Target Echo Detection Based On Time-Frequency Analysis And Image Morphology

Compared with Acoustic Imaging, Echo Analysis is a more effective method in underwater detection with a long range. However, target echo is always received by sonar with interference of reverberation in active sonar system. Apparently it is a tough task to extract target echo from reverberation in frequency domain, according to the theory of reverberation. Therefore, time-frequency analysis was adopted to achieve detection of target echo, basing on differences between echo and reverberation. Current methods of time-frequency analysis, however, are effected by cross term with a limited contribution. In this case, conventional time-frequency analysis can hardly fulfill demand of target detection with low SRR, and it became necessary to figure out a method of target detection with ability of anti-reverberation and anti-cross-term.In this thesis, echo structure of bottom object, the distribution of single-highlight echo and multi-highlight echo was demonstrated, according to theory of echo, highlight model and the characteristics of launched signal. Afterwards, the characteristic of reverberation distribution was involved. Moreover, the theory of cross term in time-frequency analysis was discussed, after which output time-frequency analysis was deduced with a multi-components linear FM input. Meanwhile, cross term was analyzed basing on Image Morphology.Combining morphology operator, characteristics of echo and reverberation, this thesis introduced the concept and practical examples of Image Morphology, and then demonstrated the feasibility of suppressions of reverberation noise and cross term, parameter estimation of target echo basing on the characteristics of time-frequency image, which provide theoretical principle of them.After that, evaluation criterions of reverberation suppression and LFM signal parameter estimation were proposed, after which three experiments of multi-components signal cross term suppression, reverberation suppression as well as echo parameter estimates were designed. The experimental results proved theories proposed in this thesis, and presents performance trend of reverberation suppression and echo parameter estimates with different SRR.At last, this thesis confirms the availability of proposed method with oceanic experiment data. Due to the complicated oceanic environment and the interference which is impossible to be got rid of, cross term suppression is not possible to be proved individually with data of oceanic experiment. However, the experimental result proved that the method proposed by this thesis is valid in oceanic experiment.