It is 0300 and an F-16 Fighting Falcon is returning to Bagram Airfield. The pilot has just completed a sortie supporting a forward observation post in northern Afghanistan. As the pilot approaches the airfield, the Identify Friend or Foe (IFF) transponder on board his F-16, like the AN/APX-113, informs anti-aircraft defense positions nearby that his aircraft is friendly. This keeps the Falcon pilot from being shot out of the sky by friendly fire.
With supersonic aircraft technology, visual identification for friend-or-foe is insufficient for both proper defense and avoiding friendly fire. An IFF system (Identification Friend of Foe) allows military interrogation systems to identify aircraft, vehicles and ground forces as friendly and can also determine target vector and range. When swept by an interrogation beam, aircraft transponders are triggered to respond, providing more clarity as to the identity and intentions of the target.
IFF works when a transponder like the AN/APX-78 (pictured to the right) in an aircraft emits a signal at a specific frequency that is identified by an interrogation system like the AN/TPX-54 as friendly. Though it is known as Identification Friend or Foe, an IFF system can only identify whether an aircraft or vehicle is friendly, prompting the operator to more closely investigate any target that does not clearly identify as friendly. The APX-78 is commonly found in the B-1 or the E-3 platforms.
In accordance with the Army/Navy Designation System, the APX of AN/APX-78 stands for A=Manned Aircraft, P=Radar, and X=Identification or Recognition. The aircraft’s solid-state transponder receives interrogations (decoder) from X-band radar systems and then transmits replies (encoder) at a greater strength within the same frequency band.
The TPX-54 is identified as T=Ground Transportable, P=Radar, and X=Identification or Recognition. Though the AN/TPX-54 is designed for maritime applications, this system is also employed in aircraft like the P-3 Orion. This interrogator receives transmissions from transponders, enabling the operator to identify an aircraft as friendly across an area covering hundreds of square miles.
As the aircraft above returns from the dangerous close air support mission, the AN/APX-113 IFF transponder responding with the correct transmission to the interrogator system means an aircraft is not mistaken for the enemy and lands safely. The difference in life and death for a pilot could be a properly functioning transponder.
APX-113 Combined Interrogator/Transponder Repair