Gap filler sensors for SAS

Like traditional side scan sonars, synthetic aperture sonar (SAS) systems have a coverage gap in an area directly underneath the sensor. Range resolution projected onto the seafloor deteriorates near the vertical, and objects do not cast shadows. Furthermore, to limit multipath in shallow water, some SAS systems do not transmit or receive at all near the vertical. It is difficult to come up with a sensor that can match the performance of a good SAS for the area around nadir - at least without increasing the cost and complexity of the system substantially, or imposing strict limits on operation. This paper investigates four possible technologies as gap fillers for SAS; acoustic as well as optical systems.