Aircraft emergency frequency

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The aircraft emergency frequency (also known in the USA as Guard) is a frequency used on the aircraft band reserved for emergency communications for aircraft in distress. The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD), International Aeronautical Emergency Frequency,^[1] or VHF Guard,^[1] and 243.0 MHz—the second harmonic of VHF guard—for military use, also known as Military Air Distress (MAD), NATO Combined Distress and Emergency Frequency,^[1] or UHF Guard.^[1] Earlier emergency locator transmitters (ELTs / EPIRBs) used the guard frequencies to transmit. As of February 1, 2009 satellite monitoring of the 121.5 and 243 MHz ELT (EPIRB) frequencies ceased, whereas an additional band from 406.0 to 406.1 MHz is now used exclusively by modern emergency locator transmitters (EPIRB).^[1]