Sonobuoy

Sonobuoy being loaded onto a USN P-3C Orion aircraft
Hand deployment of a sonobuoy in the Arctic Ocean from the aft deck of the R/V Sikuliaq

A sonobuoy (a portmanteau of sonar and buoy) is a small expendable sonar buoy dropped from aircraft or ships for anti-submarine warfare or underwater acoustic research. Sonobuoys are typically around 13 cm (5 in) in diameter and 91 cm (3 ft) long. When floating on the water, sonobuoys have both a radio transmitter above the surface and hydrophone sensors underwater.