Improvement of synthetic aperture sonar with multi-channel projector

Abstract

A synthetic aperture sonar gathers echoes from large area of seafloor per a ping, therefore the sonar needs wider width of the beam than Side scan sonar. But if the elements of the projector is just one element, it have to be small to widen the beam, and must be low sensitivity. One solutions to raise the sensitivity of the sonar is to use array as a receiver. The array is not only necessary for high sensitivity, but also the motion compensation and higher mapping rate in synthetic aperture process. Many synthetic aperture sonar are now constructed with a single transmitter with an arrayed receiver. But the projector remains small and single in many synthetic aperture sonars. The maximum detection range of synthetic aperture sonar was no longer than side scan sonar at same sonic frequency yet. To extend the sonar range, we proposed multi-channel projector whose elements lined vertically. We consider that the intensity of the projection beam increases keeping its width, by controlling a timing of the projection from each element. In addition, if the projector can receive echoes, 3D-maps can be generated simultaneously like a multi-beam echo sounder. This means synthesizing aperture process become more precise with the correct altitudes. In June 2014, we carried out survey with the synthetic aperture sonar totally for 40 hours over 5 days at off-Sanriku near border of Miyagi and Iwate prefecture. We evaluated the projector performance through detections of many artificial objects such as underwater reefs and sunken ship in the survey. We confirmed that projection beam was widen keeping projection power and maximum detection range was over 400 m in slant range. In addition, bottom detection process which is before step of 3D-mapping became easy.