IFF (Identification Friend or Foe) is the military designation of the Air Traffic Control (ATC) System that is used to identify and track military aircraft. The IFF system consists of an airborne transponder and a ground (or airborne) interrogator. The system measures the distance and heading to the aircraft, and the transponder encodes identification and position information into the response.  IFF Mode 5 is the most recent implementation of the system.

 The IFF system’s data capabilities started with a single identification number in the 1940’s, and was expanded to include altitude reporting and cryptographically secure identification in the 1960’s. This design was referred to as the Mark XII standard and incorporated Mode 4 IFF technology which remains the current standard for U.S. and NATO countries. The 1980’s saw the introduction of Mode S, which allows aircraft specific selective data communications added to support additional air traffic data and the airborne collision avoidance system.

While traditional Secondary Surveillance Radar (SSR) stations interrogate all aircraft within their range, Mode S establishes selective and addressed interrogations with aircraft within its coverage.   Such selective interrogation improves the quality and integrity of the detection, identification and altitude reporting.

These improvements translate into benefits in terms of safety, capacity and efficiency – benefits which are key to supporting the future of the high-traffic density airspace. This first step of selective interrogation is known as Mode S Elementary Surveillance.

Mode S Enhanced Surveillance builds upon the concept of Elementary Surveillance and consists of the extraction of further aircraft parameters known as Downlink Airborne Parameters (DAPs). This facilitates an increase in the safety and efficiency of the ATM operations.

The Mark XII IFF System is a cooperative question and answer friend identification system that has been operational since the 1960’s. The Mark XII System is comprised of 4 main component sections: An interrogator subsystem [e.g. AN/UPX-25(V)], transponder subsystem [e.g. AN/UPX-28(V)], associated decoders and antennas.