Underwater locator beacon
|Term||underwater locator beacon|
underwater acoustic beacon
a device fitted to aviation flight recorders such as the cockpit voice recorder and flight data recorder.
SKYBRARY (aviation safety knowledge reference tool) 
ULBs are also sometimes required to be attached directly to an aircraft fuselage. When triggered by water immersion, the ULB emits an ultrasonic pulse of 37.5 kHz at an interval of once per second, for a duration of at least 30 days.
The devices are designed not only to survive accidents, but to function correctly after impact.