On 07 March 1942 aerial antisubmarine warfare took a leap with the demonstration of an aerial dropped, radio sonobuoy off New London, Connecticut. A K-5 blimp used a sonobuoy to detect the sound of a S-20 submarine. It could detect the submarine up to 3 miles away and radio its information to the blimp up to 5 miles away.
The use of aerial dropped sonobuoys greatly complicated the submarine's mission of not being detected. Normally, a sonobuoy is used to detect submarines by acquiring the sounds produced by propellers and machinery (passive detection) or by bouncing a sonar "ping" off the surface of the submarine (active detection). Specialized sonobuoys can detect electrical fields and magnetic anomalies, as well as measuring environmental parameters like water temperature versus depth, air temperature, barometric pressure, and wave height. Sonobuoys are generally dropped from aircraft that are equipped with a means to launch them, and electronic equipment to receive and process data sent by the sonobuoy.
Watch the deployment of a sonobuoy from a P-3 Orion aircraft below...
Last we have an "old" basic training Navy film, "Coordinated Anti-Submarine Warfare," it discusses how surface ships, aircraft and submarines work together to unravel the submarine threat. The film explains both passive and active sonar, and shows how sonobuoys and other technology are deployed to locate enemy submarines.