The aircraft emergency frequency (also known as guard) is a frequency used on the aircraft radio band reserved for emergency communications for aircraft in distress. The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD) and 243.0 MHz for military use, also known as Military Air Distress (MAD). Both are in use at the international level.
In the U.S.A. 121.5 is monitored by most air traffic control towers, FSS services, national air traffic control centers, and other flight and emergency services. Separate frequencies exist for military and other government emergency frequencies.
In the UK, 121.5 is monitored by the Royal Air Force Distress and Diversion Cells (known as "D&D") at the London Terminal Control Centre and the Scottish Oceanic and Area Control Centre, from a nationwide network of antennas.
Older Emergency Locator Transmitters (ELTs) transmit on 121.5 in case of impact. Newer ELTs transmit on 406 MHz, with a low power beacon on 121.5 MHz for local homing. Satellites listen for the signals and alert local personnel to the emergency, and the beacon allows search and rescue to find the scene of the accident faster. 406 MHz beacons are encoded, allowing the vessel of origin to be determined and false alarms quickly verified. Satellite support for the 121.5-only versions is being phased out in early 2009.