Study On Key Problems In Anti-interference Signal Design For Continuous Active Sonar

The active sonar is the important part of the anti-submarine defense system.Hence,it’s a hotspot problem about how to effectively improve the performance of active sonars.The pulse active sonar technology has been developed as the most widely used sonar system.Recently,some problems of pulse active sonar have exposed with the appearance of a large number of quiet submarines.In shallow sea surroundings,the reverberation scattered from the complex interface and the direct blast caused by bistatic layout are main interference factors for active sonars.Due to these factors,the pulse active sonar reaches the demand for higher source level,the bigger blind zone of detection and the more difficult target detection.Since the above problems are constrained from the particular features of pulse signals,it’s not obvious to improve the performance of active sonars through the pulse signal design.However,the latest research shows that the continuous signal reaches some unique advantages over the pulse signal as the transmitted signal of active sonars,such as the continuous detection and tracking,the low demand of source level,the high time utilization and the low false alarm probability.These advantages make the continuous active sonar technology as a promising anti-submarine technology.By far,the study on continuous active sonars is still in its infancy which has many problems to be solved.For instance,the study on the anti-interference signal designs of continuous active sonars is important to improve the performance on the target detection.Therefore,it’s of great significance to further explore relevant problems in the anti-interference continuous signal designs in order to improve the performance of continuous active sonars and enhance their application values.In this paper,some problems in anti-interference continuous signal designs are studied,analyzed and discussed.The research includes the optimization and design of frequency-modulated continuous signals,as well as the optimization and design of phase-coded continuous signals,which both aim at the anti-interference performance of signals.Furthermore,this paper researches about the interference on continuous active sonars in shallow sea surroundings,including the bottom reverberation model of bistatic sonars in complex landforms,the characteristic analysis on continuous bottom reverberation and the influence of reverberations on the detection performance of continuous signals.Finally,the paper studies on the suppression methods of the reverberation and the direct blast.The main researches and concludes are as followings:(1)The optimization model of the frequency-modulated continuous signals is constructed.Based on the quantitative expressions of the ambiguity functions,we propose the evaluation criterion applying to any frequency-modulated continuous signals.The model which is constructed based on the criterion can optimize the key modulated parameters of signals via the genetic algorithm.The results prove that the optimization model effectively improve the anti-interference performance of the frequency-modulated continuous signals,which are specific in the obvious ‘thumbtack’AF shape and good correlation properties of sub-pulses.In addition,the new frequency-modulated continuous signal composed with optimized parameters is shown to be of good anti-interference performance.(2)The design methods of phase-coded continuous signals are studied.We convert the design of this kind of continuous signals to the design of the corresponding phase-coded sequences.According to the existing researches on phase-coded sequences,the sequence design methods under the periodic WISL metric are computationally burdensome.Therefore,based on the majorization-minimization strategy,we propose a phase-coded sequence design method under the periodic WISL metric for the first time.Then we improve the method and introduce an acceleration scheme in order to enhance the convergence speed.The simulation results demonstrate that the proposed methods can generate sequences with low periodic autocorrelation sidelobe levels in specific time-delay intervals.Also,the computational efficiency is higher than the existing methods.In addition,the continuous signals phase-coded by the proposed sequences reach good detection performance.(3)The computation about bottom reverberations caused by bistatic continuous active sonars are studied.According to the formation and transmission mechanism of the bottom reverberation,a computational model of bistatic bottom reverberation is constructed.Combined with the curve fitting technology,a fast calculation method of bistatic bottom reverberation is proposed,which can apply to any bottom profiles in shallow sea surroundings.The numerical simulations prove that the proposed calculation method significantly improve the computational efficiency and guarantee the accuracy,compared with the classical cell scattering model.In addition,two assessments are proposed to evaluate the complexities of two representative sea floor profiles.Then we discuss the influence of fitting steps on the computational accuracy of different complexity.It’s demonstrated that the fitting step should be chosen reasonably for sea floor profiles with different complexities in order to balance the accuracy and the real-time.(4)The influence of reverberation from continuous signal on the detection performance of active sonars is discussed.The characteristic of the bottom reverberation from bistatic continuous active sonars is analyzed.The result shows a large difference between the reverberation from continuous signals and pulse signals.The reverberation is affirmed to cause severe interference to the detection of continuous active sonars.Furthermore,the detection performance of continuous signals under the reverberation interference is analyzed.Through the numerical calculation,we get the change rules of the matched filter output power with respect to the targets’ extension features under the uniform-distributed reverberation.Also,the Q-function results of different continuous signals show that the detection performance on moving targets under the reverberation can be improved via the optimization model of the frequency-modulated continuous signals.Then a reverberation suppression method and a direct blast suppression method are presented in this paper.The simulation results verify the effect of two suppression methods.(5)The experiments on the detection performance of continuous active sonars have been carried out on the lake.The bistatic continuous active sonar system was constructed and the detection experiments of several kinds of continuous signals have been completed.The results of the experiments affirm that the continuous signal reaches the better anti-interference and detection performance than the pulse signal in the shallow sea surroundings.The experimental data can provide support to the applications of the anti-interference signals of the continuous active sonars.