Optimizing a three-stage autofocus system for synthetic aperture imaging using a UUV

High-resolution synthetic aperture imaging of the seabed at ranges up to 500 m requires precise compensation for vehicle motion errors. The paper shows how a combination of (a) direct measurement, (b) DPCA autofocus (i.e. cross-correlation of overlapping phase centres) and (c) Phase Gradient Autofocus (PGA) provides a robust, three-stage motion compensation scheme. The performance and reliability of the three-stage scheme depend on the degree of overlap between pings, the length of the synthetic aperture and a number of key parameters that are chosen for the DPCA and PGA algorithms. Results are presented in the context of a set of UUV trials in shallow coastal waters using a vehicle of 7m length equipped with a 1.1 m array of 192 hydrophones. Despite a water depth of only 12 m with consequent severe problems due to multipath reflections, images were obtained of target objects at ranges in excess of 250 m with resolutions of a few tens of millimetres.