The aircraft emergency frequency (also known as GUARD) is a

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

used on the aircraft band reserved for emergency

communications Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...

for aircraft in distress. The frequencies are 121.5

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

for civilian, also known as International Air Distress (IAD) or VHF Guard, and 243.0 MHz—the second harmonic of VHF guard—for military use, also known as Military Air Distress (MAD) or UHF Guard. Earlier emergency locator transmitters (ELTs) used the guard frequencies to transmit, but an additional frequency of 406 MHz is used by a more modern emergency locator transmitter.

History

The choice of 121.5 MHz was made by

ICAO The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international a ...

in conjunction with

ARINC Aeronautical Radio, Incorporated (ARINC), established in 1929, was a major provider of transport communications and systems engineering solutions for eight industries: aviation, airports, defense, government, healthcare, networks, security, and t ...

and the

ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...

Monitoring

In the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

, the emergency frequency is monitored by most

air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...

towers, FSS services, national air traffic control centers, military air defense and other flight and emergency services, as well as by many commercial aircraft. The

notice to airmen A Notice to Airmen/Notice to Air Missions (NOTAM), is a notice filed with an aviation authority to alert aircraft pilots of potential hazards along a flight route or at a location that could affect the flight. NOTAMs are unclassified notices or ...

FDC 4/4386 requires "…all aircraft operating in United States National Airspace, if capable, shall maintain a listening watch on VHF GUARD 121.5 or UHF 243.0." In the UK, 121.5 MHz is monitored by the

Royal Air Force The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...

Distress and Diversion cells (known as "D&D") at the

London Terminal Control Centre The London Terminal Control Centre (LTCC) was an air traffic control centre based in West Drayton, in the London Borough of Hillingdon, England, approximately north of London Heathrow Airport. Operated by National Air Traffic Services (NATS) it ...

and the Shanwick Oceanic Control, from a nationwide network of antennas. Depending on the aircraft's altitude and location, the personnel in the centres may be able to use

triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle me ...

to determine its exact position which can be useful to the pilot if the aircraft is lost or "temporarily unsure of position".

Use

Both guard frequencies can be used by any aircraft in distress or experiencing an emergency and in addition it can be used by

to warn aircraft if they are about to fly into restricted or

prohibited airspace A prohibited airspace is an area (volume) of airspace within which flight of aircraft is not allowed, usually due to security concerns. It is one of many types of special use airspace designations and is depicted on aeronautical charts with the l ...

. Aircraft will also be contacted on 121.5 MHz when intercepted by air defence aircraft, to ask for identification and intentions and to pass on instructions.

Locating beacons

Older emergency locator transmitters transmit on 121.5 MHz 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. Beacons operating at 406 MHz are encoded, allowing the vessel of origin to be determined and false alarms to be quickly verified. Satellite support for the 121.5 MHz–only versions was discontinued in early 2009.

References

External links

{{DEFAULTSORT:Aircraft Emergency Frequency Air traffic control Airbands Emergency communication International telecommunications Rescue ja:航空無線機#周波数および出力

History

Monitoring

Use

Locating beacons

See also

References

External links