Identification Friend or Foe - Method of Operation

Method of Operation

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 because of its similarity to 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 a military aircraft, vehicle or unit by returning a coded reply signal only when the incoming interrogation is identified as part of the friendly forces network;
• if no IFF response is generated, a civil (Selective Identification Feature, SIF) interrogation may then be generated and the aircraft, by returning various mode replies, can then be identified or sorted.

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). This is 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 three-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 if possible.

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 it does not reply. Very often pilots can have the wrong code (encryption key) selected, or the code has 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.