Test results from a multi-frequency bathymetric synthetic aperture sonar

Describes the implementation of a bathymetric synthetic aperture sonar and presents preliminary results from sea trials of the sonar. The sonar is designed for high resolution seafloor imaging in a shallow water environment. This is achieved through coherent summation of successive echo signals to synthesise an aperture many times longer than the towfish. Provided the motion of the towfish is accurately estimated and compensated, the application of aperture synthesis can result in a range independent resolution over the operating swath. While high resolution is desirable for high quality imagery, it is not always sufficient for the discrimination of targets of interest from general seafloor clutter. Therefore, the authors have attempted to use bathymetric techniques to discriminate targets from the seafloor on the basis of height. The sonar is configured to use an array of three vertically spaced hydrophones from which three independent synthetic aperture images are reconstructed. Using knowledge of the sonar geometry, the relative heights of the seafloor scatterers are estimated using the phase differences resulting from the slight range difference of a scatterer from each hydrophone. Preliminary results are presented from trials in a shallow water (nominally 16 m) harbour environments. Results are shown for two different frequency bands: the sonar transmits two simultaneous linear FM chirp signals; one covering the frequency band of 20-40 kHz and the other covering 90-110 kHz. Due to the use of a neutrally-buoyant, high-drag towfish, reasonable reconstructed imagery is obtained without autofocusing at the low frequency range. However, autofocusing is required for the high frequency data even though the synthetic apertures are shorter.