Experimental validation of autofocus algorithms for high-resolution imaging of the seabed using synthetic aperture sonar

The paper outlines the strengths and limitations of the phase gradient autofocus (PGA) algorithm and the displaced phase centre antenna (DPCA) algorithm when applied to synthetic aperture sonar imaging. The PGA algorithm was originally developed for radar, and care is required when implementing it for sonar. Due to the typically high fractional bandwidth of sonar chirp signals compared with radar, the problems of range migration are much more severe. This leads in some cases to a distribution of blur information over many range lines. Both algorithms are demonstrated in their application to seabed data collected at Brest Harbour, France, using a rail-based sonar system with a 15 m track. It is seen that PGA handles successfully an applied sinusoidal range error of 10 mm p-p amplitude (one carrier wavelength) repeating at intervals of 1 m.