In telecommunications, identification, friend or foe (IFF) is an identification system designed for command and control. It is a system that enables military and national (civilian-located ATC) interrogation systems to identify aircraft, vehicles, or forces as friendly and to determine their bearing and range from the interrogator.
IFF was first developed during World War II. The term is somewhat of a misnomer, as IFF can only positively identify friendly targets but not hostile ones. If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly, but is not positively identified as a foe.
There are many reasons for friendly aircraft not to reply to IFF, such as battle damage or equipment failure, loss of encryption keys, and wrong encryption keys. Terrain-hugging aircraft are very often poor candidates for microwave line-of-sight systems such as the IFF system. Microwaves cannot penetrate terrain, and very often atmosphere effects (referred to as anomalous propagation) cause timing, range, and azimuth issues.
Contents |
The world's first IFF, FuG-25a "Erstling" (English: "Debut"), was developed in Germany in 1940. It received the radar frequencies on 125 MHz (Freya radar) and 550-580 MHz (Würzburg radar). 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 its code. Before departure, 2 mechanical keys of 10 bits each were inserted into the shown reader. The IFF transmitter worked on 168 MHz with a power of 400 Watt (PEP). Unfortunately for the Germans, British intelligence were able to build their own IFF transmitter, called "Perfectos". When mounted into a RAF Mosquito, it could trigger the FuG-25 and therefore betrayed the position of the nightfighters. To avoid that, the FuG-25 had to be switched off very often.
IFF is used by both military and civilian aircraft. Modes 1, 2, 4 and 5 are for military use only. Modes 1, 2 and 3 are collectively known as Selective Identification Feature (SIF) modes. Civilian aircraft use modes A, C and S. Mode C which includes barometric pressure altitude information is often used in conjunction with mode A. Mode A is often referred to as mode 3/A due to the similarity to the military mode 3. Mode S is a new civilian mode developed to replace both mode A and C.
IFF is also called secondary radar, with primary radar bouncing an RF pulse off of the aircraft to determine position. Position with IFF is determined by comparing antenna dish angle and the delay from the interrogator (1,030 MHz) pulse to the received IFF pulses on (1,090 MHz).
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. Modern IFF systems use a separate specialized transponder beacon which can operate without radar. They are referred to as cross-band beacon or transponders.
An IFF transponder responds:
In an IFF network both the interrogation and the reply are verified as friendly.
Each IFF transponder also has a KIR or KIT cryptography computer associated with it. The KIR (designed for interrogators) and the KIT (designed for transponders) have an access port where the encryption keys are inserted. The military IFF system will not function without a valid key. Civilian SIF systems and mode S do not require encryption keys.
An IFF transponder receives interrogation pulses at one frequency (1,030 MHz), and sends the reply pulses at a different frequency (1,090 MHz). Just the opposite of a SIF interrogation, which is composed of two pulses spaced apart by a different amount for each mode, with the transponder reply being a long series of bits; the IFF interrogation is instead a long series of bits that contains the encrypted message and parity, and the reply is just three pulses.
The IFF message is encrypted with a secret key. IFF transponders with the same secret key will be able to decode the IFF message. Once decoded, the IFF transponder will execute the message and send back a 3 pulse reply. The interrogator then compares each reply to the challenge messages, and marks these targets friendly while also storing their azimuth and range.
A second possibility is a target being marked as a spoof target. That is, the target replies, but fails to process the IFF message correctly on a significant number of challenges. Targets marked as a spoofer can be declared hostile and, inside a battle-space, are often destroyed when able.
If no reply is received from the IFF transponder, the target continues to be an unknown. The IFF system is not used to declare a target hostile if they do not reply. Very often the pilot can have the wrong code (encryption key) selected, or the code is expired, and they will have an audible and visual alarm every time they are interrogated by IFF. If they can't clear the alarm they follow the pre-briefed safe passage procedures.
The major military benefits of IFF include preventing "friendly fire" and being able to positively identify friendly forces.
Mode 1 – provides 2-digit 5-bit mission code. (military only – cockpit selectable)[1]
Mode 2 – provides 4-digit octal unit code. (military only – set on ground for fighters, can be changed in flight by transport aircraft)[1]
Mode 3/A – provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller. (military and civilian)[1]
Mode 4 – provides a 3-pulse reply to crypto coded challenge. (military only)[1]
Mode 5 – provides a cryptographically secured version of Mode S and ADS-B GPS position. (military only)[1] Mode 5 is divided into two levels. Both are crypto-secure with Enhanced encryption, Spread Spectrum Modulation, and Time of Day Authentication. Level 1 is similar to Mode 4 information but enhanced with an Aircraft Unique PIN. Level 2 is the same as Mode 5 level one but includes additional information such as Aircraft Position and Other Attributes.
Mode C – provides 4-digit octal code for aircraft's pressure altitude. (military and civilian)
Mode S – provides multiple information formats to a selective interrogation. Typically aircraft are assigned a unique 24-bit Mode S address. The Mode S address is partitioned and a group of address ranges are allocated to each country. Some countries change the assigned address for security reasons, and thus it might not be a unique address. (military and civilian)
Modes 4 and 5 are designated for use by NATO forces.