Underwater Acoustic Target Passive Localization Techniques Based On Acoustic Field Interference Structure

The acoustic waves radiated by underwater targets propagate through underwater sound channel to receivers.Sonar's received signals contain both target's information and marine environmental information.Sonar signal processing technology based on underwater physical characteristics has become the research direction and development trend of underwater acoustic technology.Stable interference structure can be observed in the intensity plot of shallow water low frequency sound field.Waveguide invariant concept is proposed to describe interference striations.It has important application value in underwater targets' passive ranging.However,passive ranging techniques are still limited by application platforms and underwater acoustic environmental characteristics.In this paper,interference structure's formation mechanism and influence of environmental characteristics on interference structure are discussed in-depth.On this basis,passive ranging techniques which are applicable for different receiving platforms and marine environments are studied.Due to interference effect between modes,dark and bright fringes appear in the intensity plot.In this paper,fundamental theory of acoustic propagation is briefly reviewed.The normal mode theory is introduced.According to propagation path,modes are divided into two types: reflected modes and refracted modes.Interference striations in element signal's intensity plot and beam signal's intensity plot are analyzed.Compared to striations in element signal's intensity plot,striations in beam signal's intensity plot have similar slope features.Through beamforming,targets in different directions can be distinguished.In addition,beam signals could obtain array gain.Therefore,beam signals are adopted in the whole research of this paper.Waveguide invariant theory is derived.The influences of sound speed profile,source's depth and receiver's depth on waveguide invariant are analyzed.The waveguide invariant distributions corresponding to two types of modes are compared,which provides a theoretical basis for passive ranging techniques.For most shallow water waveguides,waveguide invariant ??1.In this situation,a passive ranging technique using double arrays and a passive ranging technique based on array invariant are verified through simulation research.A passive ranging technique using single maneuverable array is proposed.All of three ranging algorithms do not need to know marine environmental parameters accurately,to estimate target's velocity in prior or with the help of guide sound source.However,each ranging algorithm has its advantages and limitations.The passive ranging algorithm using double arrays can be applied in double separated arrays.It requires targets locating in the far field of receiving arrays.This algorithm has ranging blindspots in the vicinity of normal direction.The passive ranging algorithm based on array invariant utilizes invariance of beam maximums' shifts with time delay.It doesn't require multiple modes existing in the acoustic field to form interference,and can be used for real-time processing.However,this algorithm is sensitive to line spectrum.The passive ranging algorithm using single array can be applied in a moving platform which changes velocity or course in motion.The motion parameters are required to be known in this algorithm.The algorithms can be selected according to application conditions.Waveguide invariant corresponding to reflected modes approximately equals to one,but waveguide invariant corresponding to refracted modes varies in a wide range.For acoustic field dominated by refracted modes,description of interference striations is complicated.Relevant theoretical basis of passive ranging algorithm is limited.Besides,affected by noise and line spectrum,interference striation's effect is poor.The striation's trajectory can not be extracted accurately through image processing.To solve these problems,a modal horizontal wavenumber filtering technique is proposed and a striation filtering technique is adopted.Utilizing horizontal wavenumber's difference between two types of modes,refracted modes are filtered out and reflected modes are preserved.The striation filtering includes lowpass filtering in two dimensional Fourier transform domain of sound intensity and several image processing methods.The influences of high frequency noise and line spectrum are reduced,improving accuracy of striation trajectory extraction.For single receiving array which is static or uniform moving,a target's radial velocity estimation method is revised.Using periodicity of pressure cross spectrum,target's radial velocity is estimated.The limitation in signal processing parameters is removed by initial phase difference compensation.The performance of narrowband processing method and broadband processing method is compared.Target's motion parameters and range are estimated using interference striations.Combined with the striation filtering technique,target's passive ranging in complicated marine environment is completed.The target's passive ranging techniques studied in this paper are applicable in various receiving platforms and marine environments,and are expected to be applied in passive sonars such as sonar moored at seafloor,broadside sonar and towed line array,improving passive ranging performance.However,the relevant engineering application still needs extensive research.