Synthetic Aperture Sonar
Synthetic aperture sonar (SAS) is used for creating high-resolution acoustic images of the sea-floor. A high-end SAS is capable of imaging a golf-ball at 500m underwater. In Lyttelton (where we do sea-trials) divers' vision extends only about 1.5 m making sonar essential for object finding.
Resolution
Traditionally sonar resolution is determined by the size of physical receiver used. For high-resolution imaging (20 mm X 20 mm) at long range (>200 m) very large (about 10 m long) receivers are needed. SAS uses the forward motion of the sonar to synthesise the larger receiver. This lets the SAS fake using a large receiver with a small one and gives it high, range-independent resolution.