Research On Multi-beam Synthetic Aperture Sonar Imaging Technology

Multi-beam bathymetry sonar is an important tool which is widely used in sea exploration and seafloor mapping. Research on the technology of high-resolution multi-beam imaging sonar in our country has wide prospects and great significance. The along-track resolution of multi-beam bathymetry sonar is determined by the size of beam footprints, and it is low. Then, the accuracy of underwater target detection and the quality of sonar imaging is limited by the low along-track resolution. Synthetic aperture technique can use a small physical aperture to obtain high along-track resolution by signal processing. This thesis is to combine the Synthetic Aperture Sonar(SAS) and multi-beam bathymetry technology, and research on the key technology of multi-beam synthetic aperture sonar imaging. The motion compensation technology is studied at last.At the beginning, the theory which is the basis of SAS imaging technology has been reviewed, and why SAS has the high along-track resolution is explained, then the SAS echo model of point target and the imaging algorithm is introduced.Multi-receiver SAS imaging algorithm has been thoroughly studied, two kinds of inherent phase errors caused by the stop-hop-stop approximation were analyzed, and two new multi-receiver SAS imaging schemes without approximation of stop-hop-stop were given. The dot by dot algorithm and line by line algorithm for the new schemes have been studied. Computer simulations have been done to test the validity and efficiency of the two methods. The results show that, the imaging quality of the new schemes has been clearly improved.In order to improve the along-track resolution of multi-beam bathymetry sonar, the paper proposes two schemes for the realization of multi-beam synthetic aperture sonar based on the multi-beam bathymetry and aperture synthetic principle. How is the dot-by-dot imaging algorithm applied in the two new schemes are described respectively. Simulation results show that both imaging methods can solve the contradiction between the average coverage rate and the azimuth ambiguity, and the new imaging schemes can get more target information and higher signal to noise ratio than the conventional SAS.Motion errors resulting from the towed platform deviating from its straight path need to be determined and compensated accordingly. Two receiving-data-based motion compensation methods of SAS have been studied in this thesis. The first one is a method based on single receiving array, which need to estimate the range migration using a curve fitting. This method's precision is not good enough and its application scope is small. Another method is the displaced phase center algorithm, which can estimate both sway and yaw. Its results are accurate and the error estimating range is large.