Square SAS: Multi-Aspect Imaging with a Towed Synthetic Aperture Sonar

Abstract

When hunting for stealthy naval mines in highly cluttered regions, a multi-aspect capability is advantageous because multiple "looks" improve the probabilities of detection and classification. For autonomous underwater vehicles (AUVs), full azimuth coverage is possible by flying circular or spiral trajectories while keeping the target within the sonar beam. Circular navigation of a towed sensor is more challenging due to the variable tow cable dynamics. Nevertheless, towed synthetic aperture sonar (SAS) systems are preferred for many applications because they achieve a very high area coverage rate with excellent endurance, rapid transit capabilities, and high bandwidth telemetry. In this paper, we describe and demonstrate an operational concept called "Square SAS" for towed sonar platforms. Square SAS consists of fusing multi-aspect imagery from piecewise linear survey lines having an azimuthal extent of at least ±90°. The concept can be extended to include an arbitrary number of piecewise linear survey lines such as hexagonal or octagonal configurations. Experimental results are presented from Kraken’s SeaScout seabed mapping system equipped with a high speed towed SAS. The image fusion technique is generally applicable to any SAS platform, including AUVs, without any prior assumption for the target shape. However, the technique is ideal for towed systems where multiple linear passes are necessary to achieve a true multi-aspect imaging capability with a wide azimuth coverage for effective target classification.