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.
In this paper we present experimental results of a method for determining the path of a synthetic aperture sonar (SAS), with sub-centimeter accuracy, by autofocussing images of an active beacon. 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 sonar motion, with a high signal to noise ratio (SNR) and no contamination from close range objects. A simple narrowbeam autofocus algorithm is applied to estimate the sonar path, providing motion compensation (MOCOMP) data to correct the SAS images of the other objects in the scene. Independent one-way and twoway approaches to measuring the time of flight (TOF) were investigated, with the two-way approach producing better results. Significant improvement was achieved for two runs at 30kHz with a small improvement achieved in one run at 100kHz. Fitting a hyperbola, to the separate beacon raw data produced a velocity estimate for the sonar that was within ±0.025 m/s of the tow boat velocity, for both runs, when compared with the global positioning system (GPS) velocity measurements averaged over the run length.
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