Ship Wake Imaging Technology Based On Multi-beam Sonar

The research on the acoustic features of ship wakes is an urgent demand not only in the military areas such as ship blanking and ship wake homing torpedo,but also in the civilian field such as port and channel base erosion,construction safety.The research on ship wakes has been a subject of considerable interest in many science and technology research field.To follow the recent development of relative technologies in the world and focusing on the key technologies of measuring the acoustic backscatter intensity and the spatial distribution characteristics of ship wakes using multi-beam sonar,this thesis is devoted to several aspects,namely the acoustic backscattering feature and multi-beam sonar echo model of the ship wakes,multi-beam sonar signal processing,ship wake detection based on the estimation of distributed source,multi-beam imaging and feature extraction of ship wakes and the experimental research on the spatial-time distribution of ship wakes.More specific contents are as follows:First,since the acoustic scattering property of ship wakes is the theoretical base of measuring the spatial distribution of ship wakes using multi-beam sonar,to establish the multi-beam sonar echo model of ship wakes,the relationship between the bubble distribution and acoustic scattering intensity is analyzed in quantity considering the acoustic scattering property of the ship wakes.A simple echo model cannot simulate the shape and spatial distribution of ship wakes or meet the requirements of multi-beam sonar echo modeling.Considering the single bubble transfer function,the channel transfer function and the wake bubble density distribution,the transfer function of the system is modeled.The multi-beam sonar echo model of ship wakes is established and simulated for a known transmitting signal waveform,and the simulation results show that the model has the ability of simulating the shape and spatial distribution of ship wakes.Second,in order to obtain ship wake images of high quality,two aspects of research have been done around the signal pre-processing of multi-beam ship wake imaging.On the one hand,the improved multi-subarray amplitude-phase wake detection method was proposed to estimate the TOA-DOA of the echo which was used to calculate the position of ship wakes.The mean amplitude of the multi-subarray will be used for the imaging data calculation.On the other hand,to improve the imaging quality of the outter beams,the variable bandwidth filtering algorithm was proposed to improve the SNR of echo in each beam.Finally,the above methods were verified using the experimental data on the lake.Third,due to the poor performance of estimation based on point source model when estimating the spatial distributed ship wakes,the generalized MUSIC algorithm based on spatial smoothing is proposed to estimate parameters of coherent distributed source measured by the multi-beam sonar.By both simulation and the processing of experimental data,the algorithm is proved to be effective in estimating the Central DOA and angle spread parameters.Fourth,multi-beam sonar imaging of ship wakes and feature extraction method are studied.In order to reflect the spatial distribution of ship wakes,the volume scattering intensities calculated using amplitude information of multi-subarray method are used as the imaging data.The noise of the images can be reduced by averaging the multi-subarray amplitudes which effectively improved the quality of the image and clearly reflect the spatial distribution of the ship wakes.The image edge detection method was used to extract the spatial distribution of the ship wakes and these methods are validated using experimental data on the lake.Finally,the thesis is devoted to the experimental study on spatial-time distribution of the ship wakes.Under the condition that the ship running straight at the uniform speed,a typical cross-track wake image can be obtained from single measurement of the multi-beam sonar with the measuring plane orthogonal to the ship's passing direction.As the multi-beam sonar measuring continuously,the variation of the typical wake cross-section shows the space-time distribution of the ship wake which can be equivalent to the three-dimensional spatial distribution of the ship wake.In view of the above measurement scheme,an upward-oriented measuring scheme and a horizontal-oriented measuring scheme of the multi-beam sonar were designed.The experiments using the two measuring scheme were carried out on the Songhua Lake and Xin'an River separately and large amount of experimental data were collected.The algorithms and imaging method were verified using the experimental data,and the advantages and disadvantages of the two measuring schemes were analyzed.