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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 ...
123.1 MHz: Aeronautical Auxiliary Frequency (International voice for coordinated SAR operations). 138.78 MHz— U.S. military voice SAR on-the-scene use. This frequency is also used for direction finding (DF). 155.160 MHz; 172.5 MHz— U.S. Navy emergency sonobuoy communications and homing use.
In the 25 kHz channel spacing scheme, an upper audio frequency of 12.5 kHz would be theoretically possible. [19] However, most airband voice transmissions never actually reach these limits. Usually, the whole transmission is contained within a 6 kHz to 8 kHz bandwidth, corresponding to an upper audio frequency of 3 kHz to 4 kHz. [19]
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)
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).
For military users, the Emergency has a miniaturized transmitter operating on the 243.0 MHz military aviation emergency frequency. Under normal conditions—flat terrain or calm seas—the signal can be picked up at a range of up to 90 nautical miles (170 km) by search aircraft flying at 20,000 feet (6,100 m). [ 2 ]
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 .
An emergency locator beacon is a radio beacon, a portable battery powered radio transmitter, used to locate airplanes, vessels, and persons in distress and in need of immediate rescue.