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Laboratory implementation of a synthetic aperture imaging sonar

This paper outlines a programme of work at the University of Cape Town which aims at utilising expertise in synthetic aperture radar (SAR) to produce sonar images. This work has culminated in the production of images both in air and in water using equipment developed especially for the purpose. The coherent sonar transceiver operates at 40 kHz and uses a bistatic antenna arrangement. Synthetic aperture sonar can provide higher resolution imaging of the sea bed at the same, or even higher, coverage rate than conventional sonar imaging techniques. Synthetic aperture imaging differs from traditional array processing techniques, in that a large underwater aperture is synthesised as the towfish moves through the water, instead of using a large, physical array of transducers. In the results presented below, the targets used were stationary during measurements and the transceiver antennas were moved for experimental convenience. The resulting synthetic aperture images were free of ghost images, had negligible side lobes and showed a great improvement on the along-track resolution of the sonar system.