Synthetic aperture sonar for small unmanned underwater vehicles

The shallow water and very shallow water (<80 feet) environment is a very hostile region far sonar operations, however it is still considered one of the best options for mine detection and classification in this region. The wats depth and small vehicle size demands that the sonar be small and low power. A conflicting need exists for very high-resolution sonar, which implies a very large aperture, that can discriminate between small mines and natural or benign manmade objects. Resolution cells of approximately 1/16 the size of the targets smallest dimension are generally considered necessary to classify a target with high confidence level and low false classification rates. The synthetic aperture sonar (SAS) concept can meet the requirement for small size and high resolution. This paper will review the development of the SAS technology at CSS from a simple, single channel first generation SAS to the present real-time dual frequency multi-channel SAS. All these systems were designed to be compatible with small underwater vehicles. The SAS signal processing evolved from a very simple delay and sum beamformer to a more complex motion error modulated beamformer. This paper will show images of these systems as they evolved from the earliest 15-cm resolution SAS to the present system producing 2.5 cm and 7.5-cm resolution from a dual high/low frequency SAS, respectively. The present system operates in real-time and has generated the current images using a physical array that is less than 0.6 meters in length and 0.15 meters in height.