Ultra-wideband synthetic aperture radar in the VHF-band: Image formation algorithms and interferometric applications

Sammanfattning: Synthetic aperture radar (SAR) is a technology to generate high-resolution radar images for ground surveillance and mapping. By exploiting the lower VHF band, the attenuation reduces dramatically in forest areas compared to microwaves and new applications become feasible. Examples of these are detection of targets obscured by vegetation, stem volume estimation of forest stands, and generation of “bald earth” digital elevation models in forested areas. This thesis presents research results of importance for ultra-wideband VHF-band SAR, including interferometric technique. The processing steps and the results for stem volume retrieval are described in an early stage study of VHF-band SAR data. Low attenuation and backscattering levels were observed and thus a potential for forest inventory applications was recognized. Novel results of generating interferometric fringe images using VHF-band SAR data in a forest area are treated, and parts of the processing steps are discussed. The effects of the motion compensation for the interferometric coherence are investigated. The conclusion is that the coordinate system of individual SAR images should be selected as close as possible to the actual track heading. Further investigation of motion compensation effects in SAR processing is presented for quantitative geometrical error estimates of targets elevated from a nominal focusing height. An extensive investigation of bald earth height estimation in a boreal coniferous forest area is carried out and the choice of baseline is discussed. A wide range of baselines are applicable and give a similar height estimate quality. For a large area of 1 x 2 km2 an rms deviation of about 2 meters from ground truth data was obtained. Especially good results are obtained for small areas consisting of dense forest with strong backscattering and continuous high coherence, i.e. with ground truth rms deviation of less than 1 meter. As a final issue, angular interpolation is evaluated in a time domain SAR algorithm, based on fast factorised backprojection for generating high quality images. The results show that an image quality in parity with a global backprojection (GBP) reference image can be reached, using an adequate sampling rate and interpolation kernel size, while still gaining the benefit of a large reduction in the number of operations required. Using a cubic interpolator and a sampling rate of two times the maximum angular bandwidth, 2Bθ, and applying a Hamming window, the ISLR and PSLR along the one-dimensional azimuth direction is measured to -58 dB and -42 dB, respectively. Increasing the angular sampling rate to 8Bθ, results in an ISLR and PSLR of -60 dB and -48 dB, respectively, which equals the result obtained by the GBP reference algorithm.