Study On The Passive Localization Based On Time Revesal Theory In Shallow Water

This dissertation studies the theory of passive localization for underwater sources based on normal mode modelling.The performance of the sources localization is severely degraded by strong bottom reverberation and multi-path interference.The paper sets out from the angle of normal mode modeling,studies the influence placement depth of the horizontal line array on the performace of time reversal processing and analysed the geometry relationship between the positions that strong sidelobes appear in and the depth of horizontal line array.Furthermore,the selective focusing on multi-scatters using horizontal linear in shallow water is studied.On this basis,this article puts forward the robust localization method based on virtual time reversal processing and virtual time reversal processing for short array by normal mode theory.The main contributions are as follows:1.Through the normal mode theory the dissertation analyzed the difference between the vertical array time reversal processing and the horizontal array time reversal processing.The performance of the spatial sampling of a horizontal line array at different depths is investigated by normal mode modeling.It is observed that the focusing performance of a bottom-mounted horizontal line array is better than that of a horizontal line array at other depths in shallow water.2.The perfomance of the horizontal array time reversal processing in low-frequency shallow water acoustic environment with multi scatterers are studied.Firstly,the selective focusing using horizontal time reversal method which based on the decomposition of time reversal operator is proposed.The choice of the active excitation signal weights depends on the intensity of the scatterer.Secondly,the spatial and temporal refocusing properties of the horizontal time reversal methods are discussed by theoretical derivation and numerical simulations.Finally,the conventional time processing techniques unable to restrain the influence of noise due to the time reversal operator can only be decomposed into the signal subspace and the reverberation subspace to solve this problem.A optimization method with Hadamard orthogonal basis is proposed.Theoretical derivation presents the implement steps of proposed optimization method,and numerical simulations in a typical shallow water environment demonstrate the effectiveness of the proposed method.3.The robust virtual time reversal source localization in an uncertain ocean environment is studied.In normal mode model,the environmental mismatch cased some modal amplitudes changed.However,there are still some modal likelihood function shape remains.In this work,the siganl wavefront is decomposed into a predictable modal subspace and an unpredictable subspace,the predictable space can be determined by finding the horizontal wave numbers with lowest variablity in uncertain enviroment.The active excitation signal weights of perpendicular arrays are reconstructed by the predictable subspace.Finally,the data collected off the experiments in the anechoic tank is used to verify the performance of the proposed method by interference noise ratio,background ratio,correct focusing index and localization accuracy,the results show that the proposed method is better.4.In order to reduce the difficulty in the practical application of the time reversal processing,a rreconstruction of transient acoustic field method based on short array is proposed.The methodology is based on the least squares algorithm and the iterative procedure.At the beginning,the sound field received by a short vertical linear array can be decomposed into normal mode depth functions and normal mode amplitudes in the mode subspace,and the amplitudes of modes can be estimated separately by the least squares algorithm and the iterative procedure.Next,an active excitation signal weights ofan ideal virtual long array reconstructed by the estimated amplitudes.The performance of time reversal processing with the short perpendiculararray is improved because the reconstructed fields contain more environmental informations.Then,the performance of the amplitudes of modes estimated by the least squares algorithm and the iterative procedure are compared.From the estimation accuracy of the amplitudes and normal mode theory analyzed the scope of application of the two methods.Computer simulation shows that the simulation result is consistent with the result of theoretic analysis.5.A deployment method based on time reversal processing for marine surveillance mode has been studied.The numerical results have been explained by the normal mode model according to the sound intensity distribution.The numerical simulations and the theoretical data calculated illustrate the validity of this method and the comparative results provide a basis of researching on the sensitivity for target monitoring different targets by this method at a deeper level.The proposed method has performed better than the traditional surveillance method on detecting small targets or the objects scattered small echo intensity owing to the method cannot be affected by the multi-path characteristics of the shallow water channel.