Autofocus of active beacons for measuring the tow-path of a synthetic aperture sonar: preliminary sea test results

We present operational details and preliminary experimental results of a portable freely-deployed "active beacon" used to perform motion compensation on images from the KiwiSAS-IV synthetic aperture sonar. The beacon sits on the seabed, listens for sonar pings using a matched filter, and retransmits in a different frequency band after a fixed time delay. The sonar forms a separate image containing just this point-target, blurred by towfish motion, with a high SNR and no contamination from close range objects. Autofocus algorithms tailored for point targets are then applied to estimate the sonar path. The experimental results are from the first sea test in a shallow marina environment and reveal several areas where improvement needs to be made before good results can be achieved. The main problems encountered were multipath and nulls in the sonar beampattern