Research On Key Techniques Of Frequency Diversity Sonar Imaging

Synthetic aperture sonar(SAS)is an effective underwater target imaging method.It can improve the resolution of distance by increasing the bandwidth of the transmitting signal.However,the impedance of the transducer in the sonar system will increase with the increase of frequency,and the impedance of power amplifier is certain,which is difficult to match and seriously affect the output waveform.In view of the difficulties of broadband matching between the power amplifier and the transducer in sonar system with large bandwidth signal,the complexity and high cost of the transducer implementation,this paper studies the synthetic aperture sonar imaging technology based on frequency diversity theory.The technology can transmits narrow-band signals of different frequencies at the transmitter and synthesizes the wideband signals at the receiver,which can achieve wideband detection performance with narrow-band signals and reduce the requirements for sonar transducers.At the same time,the frequency distribution can be adjusted to improve the flexibility of transmitting signal.The main research contents are as follows:1?This paper introduces the imaging principle of synthetic aperture sonar,which uses the relative motion between sonar small aperture array and target to synthesize larger virtual aperture to improve the azimuth resolution of sonar.In addition,the underwater acoustic channel is briefly introduced,and the relationship between the parameters of sonar,such as sound velocity,loss and propagation medium,propagation environment is analyzed.2?The two-dimensional imaging method of frequency diversity synthetic aperture sonar is studied.The idea of frequency diversity(FD)is introduced into sonar imaging.A large bandwidth signal is divided into different frequency narrow-band signals and transmitted at different positions.Then the echoes corresponding to different pulses are processed together at the receiving end to synthesize a large bandwidth target echo,so as to obtain range imaging.This method makes the power amplifier and the transducer work in narrow band condition,easy to match,and can reduce the cost of sonar system.In the azimuth direction,the traditional synthetic aperture sonar working principle is adopted.Firstly,the two-dimensional imaging model of frequency diversity synthetic aperture sonar is established,and then the imaging performance of underwater point target is analyzed by using back projection imaging method.Finally,the simulation environment is set to image the underwater target,and the feasibility of the scheme is verified.3 ? Three dimensional imaging method of frequency diversity array synthetic aperture sonar is studied.In order to solve the problems of complex implementation and high cost of wideband transducer in the existing synthetic aperture 3-D imaging sonar array,the wideband sonar transceiver array is replaced by frequency diversity array(FDA)to transmit narrowband signal and synthesize wideband signal at the receiver.The technology can obtain wide-band detection performance with narrow-band signal and reduce the requirement of sonar transducer.In imaging,the back projection algorithm is used to image the target.The simulation results show that the SAS 3-D imaging method based on frequency diversity array is feasible.Because the imaging method of frequency diversity synthetic aperture sonar is essentially the double sparse sampling of signal in frequency domain and spatial domain,the traditional imaging algorithm based on matched filter principle will inevitably have the problem of high sidelobe.Therefore,compressed sensing(CS)theory is applied to underwater target imaging,and the reconstructed signal is used to suppress the side lobe,so as to improve the imaging quality.