Synthetic-aperture sonar (SAS) is a form of sonar in which sophisticated post-processing of sonar data is used in ways closely analogous to synthetic-aperture radar.

Synthetic-aperture sonars combine a number of acoustic pings to form an image with much higher along-track resolution than conventional sonars. The along-track resolution can approach half the length of one sonar element, though is downward limited by 1/4 wavelength.

The principle of synthetic-aperture sonar is to move the sonar while illuminating the same spot on the sea floor with several pings. When moving along a straight line, those pings that have the image position within the beamwidth constitute the synthetic array. By coherent reorganization of the data from all the pings, a synthetic-aperture image is produced with improved along-track resolution. In contrast to conventional side-scan sonar (SSS), SAS processing provides range-independent along-track resolution. At maximum range the resolution can be magnitudes better than that of side-scan sonars.

A 2013 technology review with examples and future trends is also available. For academics, the IEEE Journal of Oceanic Engineering article: Synthetic Aperture Sonar, A Review of Current Status gives an overview of the history and an extensive list of references for the community achievements up to 2009.

The length of the synthetic aperture is

${\displaystyle L_{sa}\approx R\eta {\frac {\lambda }{d}}}$

Where R is the range, ${\displaystyle \lambda }$ is the wavelength at center frequency and d is the along-track element size in the array. ${\displaystyle \eta }$ is a programmable parameter which controls the process beamwidth—the beamwidth actually processed.

The SAS system relies on a stable sensor platform, being able to determine to a high accuracy where the sensors are over several meters of travel distance—all the pings captured will be used in the formation of a synthetic aperture. Due to currents, heave or sway, a sensor platform may undergo lateral movement known as "crabbing", which have the potential to heavily impact SAS image formation. SAS arrays may not be the best choice for a sensor platform in rough terrain nor areas where one can expect currents from the sides. Mission planning and selection of sensor platform can alleviate some of these challenges.