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Underwater Locator Beacon (ULB)
From SKYbrary Wiki
An underwater locator beacon (ULB) or underwater acoustic beacon is a device fitted to aviation flight recorders such as the cockpit voice recorder and flight data recorder. ULBs are also sometimes required to be attached directly to an aircraft fuselage. Once activated by immersion in water, a ULB is designed to emit an ultrasonic pulse at 37.5 kHz every second for a duration of at least 30 days.
The devices are designed not only to survive accidents, but to function correctly after impact. Research carried out for the BEA and published in 2011 showed that they have had a 90% survival rate spanning 27 air accidents over the sea.