Suppression Of Strong Direct Interference In Complicated Underwater Environment

Multistatic network sonar system has gradually gained more attention,because of its advantages of both great detection performance and good concealment performance.It is inevitable that when the receiving platform in multistatic sonar system receives the target echo,it would also receive the direct interference signal from the cooperating ship at the same time.How to suppress the strong direct interference is an urgent problem to be solved in multistatic sonar detection.In this paper,in different application scenarios of single element,horizontal array and vertical array,the corresponding strong direct interference suppression schemes are given.When the receiving platform is one single hydrophone,using adaptive interference cancellation technique can suppress strong direct interference effectively and achieve the goal of weak target detection.The principles of interference cancellation and adaptive techniques are introduced.This paper analyzes the adaptive index of the variable step size VSSNLMS algorithm LMS and NLMS,which includes steady-state error and convergence rate.Under the condition that direct interference is 20 dB stronger than the target echo,the target can be found by using VSSNLMS adaptive interference cancellation and coherent detection technology together.When the receiving platform is a horizontal array,it is useful to suppress the strong direct interference by using the beamforming technique setting the nulls in the interference direction.Weighting vectors are designed to set nulls in directions of interference to suppress the direct interference signal based on the zero-order zero-point constraints,high-order zero-point constraints and the second-order cone programming zero-point constraints respectively.Each of the three kinds of algorithms has advantages and disadvantages.And in practical application,zero-order zero-point constraint is often used to suppress strong direct interference by setting multiple zero points on the target direction and adjacent areas.Influence factors on weak target detection are analyzed,including signal to noise ratio,the relative angle difference between target and interference,and the effect of element random disturbance.When the signal-to-noise ratio is higher than 0dB,the interval between the target and the interference is larger than that of the main lobe,using nulling beamforming can detect the weak target echo signal which is 40 dB less than strong direct interference.When the receiving platform is a vertical array,adaptive time reversal mirror can be used to suppress strong direct interference.The channel impulse response is obtained by using the Bellhop simulation toolbox based on the model of ray acoustics theory.It is proved that the multi hydrophone passive time reversal mirror technique can achieve the location of the underwater target,by utilizing the difference of channel spatial structure.The adaptive time-reversal mirror technique is used to suppress the strong direct interference signal of 30 dB more than target echo to achieve the weak target location.