A sonobuoy (a portmanteau of sonar and buoy) is a relatively small buoy – typically diameter and long – expendable sonar system that is dropped/ejected from aircraft or ships conducting anti-submarine warfare or underwater acoustic research. Theory of operation Sonobuoys 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. History 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. Air patrols (the British mostly used small airships which had the advantage of long endurance) could spot surfaced submarines and occasionally, when conditions were right, even submerged ones as the diving depth of submarines of the era was so limited. If contact was made, they would follow the submarine while summoning surface ships by radio to attack it. 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 World War I came the end to serious development of sonar in the United States, a fact that was to be fatal in the early days of World War II. However, considerable development of ASDIC took place in the United Kingdom, including integration with a plotting table and weapon. While the United Kingdom pursued the development of sonar during the interwar period, the United States Coast and Geodetic Survey during the 1920s developed the radio acoustic ranging method of fixing the position of survey ships during hydrographic survey operations by detonating a small explosive at the location of the ship, recording the time it took for the sound of the explosion to reach distant hydrophones mounted at shore stations or aboard crewed station ships, and radioing the time of receipt of