Research On Synthetic Aperture Sonar Beamforming Algorithm Based On Motion Compensation

Synthetic Aperture Sonar is a kind of imaging sonar with high resolution.It synthesizes an aperture by storing successive echoes obtained from a moving platform and by processing the results as if they had been obtained from a multi-element array enables a high azimuth resolution to be obtained from a physically small array.One of the outstanding advantage of SAS is that the sonar can focus the target in the water at any distance by using complicated beamforming techniques,meanwhile,the azimuth resolution is independent of the transmitting frequency and the distance between the sonar and the targets.The sophisticated imaging algorithm and the strict demand of the motion state is necessary to fulfill the function of SAS.Motion compensation based on receiving data is one of the key techniques in the SAS imaging algorithm.Because the motion error caused by the departure of the ideal sea route is reflected by the receiving data,the direct use of the data can't lead to a high qualified image.According to this,it is necessary to estimate and to compensate the phase error by the receiving data.This thesis studies the motion compensation algorithm which based on the receiving data.The thesis firstly reviews the history of SAS's developing and its actual state, then introduces the basic principle of SAS.In order to illuminate the key problem of SAS and introduce the main work of this paper,the imaging process of SAS is analyzed.Secondly,the thesis studies deeply the motion error of SAS based on the receiving signal,because of comparing with SAR,the unstability of the platform and the complexity of the propagation characteristic of sound in ocean which result in the motion error problem of SAS is more serious.The thesis simulate the receiving data of the single strong scatter,the category of phase error and the range of its compensational frequency is also discussed.Due to the compensation should be based on the receiving data of the whole target area instead of the single strong scatter, the influence of the position error of the phase center of the SAS's array on the receiving data is studied in this paper.Based on the analysis of the motion error,the thesis discussed how to compensate the motion error in azimuth and range direction by the Doppler parameter. The simulation on the compensation of the variational velocity in along-track is completed in this paper.The estimation of the Doppler parameter is the key technique in the motion compensation algorithm.The method for the estimation of the Doppler centroid and the problem about the Doppler centroid ambiguity is introduced.An improved method based on the time-frequency filtering for the Doppler centroid ambiguity resolving algorithm is proposed in this paper,the improved algorithm promote the accuracy of the algorithm effectively.On the other hand,the performance of the window width of the PGA algorithm which is applied in the SAS is analyzed in the paper,then the improved way to select window width is applied to the PGA algorithm based on strong targets,the improved algorithm reduce the iterative steps and promote the accuracy of the PGA algorithm.At last the author summarizes the whole work of this thesis and points out the problems to be solved in the future.