Identification friend or foe

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An IFF test set used by a United States Air Force avionics technician Technical Sergeant for testing transponders on aircraft
Model XAE IFF kit, the first radio recognition IFF system in the U.S.

Identification, friend or foe (IFF) is an identification system designed for command and control. It enables military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. IFF may be used by both military and civilian aircraft. IFF was first developed during the Second World War, with the arrival of radar, and several infamous friendly fire incidents.

Despite the name, IFF can only positively identify friendly targets, not hostile ones.[1][2][3][4] If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly, but is not positively identified as foe. There are in addition many reasons that friendly aircraft may not properly reply to IFF.

IFF is a tool within the broader military action of Combat Identification (CID), "the process of attaining an accurate characterization of detected objects in the operational environment sufficient to support an engagement decision." The broadest characterization is that of friend, enemy, neutral, or unknown. CID not only can reduce friendly fire incidents, but also contributes to overall tactical decision-making.[5]

History[edit]

With the successful deployment of radar systems for air defence during World War II, combatants were immediately confronted with the difficulty of distinguishing friendly aircraft from hostile ones; by that time, aircraft were flown at high speed and altitude, making visual identification impossible, and the targets showed up as featureless blips on the radar screen. This led to incidents such as the "Battle of Barking Creek", over Britain,[6][7][8] and the "air attack on the fortress of Koepenick", over Germany.[9][10]

Britain and USA[edit]

Radar coverage of the Chain Home system by 1939

Early concepts[edit]

Already before the deployment of their Chain Home radar system (CH), the RAF had considered the problem of IFF. Robert Watson-Watt had filed patents on such systems in 1935 and 1936. By 1938, researchers at Bawdsey Manor began experiments with "reflectors" consisting of dipole antennas tuned to resonate at the primary frequency of the CH radars. When a pulse from the CH transmitter hit the aircraft, the antennas would resonate for a short time, increasing the amount of energy returned to the CH receiver. The antenna was connected to a motorized switch that periodically shorted it out, preventing it from producing a signal. This caused the return on the CH set to periodically lengthen and shorten as the antenna was turned on and off. In practice, the system was found to be too unreliable to use; the return was highly dependent on the direction the aircraft was moving relative to the CH station, and often returned little or no additional signal.[11]

It was suspected this system would be of little use in practice, and when that turned out to be the case the RAF turned to an entirely different system that was also being planned. This consisted of a set of tracking stations using HF/DF radio direction finders. Their aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the tracking stations ample time to measure the aircraft's bearing. Several such stations were assigned to each "sector" of the air defence system, and sent their measurements to a plotting station at sector headquarters, who used triangulation to determine the aircraft's location. Known as "pip-squeak",[12] the system worked but was labour-intensive and did not display its information directly to the radar operators. A system that worked directly with the radar was clearly desirable.

IFF Mark II[edit]

The first active IFF transponder (transmitter/responder) was the IFF Mark I which was used experimentally in 1939. On receipt of a signal from the CH radar (20-30 MHz), an oscillator in the system began to ring with the same frequency. This signal was amplified and sent out an omnidirectional monopole antenna, where it was received by the CH station. The oscillator circuit rang only for a short time, causing the signal to quickly disappear again. Since the signal was received at the same time as the original reflection of the CH signal, the result was a distorted "blip" on the CH display which was easily identifiable. In testing, it was found that the unit went would often overpower the radar or produce too little signal to be seen, and at the same time, new radars were being introduced using new frequencies.

Instead of putting Mark I into production, a new IFF Mark II was introduced in early 1940. Mark II had a series of separate tuners inside tuned to different radar bands that it stepped through using a motorized switch, while an automatic gain control solved the problem of it sending out too much signal. Mark II was technically complete as the war began, but a lack of sets meant it was not available in quantity and only a small number of RAF aircraft carried it by the time of the Battle of Britain. Pip-squeak was kept in operation during this period, but as the Battle ended, IFF Mark II was quickly put into full operation. Pip-squeak was still used for areas over land where CH did not cover, as well as an emergency guidance system.[13]

IFF Mark III[edit]

Even by 1940 the complex system of Mark II was reaching its limits, and a number of sub-models were introduced covering different combinations of radars, common naval ones for instance. But the introduction of the cavity magnetron and radars using it in the microwave range rendered this obsolete, there was simply no way to make a responder operating in this band. In 1940, English engineer Freddie Williams began work on the IFF Mark III system at the Telecommunications Research Establishment, which was to become the standard for the Western Allies for most of the war.

Mark III transponders were designed to respond to specific 'interrogators', rather than replying directly to received radar signals. These interrogators worked on a limited selection of frequencies, no matter what radar they were paired with. The system also allowed limited communication to be made, including the ability to transmit a coded 'Mayday' response. The IFF sets were designed and built by Ferranti in Manchester to Williams' specifications. Equivalent sets were manufactured in the US, initially as copies of British sets, so that allied aircraft would be identified upon interrogation by each other's radar.[13]

Germany[edit]

Code generator from German WW II IFF-Radio FuG 25a Erstling

FuG 25a "Erstling" (English: "Firstborn", "Debut") was developed in Germany in 1940. It had two bands tuned to the low-VHF band at 125 MHz used by the Freya radar and the low-UHF-banded 550–580 MHz used by Würzburg).

Before flight, the transceiver was set up with a selected day code of ten bits which was dialled into the unit. To start the identification procedure, the ground operator switched the pulse frequency of his radar from 3,750 Hz to 5,000 Hz. The airborne receiver decoded that and started to transmit the day code. The radar operator would then see the blip lengthen and shorten in the given code, ensuring it was not being spoofed. The IFF transmitter worked on 168 MHz with a power of 400 watts (PEP).

Although the system included a way for ground controllers to determine whether an aircraft had the right code or not, it did not include a way for the transponder to reject signals from other sources. British military scientists found a way of exploiting this by building their own IFF transmitter called "Perfectos", which were designed to trigger a response from any FuG 25a system in the vicinity.

When an FuG 25a responded on its 168 MHz frequency, the signal was received by the antenna system from an AI Mk. IV radar, which originally operated at 212 MHz. By comparing the strength of the signal on different antennas the direction to the target could be determined. Mounted on Mosquitos, the "Perfectos" severely compromised German use of the FuG 25a.

Further wartime developments[edit]

IFF Mark IV and V[edit]

The United States Naval Research Laboratory had been working on their own IFF system since before the war. It used a single interrogation frequency, like the Mark III, but a separate responder frequency. Responding on a different frequency has several practical advantages, but requires a complete radio transmitter for the responder side of the circuitry. This technique is now known as a cross-band transponder.

When the Mark II was revealed in 1941 during the Tizard Mission, it was decided to use that instead, and further improve their experimental system. The result was what became the Mark IV. The main difference between this and earlier models is that it worked on higher frequencies, around 600 MHz, which allowed much smaller antennas. Unfortunately, this also turned out to be close to the frequencies used by the German Würzburg radar, and there were concerns that it would be triggered by that radar and the transponder responses would be picked on its radar display and thereby give away the operational frequencies.

This led to a US-British effort to make a further improved model, the Mark V, also known as the United Nations Beacon, or UNB. This moved to still higher frequencies around 1 GHz, but operational testing was not complete when the war ended. By the time testing was finished in 1948, the much improved Mark X was beginning its testing, and Mark V was abandoned.

Postwar systems[edit]

IFF Mark X[edit]

Mark X started as a purely experimental device operating at frequencies above 1 GHz, but as development continued it was decided to introduce an encoding system known as the "Selective Identification Feature", or SIF. SIF allowed the return signal to contain up to 12 pulses, representing four octal digits of 3 bits each. Depending on the timing of the interrogation signal, SIF would respond in several ways. Mode 1 indicated the type of aircraft or its mission (cargo, for instance) while Mode 2 returned a tail code.

Mark X began to be introduced in the early 1950s. This was during a period of great expansion of the civilian air transport system, and it was decided to use slightly modified Mark X sets for these aircraft as well. These sets included a new Mode 3 which was paired with a civilian Mode A, which operated similar to the original Mode 2 and returned a four-digit identifier. Because Mode 3 and A are identical, they are normally referred to as Mode 3/A. Mode C returned the altitude encoded in a single 12-bit number in Gillham code, which represented the altitude as (that number) x 100 feet - 1200. Mode B and D were specified but never used.

IFF Mark XII[edit]

The current IFF system is the Mark XII. This works on the same frequencies as Mark X, and supports all of its military and civilian modes.

The main reason for the creation of Mark XII was the addition of the military Mode 4. Before Mark XII, the transponders would respond to any properly formed interrogation signal, broadcasting a reply that could be picked up by any receiver. Using triangulation, an enemy could determine the location of the transponder. The British had already used this during WWII, and it was used by the USAF against VPAF aircraft during the Vietnam War.

Mode 4 started with an interrogation similar to Mode 3, but then followed that with an encoded pulse chain similar to the one used in Mode 3/A. The receiver side of the transponder checks this code against a known day code, and only responds if the two match. The pulses in the reply are delayed based on the received code. This largely eliminates the ability for the enemy to trigger the transponder.

During the 1980s, a new civilian mode, Mode S, was added that allowed greatly increased amounts of data to be encoded in the returned signal. This was used to encode the location of the aircraft from the navigation system. This is a basic part of the traffic collision avoidance system (TCAS) system that allows commercial aircraft to know the location of other aircraft in the area and avoid them without the need for ground operators.

The basic concepts from Mode S were then militarized as Mode 5, which is simply a cryptographically encoded version of the Mode S data.

The IFF of World War II and Soviet military systems (1946 to 1991) used coded radar signals (called Cross-Band Interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft illuminated by the radar. Radar-based aircraft identification is also called secondary radar in both military and civil usage, with primary radar bouncing an RF pulse off of the aircraft to determine position. George Charrier, working for RCA, filed for a patent for such an IFF device in 1941. It required the operator to perform several adjustments to the radar receiver to suppress the image of the natural echo on the radar receiver, so that visual examination of the IFF signal would be possible.[14]

By 1943, Donald Barchok filed a patent for a radar system using the acronym IFF in his text with only parenthetic explanation, indicating that this acronym had become an accepted term.[15] In 1945, Emile Labin and Edwin Turner filed patents for radar IFF systems where the outgoing radar signal and the transponder's reply signal could each be independently programmed with a binary codes by setting arrays of toggle switches; this allowed the IFF code to be varied from day to day or even hour to hour.[16][17]

Early 21st century systems[edit]

NATO[edit]

The United States and other NATO countries started using a system called Mark XII in the late twentieth century; Britain had not until then implemented an IFF system compatible with that standard, but then developed a program for a compatible system known as successor IFF (SIFF).[18]

Modes[edit]

  • Mode 1 – military only; provides 2-digit octal "mission code" that identifies the aircraft type or mission.[19]
  • Mode 2 – military only; provides 4-digit octal unit code or tail number. (usually can't be changed in flight. Some aircraft like the C-17 Block 17 and higher have the capability to do so)[19]
  • Mode 3/A – military/civilian; provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller.[19]
  • Mode 4 – military only; provides a 3-pulse reply, delay is based on the encrypted challenge.[19]
  • Mode 5 – military only; provides a cryptographically secured version of Mode S and ADS-B GPS position.[19]
Notes

Modes 4 and 5 are designated for use by NATO forces.

See also[edit]

References[edit]

  1. ^ "Combat Identification IFF Systems" (PDF). Tellumat. Retrieved 13 July 2016. [permanent dead link]
  2. ^ "MEADS System Gains Full Certification for Identifying Friend or Foe Aircraft". Lockheed Martin. Retrieved 31 May 2015. 
  3. ^ "Identification Friend or Foe". Global Security. Retrieved 31 May 2015. 
  4. ^ "Combat Identification (IFF)". BAE Systems. Retrieved 31 May 2015. 
  5. ^ "Joint Publication (JP) 3-09, Joint Fire Support" (PDF). US DoD. 30 June 2010. p. III-20. Retrieved 27 December 2013. 
  6. ^ Christopher Yeoman & John Freeborn, Tiger Cub – The Story of John Freeborn DFC* A 74 Squadron Fighter Pilot In WWII, Pen and Sword Aviation, 2009, ISBN 978-1-84884-023-2, p45
  7. ^ Bob Cossey, A Tiger's Tale: The Story of Battle of Britain Fighter Ace Wg. Cdr. John Connell Freeborn, ISBN 978-1-900511-64-3, chapter 4
  8. ^ Hough, Richard and Denis Richards. The Battle of Britain: The Greatest Air Battle of World War II, WW Norton, 1990, p.67
  9. ^ Galland, Adolf : The First and the Last p 101(1954 reprinted ..) ISBN 978 80 87888 92 6
  10. ^ Price, Alfred : Battle Over the Reich pp95-6(1973) ISBN 0 7110 0481 1
  11. ^ "General IFF principles". United States Fleet. 1945. Retrieved 2012-12-17. 
  12. ^ "The British invention of radar". Retrieved 2012-12-17. 
  13. ^ a b Lord Bowden (1985). "The story of IFF (identification friend or foe)". Physical Science, Measurement and Instrumentation, Management and Education - Reviews, IEE Proceedings A. 132 (6). Retrieved 13 July 2014. 
  14. ^ George M. Charrier, Recognition System for Pulse Echo Radio Locators, U.S. Patent 2,453,970, granted Nov. 16, 1948.
  15. ^ Donald Barchok, Means for Synchronizing Detection and Interrogation Systems, U.S. Patent 2,515,178, granted July 18, 1950.
  16. ^ Emile Labin, Magnetostrictive Time-Delay Device, U.S. Patent 2,495,740, granted Jan. 31, 1950.
  17. ^ Edwin E. Turner, Coded Impulse Responsive Secret Signalling System, U.S. Patent 2,648,060, granted Aug. 4, 1953.
  18. ^ [1]
  19. ^ a b c d e NATO STANAG 4193

External links[edit]