The Research Of Propagation Path Effects On VHF SAR Imaging

Synthetic Aperture Radar(SAR) is an imaging radar developed after the World War II. It has the ability of imaging in far distance with high resolution and in all-weather condition for all days long. It is an important branch of radar technology and now has become the hot spot of research in radar area since last two decades with tremendous applications involved. When working in VHF frequency band, SAR signals can penetrate foliage and ground with low or even middle moisture. So VHF SAR has found wide applications in both civil and military areas, such as growing crops monitor, foliage or biomass investigation, counter-camouflage detection and buried unexploded ordnance detection. But VHF SAR imaging is easily affected by the propagation path. The thesis focuses mainly on the effects of soil and ionosphere on VHF SAR imaging. The work includes three parts:1. The first part conducts SAR imaging simulation using Range-Doppler(RD) algorithm and Chirp Scaling(CS) algorithm, and imaging processing of real SAR data from RADARSAT using CS algorithm. A new method is proposed for Doppler Ambiguity Number estimation in spaceborne SAR imaging by finding the minimum of image entropy or the maximum of image variance based on CS algorithm. The data of RADARSAT are used to test the method.2. The second part studies electromagnetic wave propagation in layered lossy medium and carries out simulation of VHF UWB SAR imaging for buried target. Several factors impacting the quality of SAR image are analyzed, such as the refraction effect which leads to location bias, the attenuation effect which leads to the raise of sidelobes in range and the broadening of mainlobe in azimuth, and the dispersion effect which leads to the asymmetry of sidelobes in range.3. The last part performs research on ionosphere's effect on VHF SAR imaging. The effect includes an unknown propagation time delay which caused image shift and dispersion which causes the mismatch in range compression. Simulations of point target imaging are presented. The results show that the crucial factors on imaging quality are carrier frequency, band width and TEC (total electron content). Under some ionosphere condition, lower frequency and wider band mean more image shift and more resolution degradation.