A system that uses acoustic signals propagated through the water to detect, classify, and localize underwater objects is referred to as a sonar system.‡ Sonars are typically on surface ships (including fishing vessels), submarines, autonomous underwater vehicles (including torpedoes), and aircraft (typically helicopters). A sonar system generally consists of four major components. The first component is a transmitter that (radiates or) transmits a signal through the water. For active sonars, the system transmits energy to be reflected off objects. In contrast, for passive sonar, the object itself is the radiator of acoustic energy. The second component is a receiving array of hydrophones that receives the transmitted (or radiated) signal which has been degraded due to underwater propagation effects, ambient noise, or interference from other signal sources such as surface war ships and fishing vessels. A signal processing subsystem which then processes the received signals to minimize the degradation effects and to maximize the detection and classification capability of the signal is the third component. The fourth component consists of the various displays that aid machine or human operators to detect, classify, and localize sonar signals.