A typical sonar system works by emitting ultrasonic pulses from an acoustic projector. A sensitive microphone, or hydrophone, listens and analyzes the signal that was reflected off a submarine or obstacle. There are three main types of sonar: active, passive, and acoustic communication systems.

Passive sonar systems are made of sensors that receive the noise made by an object, generally a submarine or surface ship. Since passive sonar does not create any sound, it is primarily used in submarines. The sound waves are analyzed to find the type of ship and identify its direction, distance, and speed. Other objects can also be detected such as certain species of whale that produce distinct patterns of sound. Passive projectors are usually deployed from sonobuoys (hydrophone mounted in floating buoy), towed behind a ship, or mounted on a ship's hull. Sometimes, buoys may also be laid on the ocean floor, but the most advanced system, called a towed array, uses a long cable on which hydrophones are attached. The disadvantage to passive sonar is that it cannot detect the range. An estimate can be calculated by measuring the curvature of the received sound wave.     

Active sonar is sending a very loud 'ping' into the ocean. A pulse signal is sent to a transducer which changes the electrical signal into a sound signal. This ping can be set to different frequencies, bearings, or angles. You can also
'beamform' the ping to track an area or change the pulse length. The ping has a speed of about 1500 m/s. If an object is
in path of the narrow beam (15-20°), then the sound energy is reflected back, some of it reaching the sonar dish. A receiver
amplifies the soft echoes and measures the distance with the following formula: range = sound speed x travel time / 2

The limitation of this echo-ranging sonar is the long time needed to adjust the projector, send out a ping, and listen for an echo. A reason for this is the low speed of sound in water (approx. 5000 ft/sec). 

Active sonar allows great flexibility to what you want the pulse to do. Some examples:

...........Direct Path (DP): 'Headlights' for water; search directly ahead of ship (or any other ...........bearing) to see path in front;  medium range, short depth.

...........Omni-Directional Transmission (ODT): 360° ping; see if there is anything around the ...........ship; short range, medium depth.

...........Convergence Zone (CZ): More than one frequency; meant to go very deep and very ...........far; pulse does not go straight because of refraction from different water ...........temperatures.

Acoustic communication systems are still different. They require a projector and a receiver at both ends of the acoustic path to enable ships to communicate with submarines or divers, or whales and dolphins to communicate among their own species.

The Sonar Buoy was yet another American invention, used by Aircraft to drop into the ocean in a pattern of five, each Buoy floated, and listened for noise from a Submanne. Each Buoy was colour coded, Purple. Orange, Blue, Red, and Yellow, ( P.O B.R.Y.) and was monitored in turn by the dropping Aircraft, ifa Submarine was within range of these Buoys, it could be tracked for course and approximate speed.

During my T.A.S. course in U.K. over 1947 and into 194g, I spent lime at Londonderry in Northern Ireland in R.A.F. Liberators, dropping Sono Buoy patterns over suspected Submarine positions, and monitoring the results, it was quite exciting to hear tlte noise level from the target when one happened to drop a pattern over the Submarine, and theit track it, and calculate its course and speed reasonably correctly.

The mission of the US Navy's Mobile Inshore Undersea Warfare units is to provide surface and subsurface surveillance in littoral areas throughout the world. Secondary mission capabilities include command, control and communication functions. Naval Reserve MIUW units provide the sole capability for this mission within the United States Navy.