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Using a Multi-Frequency Synthetic Aperture Sonar for Bathymetry

This paper describes the implementation of a bathymetric synthetic aperture sonar and presents imagery from preliminary sea trials of the sonar. The sonar is designed for high resolution seafloor imaging in a shallow water environment. To estimate seafloor topology, the sonar ssytem uses the phase difference between elements of a vertical hydrophone array. Since the interferometric process uses a phase difference between vertically separated receivers, any roll motion of the sonar towfish produces a large error in the angle estimate. This paper shows how the phase errors due to roll of the sonar towfish can be estimated from the echo data and then corrected. A unique feature of the sonar is that it transmits two simultaneous frequency chirps in different frequency bands. This allows the ambiguity in the high frequency (100 kHz) phase estimates to be resolved using the low frequency (30 kHz) phase estimates. Preliminary results are presented from sea trials in shallow water (nominally 10 m) harbour environments showing the effects of towfish roll before and after compensation. Even in adverse sea conditions during trials in the Hauraki gulf, the roll compensation was found to work remarkably well. Bathymetric images are also shown of the changing seafloor depth across the channel in Lyttelton harbour and of objects proud of the seafloor.