identification friend or foe

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identification friend or foe (IFF)

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A secondary radar to identify whether the aircraft, or any other vehicle being interrogated, is friendly or hostile. The interrogator sends a radio signal on a predetermined frequency, which is received by the aircraft. It replies in code, giving the requisite information. It normally operates in conjunction with the primary radar. An IFF normally has four modes. Mode 1 is used for ground identification; the interrogation consists of a pair of pulses, and the reply is a single pulse. Mode 2 is detailed identification for a specific aircraft; the ground query consists of a pair of pulses, as does the reply. Mode 3 is used to specify the functional class of aircraft and has a direct ATC (air traffic control) function when used in conjunction with the selective identification feature. The pulse pattern is the same as Mode 1, but the interval between the pulse pair is longer. To indicate a distress condition, an emergency mode of interrogation can be used, which replies to any mode of interrogation by a sequence of pulses indicating an emergency to the ground station. Similar to the aeronautical transponder (SSR–secondary surveillance radar) system.
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Air traffic controllers became suspicious when the aircraft failed to communicate and the Identify Friend or Foe tracker was showing a wrong signal.