Detection And Parameter Estimation Of Moving Target In Strong Interference Underwater Environment

Signal detection and parameters estimation is an important task for active sonar system.However,the detection probability and accuracy of parameters estimation are easily affected by the multi-path waveguide,ocean environment and reverberation.In order to achieve high detection probability and accuracy estimation,in this paper,a modal signal processing method based on a vector hydrophone array is proposed for suppressing oceanic noise.With the characteristics of multi-path Doppler shift and moving targets,a detection method and two parameters estimation methods which are based on fractional Fourier transform are proposed.The proposed methods are meaningful for active sonar system work.The main work is followed:First of all,a modal signal processing method for vector hydrophone sensors array based on spherical harmonic decomposition and characteristic of Legendre function are used to obtain the modal factors of sound field.As the modal factor is depended on the target echo information,it can be applied for beamforming and direction of arrival of targets.It is helpful for suppressing the noise and increases the signal noise ratio(SNR)and signal reverberation ratio(SRR)values.Compared with traditional array signal processing method,the modal signal processing method is not limited by the array scale.Consequently,modal signal processing method can realize high resolution beamforming in low frequency and small scale array.By choosing suitable truncation number,number of sensors and scale of array,the oceanic noise and reverberation can be effectively suppressed.What is more,the vector array performs better than traditional array on noise suppression.Consequently,it is meaningful for giving the modal array signal processing method for vector array.Secondly,considering effects which are produced by the multi-path condition,time-varying channel and reverberation,a detection method for moving targets based on the distribution characteristic of multi-path Doppler shift is given.In this method,a copy signal is constructed with different delay time and Doppler shift.By matching the received signal with copy signal,a Doppler-delay energy matrix is built.Considering the multiple path channel,the Doppler shift distribution of multi-path echo is used to detect target in low SNR and SRR environment.The simulation and experiment results show that the proposed detection method is effective and stable.Thirdly,as low echo intensity,wide velocity changing range,low SNR and SRR condition lead to poor parameters estimation of moving targets.Two time delay and Doppler shift estimation methods based on fractional Fourier transform are proposed.The first method is optimal order fractional Fourier transform parameters estimation method.By deducing the relationship among delay time and Doppler shift of target echo signal focusing point in fraction domain and optimal fractional order,the delay time and Doppler shift expansions are given.The other method is fixed order fractional Fourier transform parameters estimation method.This method uses the relationship among Doppler shift,delay time,target echo width in fraction domain and peak location of target echo in fractional domain to estimate the parameters.The simulation results show that the detection probability of optimal order fractional Fourier transform parameters estimation method is much higher the traditional matched filter in low SNR and SRR environment.The second proposed method is easily affected by the Gaussian noise and reverberation.However,as the fixed order fractional Fourier transform method only require once transform,the computation is less than other methods.The results of experiment data processing by optimal parameters estimation methods show that the accuracy of parameters estimation is much better than matched filter.The performance of fixed order fractional Fourier transform parameters estimation method is not good as the influence of oceanic noise and reverberation.At last,by combining the modal signal processing and fractional Fourier filter,a direction of arrival(DOA)method for multiple targets is proposed.This method transforms the element space-frequency signal to modal-fractional signal.With fractional filter,multiple targets are separated in fractional domain.Then,by applying the multiple signal classification(MUSIC)method to fractional domain,the DOA estimation can be achieved.As the fractional domain contains time delay and velocity information,fractional filter can get better performance than traditional filter.The simulation results show that the proposed method can get higher accuracy of DOA estimation in terms of multiple targets in low SNR and SRR environment than traditional DOA method.Signal detection and parameters estimation in underwater environment are very important not only in military but also in civilian field.In this paper,detection and parameters estimation methods are proposed which are based on vector sensors array and fractional Fourier transform theory.It provides the theoretical and technological support for multiple moving targets detection in strong interference environment with small scale sonar system.