While IFF (Identification Friend or Foe) systems were around since the advent of radar in World War 2, the systems were really misnamed.  The early systems could really only identify friendly aircraft which contained transponders that would answer the challenge from a ground radar station.  The first active IFF transponder was the 'IFF Mark I' and was put into operation in 1939. On receipt of an challenge from the Chain Home radar system (20-30 MHz), consisting of a radar distinctive 'peaked' pulse tuned to a specific frequency, the unit would respond with a signal of steadily increasing amplitude, allowing the radar operators to identify it as friendly.

Over the years before the conflict in Vietnam, IFF changed in complexity, but it still only identified friendly aircraft.  But all that changed during the Six Day War of 1967 in the Middle East.  The Israelis acquired Soviet SRO-2 IFF transponders salvaged from Arab MiGs shot down by the Israeli Air Force.  The shared this intelligence with the U.S. and for the first time it was possible to make IFF interrogators that would actually work on Soviet aircraft.

This capability was needed in the crowded skies over North Vietnam. As one wing commander put it in his after cruise summary, “The capability of the air crew of an F-4 to interrogate friendly and enemy IFF would be of considerable value in the air-to-air environment as it would often preclude the necessity for an identification pass on the part of the friendly and it would minimize radio transmissions concerning unidentified contacts thereby reducing overall net traffic.”

The F-4 armed with the AIM-7 was intended to be a BVR (Beyond Visual Range) weapon system. But with the numbers of aircraft over the skies of Vietnam (most of which were friendlies) the rules of engagement required pilots visually make sure of their target before firing upon it.  The requirement for visual identification hampered the Phantom as it was designed and forced it to fight in the parameters that were favorable to the older and slower but more nimble MiGs employed by the North Vietnamese Air Force.  An airborne IFF interrogator was needed to correct this flaw by identifying non-friendly aircraft by interrogating their IFF (The MiGs used their IFF to prevent friendly fire and also to perform ground controlled intercepts) and then alerting the pilot which targets were hostiles. 
All the F-4Bs on the USS Enterprise on its 19 Nov 1966 – 6 Jul 1977 cruise were fit with the GAINTIME Airborne, Air-to-Air IFF Interrogator. GAINTIME was an IFF interrogator capable of interrogating MK X IFF Signals.  The interrogations were fed through a hybrid duplexer and a flexible cable to a special loop antenna located near the feed of the X-band APQ-72 antenna.  These were the only aircraft fit with this equipment (the Navy only purchased 30 units) because new, more capable systems were being developed as the TEASER I and TEASER II IFF Interrogators. 

Revision History:
  •  13 MAR 2014 - Original Post