Research On Realizing Method Of Synthetic Aperture Sonar Imaging

This paper addresses the problem of the realization of Synthetic Aperture Sonar Imaging ,and concerned with the analysis of image reconstruction principle ,especially formulate beam-forming method in sonar array signal processing thoroughly .It uses digital system instead of analog system or compound of them.　Meanwhile it realizes the process of image reconstruction based on the mathematical model of beam formation The whole text concludes six chapters.The first chapter gives a outline of the history of Synthetic Aperture Sonar Imaging and current research status about internal and external.The second chapter deals with the feature and principle of the synthetic aperture image reconstruction.The third chapter presents the image-forming system through Side-Loking SAS and the mathematical principle of beam-forming.The system uses quadrature demodulation and range focus first , then uses azimuth focus to reconstruct image .The chirp signal which SAS emits can be expressed as : in formula: otherIn it,fc is center frequency ,k is frequency-modulating rate．If SAS moves along x axis,coordinate can be expressed by x,and the real part can be expressed by Re{.}.If the nearest distance between a point target and SAS moving-track is R0 and the coordinate of Y axis is y0,the echo delay time is:The response of the point target is:The response of quadtature demodulation is:Using the principle of phase continuance,we can get the spectrum of chirp modulation:　　The result after pulse compression is: In it, k is constant,sinc(.) is a sampling functionFocus in azimuth: The forth chapter examines demodulation and compression of linear band pass LFM signal and realize the function through simulation.Assuming the input of band pass frequency modulation signal is:a(t) is amplitude modulation , is phase modulation．Assuming the local voltage is :The output of multiplier in branch I is: After frequency part is filtrated through low pass filter ,we get:Because of the action of phase switcher in branch Q,the local voltage in branch Q is :So it multiply the input signal, we get:Through low pass filter, the output is:The branches of Q and I get two single magnitude in any time ,which express the signal-amplitude a(t) and signal-phase ,because The pulse compression uses the principle of matched filter .At the condition of additive zero-mean white image-Gaussian noise , the impulse response of the pulse compression is the function- in-mirror of input signal s(t), h(t)=s(t0-t)　　　　　when the input is x(t)=s(t)+n(t),the output of the pulse compression is: The impulse response of the matched filter is the conjugation of emitter signal s(n) that time has been converted , so:The output of the digital matched filter is:That is the mathematical model of realizing digit pulse compression using FFT.In the fifth chapter we study beam-forming method through the simulation and give function frame diagram. The conclusion get to the aim, the method of image reconstruction is correct.