APX-80 "Combat Tree" Enemy IFF Interrogator
Programme startet as "Seek See" in 1968 after acquisition of some Soviet SRO-2 (Odd Rods) IFF Transponders from Arab MiGs shot down by the Israelis during 1967. Combat Tree was escpecially developed to interrogate the SRO-2.
IFF basics /about Transponders :
Transponders use four "modes". Military modes are 1, 2, 3 and 4. Civil modes are A, B, C and D. Civilian IFF is called SSR (Secondary Surveillance Radar).
The basic civilian IFF mode is
Mode A, which is essentially identical to the military mode 3; therefore, this mode is commonly referred to as mode 3/A.
Mode B has very limited use and has no military equivalent.
Mode C is reserved for automatic pressure altitude transmission and has been adopted for both civil and military altitude reporting.
Mode D has not been established internationally.
Military modes 1, 2, and 4 have no civilian equivalent.
Mode 1 is known as the general identification or mission signal .
Mode 2 provides the personal identification (PI) code for a specific airframe
Mode 3/A uses codes for air traffic control within the Continental United States (CONUS)/ outside CONUS.
Mode 4 is used only to verify friendly status.
Soviet IFF transponders
also have several modes, but those are probably not the direct equivalent to one of the Western modes mentioned above. But the transmitted data is the same, i.e. height, id, etc.
Again, Combat Tree was developed to read/trigger the Soviet SRO-2 transponder.
APX-80 Components:
APX-80 = APX-76 + APX-81
APX-76
Interrogates the four IFF modes of friendly aircraft to avoid "blue-on blue" mishaps.
APX-81
Interrogates the modes of the Soviet SRO-2 IFF system
The antennas for the APX-80 system are mounted on the front of the radar dish in the fighter aircraft (F-4D/ E/ N/ S, F-14A, F-15A)
How It Works
The APX-80 is operated from consoles in the WSO cockpit (F-4D/ E Phantom II). The indications are displayed on the radar scopes in the front and rear cockpits. It could operate passively, receiving signals from the MiG's IFF or actively by transmitting an interrogation signal to it.
The VPAF (Vietnamese Peoples Air Force)used GCI (Ground Controlled Intercept) to guide his fighters into position to intercept the US aircraft. Preferred for the MiG-21 was a "high six o'clock position for a high-speed pass on the US Strike Package. To do so the Vietnamese GCI had to know the position of his own and the enemy aircraft at all time. To keep the two forces apart the MiG's had to have their transponders always "on", transmitting the needed data which consisted of a mode with an code. (example : to get a "friendly" reply from an US aircraft it needs to reply with mode 4 with the day's correct code)
US APX-80 equipped aircraft could read this data too.The indications on the scope were two "bars", known as "grass". The position of these bars on the scope showed Range and Azimuth of the MiG(s)but no height.
APX-76 bars display for friendly aircraft
APX-76 could interrogate only one mode at a time. The mode could be choosen on one of the control consoles in the rear cockpit.
The bars display looks like this :
_
-
If the aircraft is also located with the radar it looks like this :
_
*
-
* is the radar target
The lower bar indicates correct mode reply
The upper bar indicates correct code reply
For enemy aircraft it seems like some other symbology is used with indicates type, id etc. Looks like this stuff is still classified.
Ranges of APX-80 detections were up to 60 miles (compared with 20 to 30 max miles for radar detections). That's because no reflection of the signal is needed. The transmission is only one-way, from the interrogator to transponder. The transponder then sends his own signal back to the interrogator. All aircraft with active IFF were located, that means also those on the ground taxiing or testing the equipment. Later in the war the VPAF became aware that "something is wrong" and changed the codes each day. The transponders were also set to active only during the critical phase of the intercept (or used temporary only at all). But this didn't help much. Combat Tree has an "active" mode which could be activated by pushing a button. It transmitted an signal which interrogated the SRO-2 transponder. The MiG pilots do not know if the aircrafts transponder is triggered.
Sources :
Forum on
http://www.ACIG.org 2 or 3 other websites about IFF basics
Ospreys book "USAF F-4 Phantom II MiG Killers 1972-73"
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