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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 ...
Some channels between 123.100 and 135.950 are available in the US to other users such as government agencies, commercial company advisory, search and rescue, military aircraft, glider and ballooning air-to-ground, flight test and national aviation authority use. A typical transmission range of an aircraft flying at cruise altitude (35,000 ft ...
The civilian aircraft frequency for voice distress alerting is 121.5 MHz. Military aircraft use 243 MHz (which is a harmonic of 121.5 MHz, and therefore civilian beacons transmit on this frequency as well).
The transmitter component was the BC-778. The frequency was 500 kHz at 4.8 watts, giving it a range of 200 miles (300 km; 200 nmi). Keying could be automatic SOS (including the 4-second long dash for autoalarm), or manual. Crystals for frequency control were a scarce item for the U.S. during the war and the SCR-578 was not crystal-controlled.
This frequency is monitored by all U.S. Navy ASW aircraft assigned to a SAR mission. 282.8 MHz— Joint/combined on-the-scene voice and DF frequency used throughout NATO 406 MHz / 406.1 MHz - Cospas-Sarsat international satellite-based search and rescue (SAR) distress alert detection and information distribution system
121.5 MHz VHF ± 6 kHz (frequency band protected to ±50 kHz) [66] (Satellite detection ceased on 1 February 2009, [67] but this frequency is still used for short-range location during a search and rescue operation) 243.0 MHz UHF ± 12 kHz (frequency band protected to ± 100 kHz) [66] [68] (prior to 1 February 2009 – COSPAS-SARSAT Compatible)
All AN/PRC-113s contain a "Guard Receiver" tuned to the military rescue/survival frequency of 243 MHz but not to the civilian counterpart frequency of 121.5 MHz. [4] These radios have been phased out and replaced by the newer manpack ASIP and AN/PRC-148 MBITR radios that cover everything between 2 and 512 MHz. [4]
Multi-frequency signaling (MF) is a group of signaling methods that use a mixture of two pure tone (pure sine wave) sounds. Various MF signaling protocols were devised by the Bell System and CCITT .