Synthetic-aperture sonar imaging with a multiple-element receiver array

The application of synthetic aperture techniques in sonar imaging of the ocean floor has been limited by sonar platform instability. The authors present a robust algorithm for motion estimation and compensation to perform synthetic aperture image formation. First, the algorithm uses the acoustic returns from a receiver array to form a complex-valued physical-aperture image for each transmit burst. Perturbations of the sonar platform velocity or attitude manifest themselves as registration errors and phase errors between successive images. The registration errors are estimated by cross-correlation-based techniques, and the images are aligned spatially. The phase errors are then removed. Finally, after phase correction, the registered images are coherently superimposed to form a high-resolution synthetic aperture sonar image. Results of experiments at sea with a prototype synthetic aperture sonar system are presented. A small point reflector on a sandy bottom at 70 m range was imaged using a 400 wavelength physical aperture and the synthetic aperture technique to compare the performance in detecting small targets near the maximum operating range of the sonar. The synthetic aperture image exhibits superior cross-range resolution and higher signal-to-noise ratio compared with the conventional techniques.<>