Lars Ulander

Low-frequency ultrawideband array-antenna SAR for stationary and moving target imaging

The paper discusses the necessity and feasibility of designing a SAR system for detection of low-signature stationary and moving targets, e.g. concealed by foliage. In particular, we investigate the basic requirements, measurement principles and signal processing algorithms. We show that the problem can be formulated using the same principles as ultra-wideband SAR by adding multiple antenna elements. Time-domain signal processing is emphasized, and we formulate such SAR algorithms to simultaneously focus both stationary and moving targets. The computational effort to the basic algorithm is dramatically reduced by factorization into a number of recursive stages. In this manner, the time-domain algorithm has essentially the same numerical efficiency as FFT-based methods. We also include a brief description of the present development of an airborne SAR system with an antenna array and operating in the 200 - 800 MHz band called LORA.

Detection of concealed ground targets in CARABAS SAR images using change detection

The paper describes a new method to detect man-made objects hidden under foliage or camouflage. The method is based on change detection and thus multiple revisits of the same area. It uses SAR image data provided by the low-frequency and ultra-wideband CARABAS SAR system which operate in the 20 - 90 MHz frequency range. Experimental results show a drastic reduction in false-alarm rate compared to methods based on single-pass SAR images. Small- to medium-sized trucks are consistently detected with a false-alarm rate of the order of 0.1 - 1 per km2. This level of false-alarm rate is quite sufficient for most military or civilian applications of interest.

SAR interferometry for detecting forest stands and tree heights

ERS-1 SAR interferometry has a large potential to map out forested areas. Different coherence maps show a stable pattern, even with high wind speeds and melting temperatures on the ground. With a DEM of the ground, ERS-1 SAR interferometry can be used to measure interferometric tree heights in a forest. Measurements yield a tree height even when the coherence is low, but have a tendency to decrease with the area coherence. Two boreal forests have been measured showing an interferometric tree height about half of what was measured in-situ. The fact that interferometric measurements are lower than the real tree heights may have been caused by either high wind speeds lowering the effective stable scattering center in the trees or by existence of a ground contribution to the forest backscatter of the radar return.