Published in IET Radar, Sonar and Navigation Received on 25th February 2009 Revised on 23rd October 2009 doi: 10.1049/iet-rsn.2009.0039 EuSAR Special Section ISSN 1751-8784 Signal-to-clutter ratio enhancement in bistatic very high frequency (VHF)-band SAR images of truck vehicles in forested and urban terrain L.M.H. Ulander 1 A. Barmettler 2 B. Flood 1 P.-O. Fro ¨ lind 1 A. Gustavsson 1 T. Jonsson 1 E. Meier 2 J. Rasmusson 1 G. Stenstro ¨ m 1 1 Division of Information Systems, Swedish Defence Research Agency (FOI), Linko ¨ping, Sweden 2 Department of Geography, University of Zurich, Zurich, Switzerland E-mail: ulander@foi.se Abstract: Multiple scattering is often a dominating scattering mechanism in VHF-band SAR, in particular in scenes including forested and urban terrain. Most important in this context is multiple scattering generated by double- or triple-bounce scattering from structures with orthogonal corners which often dominate for monostatic SAR geometries. Examples include double-bounce scattering from vertical tree stems or building walls on horizontal ground. In such situations, bistatic SAR may offer a significant advantage compared to monostatic SAR by choosing a suitable bistatic imaging geometry. We give experimental results which show increased signal-to-clutter ratio in bistatic VHF-based SAR (28–73 MHz). Results show that the signal-to-clutter ratio of a vehicle truck in a background of forest and urban clutter increases by up to 10 dB when the broadside bistatic elevation angle increases from 4 to 208. The results indicate that significant improvement in detection performance can be expected for vehicle-sized objects in forest and urban terrain by using bistatic geometries in VHF-band SAR. 1 Introduction Airborne ultra-wideband (UWB) synthetic-aperture radar (SAR) operating in the VHF band has emerged as a promising technique for wide-area surveillance of man- made objects. In particular, the CARABAS-II SAR using HH-polarisation and the 20–90 MHz band (high-end HF to low-end VHF) has demonstrated a unique standoff capability of detecting vehicle-sized objects in forest concealment [1]. The CARABAS-II antenna system consists of two bicones oriented in the along-track direction and located side-by-side. It produces HH- polarisation in the zero Doppler plane but includes a vertical component which slowly increases towards the beam edges. The spatial resolution is 2.5 m with a processed beamwidth of 608. The best detection performance has been obtained by using change detection, that is, background clutter suppression using multiple SAR images of the same scene with changing objects. Change detection based on combining images from two SAR passes typically results in two orders of magnitude lower false-alarm rate compared to detection using single-pass SAR imagery only. These results are a consequence of using low radar frequencies in combination with large fractional bandwidth and aperture angle [2], which facilitates low forest attenuation [3, 4], high resolution [5], as well as temporally and spatially stable backscattering from the forest [6, 7]. For the frequencies of interest (20 – 90 MHz), only the largest structures in a forest contribute significantly to the scattering [8, 9]. Structures that are much smaller than the 438 IET Radar Sonar Navig., 2010, Vol. 4, Iss. 3, pp. 438–448 & The Institution of Engineering and Technology 2010 doi: 10.1049/iet-rsn.2009.0039 www.ietdl.org