動揺下における合成開口ソーナの性能向上に関する研究

摘要

A synthetic aperture sonar has a high azimuth resolution with a short array and is easy tocontrol, either using a computer or by generating a long virtual array. When a sonar platform isrocked by ocean waves, the phase of the received signals becomes disorderd. Then, it is difficultto synthesize the virtual array, and the sonar performance deteriorates.In this dissertation, the author discusses schemes for improving the performance of the syntheticaperture sonar when it is being rocked.First, a classification of sonar systems and the sonar equation, which is essential for sonardesign, are given. In addition, the basic principles of the synthetic aperture, on which is basedand advanced in radar systems, are outlined, and its application to sonar systems was discussed,with an emphasis on similarities and differences between radar and sonar.Next, we describe some experiments that were performed in a water tank to study the fundamentalprocesses and characteristics of the synthetic aperture sonar. In the experiments, asynthetic aperture process based on a 2D correlation, which can properly take into account thedoppler effect, achieved theoretically predicted resolutions despite a limitation in the chirp rateand the transient characteristics of the transducer.It is difficult to negate the effects of ocean waves on a synthetic aperture sonar that hasto operate in the ocean. Thus, computer simulations were performed to reveal the effect ofthese waves, which were simplified to periodic waves. In the computer simulations, the fuel-cellunderwater vehicle Urashima was selected as a typical platform for the sonar system. The effectsof the periodic waves on the sonar were calculated, and the resolutions, SN ratio, and azimuthambiguity were studied, in terms of the vehicle attitude and the properties of typical oceanwaves. The results showed that the quality of the sonar images depended on not only the waveamplitude but also the wave frequency. In particular, we showed that ocean waves of certainfrequencies hardly affected the sonar images.Considering these results, a simple motion-compensation method, in which acceleration informationis filtered with the motion and integrated twice into special traces for motion compensation,is proposed in this paper. This method does not require any alignment at the start ofmeasurements, and the drift error does not diverge. This simple and convenient method wasapplied to a prototype synthetic aperture sonar and a sea trial was carried out. The results wereremarkable.Although we proved the high potential of the synthetic aperture sonar in theory and experiments,our computing power was insufficient to utilize sonar, particularly with motion compensation.To complete the synthesizing aperture process quickly, the received signal was decimatedto condense the synthetic aperture process. As a result, the synthetic aperture process based ona 2D algorithm worked properly and was faster after decimation, but the sonar image remainednoisy upon using a process based on a 1D algorithm.