IFF (Identification Friend or Foe)

IFF (Identification Friend or Foe)

Identification friend or foe (IFF) systems are methods of identifying aircraft using electronic means. Applied by both military and civilian entities, IFF—which in its civilian form is more properly known as the air traffic control radar beacon system, or ATCRBS—uses radar to identify aircraft, which are assigned unique identifier codes. There are various modes of operation for IFF, depending on the level of security desired.

Challenge and response. In World War II, as radar was emerging, Allied aviators and ground controllers soon became aware of a shortcoming in the existing electronic identification system; radar could only recognize that a plane was in the sky, but could not differentiate friendly planes from those of the Axis. The Germans were the first to develop a crude IFF system, which required pilots to roll their planes in midflight as a means of creating a distinctive radar blip that would identify them as Luftwaffe craft to radar operators. The Allies developed their own active systems: first Mk I in 1940, and later the much more effective Mk III, which greatly enhanced identification technology by adding a separate transmitter that tuned through radar bands even as the receiver in the air did the same. Mk III also was made to respond to as many as six different codes.

From the beginning, IFF was a system of challenge and response, operating on much the same principal as a guard demanding a password before allowing entrance. In the case of IFF, the radar on the ground (the interrogator) transmits on one frequency, and receives a coded signal from the plane's transponder on another frequency. In the United States, these transmissions typically take place at 1030 megahertz (MHz) for challenges, and 1090 MHz for replies.

Security and modes of operation. There are several modes of operation, as well as an important submode, relating to levels of security in military IFF operation. Mode 1, for instance, is a nonsecure, low-cost mode used by ships to track aircraft and other ships, while Mode 2 is used by aircraft making carrier-controlled approaches to ships during times of inclement weather.

Commercial aircraft chiefly use Mode 3, the standard system by which they relay positions to ground controllers across the globe. The Federal Aviation Administration (FAA) requires that all aircraft, military or civilian, that fly at 10,000 feet or higher must be equipped with working IFF transponder systems capable of reporting altitude. To do so requires Mode C, a submode that automatically includes an altitude report.

At the highest security level is Mode 4, the only true IFF system, as opposed to a mere means of differentiating obviously friendly aircraft in order to enable greater control of traffic from the ground. This mode, used on warfighting planes, utilizes sophisticated encryption that includes a long challenge word with a preamble to inform the transponder that it is about to receive a secure message. If the plane's transponder is incapable of deciphering the challenge, it effectively identifies the aircraft as something other than a friend. Key codes are periodically changed and reentered into transponders and interrogators so as to ensure the continued security of codes.



Launius, Roger D. Innovation and the Development of Flight. College Station: Texas A&M University Press, 1999.

Murray, Williamson, and Allan Reed Millett. Military Innovation in the Interwar Period. New York: Cambridge University Press, 1996. Rihaczek, August W., and Stephen J. Hershkowitz. Theory and Practice of Radar Target Identification. Boston: Artech House, 2000.


Identification Friend or Foe (IFF) Systems. 551st and 552nd AEW&C Wings, U.S. Air Force. < http://www.dean-boys.com/extras/iff/iffqa.html > (March 16, 2003).


Codes and Ciphers
Electromagnetic Spectrum
FAA (United States Federal Aviation Administration)
Radio, Direction Finding Equipment

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