Research On Localization Of Short/Medium Range Targets

Recently, acoustic positioning technology has become one of the research focuses in the field of underwater acoustics. An approach to measure low-frequency target echo highlight which belongs to active near-range positioning technology, and an approach of passive ranging based on guide source and single acoustic vector hydrophone which can be used to active medium-range positioning are studed in this paper. These two methods can be applied not only to the field of underwater acoustic positioning, but also basing for the new technologies of detecting sonar signal processing. And two new methods of this thesis can use for acoustic measurement.The key technology of the first approach is the focused beamforming. Different from the conventional beamforming, the focused beamforming can be used to locate the targets. The output of it is a function of direction and range of the targets once their depth is already known. The basic principles of focused beamforming in polar coordinates is introduced, including the ideal single highlight model, the ideal double highlights model and the multi-path highlight model. Three factors namely operating frequency, array aperture and range of targets which can affect the focued peak scale are analyzed.Based on the single protuberance model, the first approach is discussed detailedly in next section. MVDR focused beamforming, which is proved to own sharper focued peaks and lower sidelobe level than conventional focused beamforming by simulation results, is deduced. And then, MVDR focused beamforming, copy-correlation MVDR focused beamforming and STMV focused beamforming are proposed sequently adapt to the measurement of broadband target echo highlight. In the same conditions, studies show that these three methods can achieve better performance than the broadband conventional focused beamforming, and the capability of copy-correlation MVDR focused beamforming and STMV focused beamforming is better than MVDR focused beamforming.Finally, the approach of passive ranging based on guide source which is robust and free from the fine structure of the acoutic field is studied. One of its key technologies is the extraction of waveguide invariant. Two interference fringes equations of vector acoustic field are deduced. These equations can be used to extract the waveguide invariant together with cross-spectrum acoustic intensity averager and the Hough transform. A novel method based on a single vector hydrophone is proposed to estimate the range of the target. What this method only need is the ideas of waveguide invariant and the signals with broadband spectrum from guide source and targets. The method is proved to be feasible by simulation results.