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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. -source Sonobuoy wikipedia.