Research On Imaging Algorithm Of Focusing Synthetic Aperture Sonar

Aperture synthesis sonar,as its name implies,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.The technique has been highly successful in radio astronomy,and in both satellite and aircraft borne radar. However,the use of this Technique has been very limited in the sonar environment mainly because of difficulties of maintaining a stable track underwater and problems of under-sampling of the aperture arising from the relatively slow velocity of acoustic waves in water.To increase its feasibility will be an important aspect in our sonar technology development.The thesis reviews the theory which is the basis of synthetic aperture processing. Then the theory of the SAS imaging is introduced in this paper in the view of how the high resolution of the range direction and azimuth direction is achieved by the sonar in physical conception,and followed by a discussion of the usual SAS imaging algorithm.How to improve the quality of the target and locate the target properly become an important task in this paper.Synthetic aperture sonar imaging system involves 2D Fourier transform in the processing of compression.Because of the limited signal bandwidth in 2D frequency domain,the envelope of time domain signal in the range and azimuth directions is the product of two sinc functions,which is interfered by its sidelobes.It is introduced in this paper a kind of adaptive algorithm---Spatially variant Apodization which can improve the quality of imaging scene.As a super resolution technique,Spatially Variant Apodization(SVA) can reduce sidelobe levels while maintaining the mainlobe resolution.Introducing SVA into synthetic aperture sonar imagery can get better imaging result than the conventional weighting approach. This paper discusses generalized apodization algorithm at the nonstandard Nyquist sample rates,improves it by iterative extrapolate method,then applies it for SAS imaging processing.The simulations concerned are also presented.