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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 ...
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.
Pan-pan calls may be made on the aircraft emergency frequency, but they are more often made on the frequency already in use, or another appropriate frequency. ICAO Annex 10, Volume V, § 4.1.3.1.1 states "the emergency channel (121.5 MHz) shall be used only for genuine emergency purposes".
A typical aircraft VHF radio. The display shows an active frequency of 123.5 MHz and a standby frequency of 121.5 MHz. The two are exchanged using the button marked with a double-headed arrow. The tuning control on the right only affects the standby frequency.
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). Aircraft can also signal an emergency by setting one of several special transponder codes, such as 7700.
This frequency is also used for direction finding (DF). 155.160 MHz; 172.5 MHz— U.S. Navy emergency sonobuoy communications and homing use. 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
Prior to the founding of Cospas-Sarsat, the civilian aviation community had already been using the 121.5 MHz frequency for distress, while the military aviation community utilized 243.0 MHz as the primary distress frequency with the 121.5 MHz frequency as an alternate.
Radio direction finding is used in radio navigation for ships and aircraft, to locate emergency transmitters for search and rescue, for tracking wildlife, and to locate illegal or interfering transmitters. During the Second World War, radio direction finding was used by both sides to locate and direct aircraft, surface ships, and submarines.