Side-scan SONAR was developed to address the limitations of single-beam SONAR. Side-scan SONAR, like a single-beam echo sounder, transmits sound energy and analyzes the strength of the return signal (echo), which has bounced off objects or the seafloor.

The sound energy (ping) is transmitted from the sides of a towfish equipped with side-scan SONAR, in a fan-like beam that sweeps the seafloor many times per second. Side-scan SONAR return signals (echoes) are recorded, processed in real-time, and stacked, creating an image of the seafloor and any other objects within range of the pings.

While contours of the seafloor and objects on it can be imaged quite well, most side-scan SONAR systems cannot provide accurate bathymetric (depth) data. See Multibeam SONAR.