A sonobuoy -short for sonar-buoy- is a relatively small expendable sonar system that is dropped from military aircraft performing anti-submarine warfare. The buoys are ejected from aircraft in canisters and deploy upon water impact. An infatable surface float with a radio transmitter rises to the surface for communication with the aircraft, while hydrophone(s) sensors and stabilizing equipment descend below the surface to a depth that is variable depending on environmental conditions and the search pattern. The buoy relays acoustic information from its hydrophone(s) via radio frequency to operators onboard the aircraft.

History: searching the seas
Sonobuoys were invented during World War II to help locate and track submarines. Since the oceans are so large, and Navy vessels so few, sonobuoys were introduced to conduct broad searches cheaply and quickly. The Maritime Patrol Aircraft (MPA) also known patrol planes, drop sonobuoys by the dozens over tens of nautical miles and can secure large areas at a time, in remote regions of the oceans. The Cold War motivated significant improvements and development of different sonobuoy types. The technology has developed from a simple passive hydrophone attached to a radio transmitter with battery monitored aurally by a human operator, to complex systems with active sonar capability, multiple hydrophones arranged in arrays, on-board signal processing and more.

Concept of Operation
Sonobuoys extend the Navy fleet sonar capabilities out from the vessels to distances an aircraft can cover.

Buoys are deployed on the surface in patterns to allow precise location by triangulation. Sometimes the pattern takes the shape of a grid or other array formation and complex signal processing is used to transcend the capabilities of single hydrophones.

There are several different kinds of sonobuoys, with one major distinction being active sonar or passive sonar.

Other air deployed sensors include bathythermograph, which provides temperature and thermal gradient information of the water column, also communication devices for transmitting information to friendly submarines.