Imagine, as a fighter pilot, being able to see your enemy without them knowing you're even in the area. Sounds like some newfangled stealth capability you'd expect to come stock on a fifth generation fighter, like the F-22 Raptor or the F-35 Lightning II, right?
But what if I were to tell you that the US Air Force possessed such a capability as far back as the early 1970s, far before the F-22 and concepts of its ilk were even on the minds of engineers who'd eventually design them? Heck, more than half of those engineers and designers were probably still finishing off college or hadn't yet completed grade school.
Called the APX-80, but more popularly known by its codename, "Combat Tree", this top secret technology was first equipped on McDonnell Douglas F-4D Phantom IIs, the US Air Force's primary fighter-bomber aircraft. Today, we call the system involved "Non-Cooperative Target Recognition", after having developed it for years. Back then, Combat Tree was a next-generation game-changer which would only be equipped on a select number of F-4Ds, which would fly in hunter/killer packs with other F-4Es (Phantoms built with internal rotary cannons). The precise details of how Combat Tree worked are still classified to this very day, but we do know, to an extent, how Phantom aircrews used it.
Instead of activating the powerful radar scanner in the nose of the Phantom, weapon systems officers (WSOs) in the rear cockpit of the fighter would use Combat Tree to look around the sky for specialized transponders built into enemy aircraft flown by the Vietnamese People's Air Force (VPAF; North Vietnam's military aerial element). These transponders were actually designed to prevent friendly-fire incidents, where North Vietnamese ground-controlled interception (GCI) stations and surface-to-air missile (SAM) emplacements would accidentally target and hit friendly fighters in a bid to shoot down enemy American aircraft. Referred to as "IFF" transponders or (Identification Friend or Foe), these beacons would relay a code to scanners built into SAM and GCI search radar computers, allowing their crews to distinguish between their own fighters and marauding jets of the USAF, US Navy and Marine Corps.
Combat Tree would "challenge" or "interrogate" each transponder it came across, asking in return whether or not the aircraft mated to the transponder was allied or otherwise. As soon as Combat Tree ascertained the allegiance of the aircraft after receiving the automatic response from the VPAF MiG-21's transponder (completely unbeknownst to the MiG's pilot, mind you), it would accurately plot its quarry's location on a display in the rear cockpit of the F-4, and open up the hunt for the pilot flying in the front seat of the Phantom. Conversely, using the Phantom's radar would have likely tipped off enemy fighters that they were being "painted" or tracked by other aircraft in the sky, thus losing any edge of surprise that the American fighters would have previously owned. Not only did this make MiG interceptions by Phantoms "stealthier", it also allowed F-4 pilots to engage VPAF MiG-21s at greater distances, beyond visual range (BVR).
Prior to the existence and fielding of Combat Tree, all US military fighter pilots operating in Vietnamese skies were forced to get closer to VPAF MiG fighters to gain a positive identification on enemy aircraft before attacking them. Since radar only determines whether or not there are other aircraft in the sky ahead of your own, a visual identification is required to figure out whose aircraft those are. While American F-4 Phantom IIs were much more technologically advanced, they were still less maneuverable within the parameters of a close-in dogfight than a MiG-21 or the older MiG-19, also flown by the VPAF. This led to frustratingly high loss rates for American fighters. Combat Tree exponentially enhanced the margin of safety for American pilots by allowing them to gain positive identifications without pushing them into envelopes which greatly favored North Vietnamese MiG drivers.
The North Vietnamese eventually wised up to the presence of such a technology, though, they didn't quite know what it was or how it functioned. The VPAF's ranking officers began noticing a sharp increase in attrition rates with their fighter forces, especially those that found themselves tangling with US Air Force fighter jets. Cells of MiG-21s were reportedly being engaged at distances never before seen during the war, and with deadly accuracy. Radio transmissions between pilots, intercepted by picket stations, were able to pinpoint the reason for the suddenly high MiG-loss rate the North Vietnamese were sustaining – their aircraft's IFF transponders. The VPAF's pilots were instructed, there on out, to only turn them on when absolutely necessary, but to otherwise fly without any IFF protection, making them vulnerable to their own surface-to-air missiles in addition to the threat posed by American fighters in the area of operations.
Combat Tree's effectiveness as a device that allowed American pilots to own the first look/first shot/first kill advantage wasn't completely diminished by this discovery, however. By the end of American involvement in Vietnam in 1975, Combat Tree had earned assists in a number of US Air Force kills against North Vietnamese aircraft. In fact, Combat Tree was was responsible for helping Air Force legends Richard "Steve" Ritchie and Charles "Chuck" DeBellevue reach ace status (achieving five confirmed kills) between May to September, 1972. Since the early 1970s, the APX-80, or at least the lessons learned from Combat tree, has likely been redeveloped and extensively modernized for use with America's current fighter fleet. Combat Tree, in a way, can be considered the forerunner of the modern sensors you'd find today on an F-35 or the F-22, which allow the aircraft to "see" the enemy before they even enter the playing field.
Originally published on The Tactical Air Network in January 2017.