The Unerwater Locator 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. When triggered by water immersion the ULB emits an ultrasonic pulse of 37.5 kHz at an interval of once per second. This signal can be heard and located by a variety of receivers. The beacon will operate for 30 days to a depth of 20000 feet (6 km).