Autofocus of active beacons for measuring the tow-path of a synthetic aperture sonar

We present design details and simulation 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 simulation incorporates the statistics of the beacon measurement error and medium fluctuations measured during previous sea trials with the beacon and a fixed transmitter.