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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 ...
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]
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.
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).
All have migrated from using 121.500 MHz as their primary frequency to using 406 MHz, which was designed for satellite detection and location, however most models still broadcast a secondary signal on 121.5 MHz as well, as this helps rescue teams pinpoint the location of survivors once in their vicinity with more accuracy (within 2km) than the ...
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 .
2182 kHz is analogous to channel 16 on the marine VHF band, but unlike VHF which is limited to ranges of about 20 to 50 nautical miles (40 to 90 km) depending on antenna height, [3] communications on 2182 kHz and nearby frequencies have a reliable range of around 50 to 100 nautical miles (90 to 190 km) during the day and 150 to 300 nautical miles (280 to 560 km) or sometimes more at night.