A sonobuoy (a portmanteau of sonar and buoy) is a relatively small (typically 5 inches or 13 centimetres, in diameter and 3 ft or 91 cm long) expendable sonar system that is dropped/ejected from aircraft or ships conducting anti-submarine warfare or underwater acoustic research.
Theory of Operation 
The buoys are ejected from aircraft in canisters and deploy upon water impact. An inflatable surface float with a radio transmitter remains on the surface for communication with the aircraft, while one or more hydrophone sensors and stabilizing equipment descend below the surface to a selected depth that is variable, depending on environmental conditions and the search pattern. The buoy relays acoustic information from its hydrophone(s) via UHF/VHF radio to operators on board the aircraft.
With the technological improvement of the submarine in modern warfare, the need for an effective tracking system was born. Sound Navigation And Ranging (SONAR) was originally developed by the British—who called it ASDIC—in the waning days of World War I. At the time the only way to detect submarines was by listening for them (passive sonar), or visually by chance when they were on the surface recharging their battery banks or by massive air patrols with lumbering airships and biplanes. Sonar saw extremely limited use and was mostly tested in the Atlantic Ocean with few naval officers seeing any merit in the system. With the end of WWI came the end to serious development of sonar in the US, a fact that was to be fatal in the early days of World War II. However, considerable development of ASDIC took place in the UK, including integration with a plotting table and weapon.
The ravaging wolf-packs of U-boats in WWII made the need for sonar a priority. With millions of tons of shipping being sunk in the Atlantic, there was a need to locate submarines so that they could be sunk or prevented from attacking. Sonar was installed on a number of ships along with Radar and High-frequency direction finding ("Huff-Duff") to detect surfaced submarines. While sonar was a primitive system, it was constantly improved.
Modern anti-submarine warfare grew from the WWII convoy and battle group movement through hostile waters. It was imperative that submarines be detected and neutralized long before the task group came within range of an attack. Aircraft-based submarine detection was the obvious solution. The maturity of radio communication and sonar technology made it possible to combine a sonar transducer, batteries, a radio transmitter and whip antenna, within a self-contained air-deployed floating (sono)buoy.
Early sonobuoys had limited range, limited battery life and were overwhelmed by the noise of the ocean. They first appeared during WWII where they were first used in July 1942 by RAF Coastal Command under the code name 'High Tea', the first squadron to use them operationally being No. 210 Squadron RAF, operating Sunderlands. They were also limited by the use of human ears to discriminate man-made noises from the oceanic background. However, they demonstrated that the technology was viable. With the development of better hydrophones, the transistor and miniaturization, and the realisation that very low frequency sound was important, more effective acoustic sensors followed. The sonobuoy went from being an imposing six feet tall, two feet diameter sensor to the compact suite of electronics it is today.
Sonobuoys are classified into three categories: active, passive and special purpose.
- Active sonobuoys emit sound energy (pings) into the water and listen for the returning echo before transmitting—usually range and bearing—information via UHF/VHF radio to a receiving ship or aircraft. The original active sonobuoys pinged continuously after deployment for a predetermined period of time. Later, Command Activated Sonobuoy System (CASS) sonobuoys allowed the aircraft to trigger pings (or buoy scuttling) via a radio link. This evolved into DICASS (Directional CASS) in which the return echo contained bearing as well as range data.
- Passive sonobuoys emit nothing into the water, but rather listen, waiting for sound waves (for instance, power plant, propeller or door-closing and other noises) from ships or submarines, or other acoustic signals of interest, to reach the hydrophone. The sound is then transmitted via UHF/VHF radio to a receiving ship or aircraft.
- Special purpose sonobuoys relay various types of oceanographic data to a ship, aircraft, or satellite. There are three types of special-purpose sonobuoys in use today. These sonobuoys are not designed for use in submarine detection or localization.
- BT—The bathythermobuoy (BT) relay bathythermographic or salinity readings, or both, at various depths.
- SAR—The search and rescue (SAR) buoy is designed to operate as a floating Radio frequency beacon. As such, it is used to assist in marking the location of an aircraft crash site, a sunken ship, or survivors at sea.
- ATAC/DLC—Air transportable communication (ATAC) and down-link communication (DLC) buoys, such as the UQC, or "gertrude", are intended for use as a means of communication between an aircraft and a submarine, or between a ship and a submarine.
This information is analysed by computers, acoustic operators and TACCOs to interpret the sonobuoy information.
Active and/or passive sonobuoys may be laid in large fields or barriers for initial detection. Active buoys may then be used for precise location. Passive buoys may also be deployed on the surface in patterns to allow relatively precise location by triangulation. Multiple aircraft or ships monitor the pattern either passively listening or actively transmitting to drive the submarine into the sonar net. Sometimes the pattern takes the shape of a grid or other array formation and complex beamforming signal processing is used to transcend the capabilities of single, or limited numbers of, hydrophones.
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