etc. The main problem is to obtain sufficient penetration and then generate the three-dimensional structure from the received signal. Studies of wave propagation through different building materials shows that the most difficult materials are metal and reinforced concrete. Results show, however, that it is feasible to penetrate the latter by choosing a suitable radar frequency in the band 300 MHz - 3 GHz. Conventional SAR provides imaging in two dimensions and the extension of the technique to three dimensions (3D SAR) has therefore been studied and shown to be promising but needs further improvement. The challenge of imaging in three dimensions remains. ? Detection of concealed targets in dense forest. Past research has shown that the frequency band 20-500 MHz is suitable for this applications and provides both adequate foliage penetration and spatial resolution. However, the forest clutter severely limits detection performance if change detection is not used. Bistatic SAR, i.e. with geometrically separated transmitter and receiver, is proposed to reduce clutter levels. The project has therefore developed the required techniques for system synchronisation based on GPS signals. The first experiments with bistatic SAR have been conducted at VHF-band and the results show a significant clutter reduction by choosing suitable geometries.