QRC-248 and "Combat Tree" AN/APX-81

Oops... I didn't notice that there was a separate category for this, so a repeat of what I wrote in the Soviet IFF thread:

I think that the book "Clashes" by Michel might be a good starting point for Combat Tree research. The system seems to be rather secret on the F-15, but not nearly so regarding its use by the F-4 in Vietnam.
According to now declassified U.S. sources, Combat Tree was an IFF
interrogator that could interrogate MiG IFF transponders. It was first
installed on a small number (8?) of F-4Ds in late'71 or so, which were
operated by the 432nd TRW (The serial numbers are given in Anthony
Thornborough's "The Phantom Story"). I've checked his list against the Red
Baron reports, which identify whether and which a/c in a flight are Combat
Tree equipped and which contacts were due to Combat Tree, and cross-checked
those incidents with listings of those kills which include the a/c serial
numbers. They match).

Because of the small numbers available, typically only the flight lead (01)
and (when available) the element lead (03) would be in 'Tree'-equipped birds.
02 and 04 were often in F-4Es, to provide some gun capability without
forcing the F-4Ds to carry gun pods. Because of the loss of several of these
a/c, some more (about 20?) were modified in mid '72 (available July, I
think). Later, when the Rivet Haste slatted F-4Es showed up in theater to
re-equip the 555th, they were apparently all equipped with it.

Combat Tree had several positive effects. First, it allowed U.S. fighters to
acquire MiGs at much greater ranges than they could from a primary return
(i.e. a skin paint). Second, it allowed them to make contact looking down in
many cases. This meant that free-roving MiGCAPs were much more effective
than they had been; previously, the NVn would just vector there MiGs around
U.S. MiGCAPs by keeping them low until they'd gotten out of our a/c's radar
arcs. It also meant that we were allowed to take more BVR shots with the
AIM-7 than we'd previously been able to. An example the initial engagement
of Oyster flight on 10 May 1972. Lodge in 01 and Ritchie in 03 were both
flying Tree-equipped a/c, and Lodge's and Markle's kills were both fired BVR
because of a Combat Tree ident.

It wasn't perfect. There were limits as to how close a Tree contact could be
to other a/c, before a BVR shot was allowed. In at least one case, an a/c
ID'ed as a MiG by Tree was visually ID'ed by TISEO gear as an F-4,
fortunately before a shot was taken. In addition, the NVN was montitoring our
radio coms. When our pilots started to report contacts on MiGs at previously
unheard of ranges, they started to get suspicious, and curtailed their use of
transponders, only turning them on at turn points or the like instead of
using them continuously as they had previously.

For the best, most accessible discussion of Combat Tree, QRC-248 (a similar
interrogator installed on board the EC-121s from 1967; Michel, quoting from a
declassified document, says theat QRC-248 would tyypically allow an EC-121
Connie which might have trouble detecting a MiG at 100nm at medium altitude,
to detect them at 175nm at low altitude) and the rest of the Vietnam air
campaign, you should find a copy of "Clashes," by Marshall Michel (sp?).
This is based on declassified U.S. documents now available from the
Historical Research Center in the archives at Maxwell AFB, checked by
Michel's own experience. The HRC has a website listing some of what's
available, although it tends to be a bit out of date, and last time I
checked, none of it was on-line; yopu have to order the stuff by mail, or go
there. I have a friend who works there, and he's made copies for me.

I got my copy of "Clashes" from amazon.com for 30% off a year or two ago,
when it was published. If you're interested in how an air campaign is
fought, this is one book you've got to have. Another excellent book for
details of the F-4's weapons is the one by Thornborough mentioned above.

Prior to the
active use of QRC-248, the MiGs had really got their act together, and first
demonstrated their new tactics on August 23rd, 1967. Quoting from "Clashes":

"The MiGs [two Mig-21s] stayed in the ground clutter to avoid interception by
U.S. airborne radars, but their transponders were picked up by one of the
orbiting EC-121's QRC-248 [in passive mode]. The EC-121 called out the MiGs'
location, but the escort did not react. Once the North Vietnamese GCI saw
the MiGs were abeam the strike flights and outside the radar range of the
MiGCAP F-4s [actually escorts], they instructed the MiGs to climb quickly to
28,000 feet. This put the MiGs above the overcast and on one of the flanks
of the strike force, still unseen by the F-4 radars. The MiGs closed on the
force and, on GCI's command, dived out of the overcast and swept down at high
speed on Ford, one of the rear F-4 strike flights. The MiGs attacked with
Atolls; the first warning the F-4s had was when Ford 3 saw a missile hit Ford
4, which blew up in a ball of flame. At the same time, Ford 2 watched an
Atoll pass by his wing and destroy Ford 1. The MiGs escaped unscathed."

With the Combat Tree in your Phantom you don't need to activate your radar. You just point the radar antenna into direction you want to scan, and activate the APX-80. It will immediately show you (it uses the radar display) all the "MiGs" with active IFF-transponders out to 50-60kms in the air and on the ground in front of you. I.e. it will show you also those MiGs which your radar couldn't detect except they are flying at ranges closer than 15-20kms. All of this regardless of the level at which the MiGs operate (the system detects even those SRO-2s on the planes which are on the ground, but are eventually testing them, or rolling to take off etc.).

And then, by pushing "a button", you can also trigger IFF-transponders of enemy fighters which are not set on active.

With other words: you know what is going on ahead of you, where are potentional enemies, and what are they doing, but they don't.

Tom on acig.org, somewhat overstating matters as usual.
RC-248 could interrogate only SRO-2 IFF. But on single EC-121K was mounted Rivet Top system, and in Clashes is writen that it could also interrogate SRO-1 and SOD-57 IFFs. What were designations of Rivet Top EIFF systems? Somewhere I have found that APX-84, and 85 were used. Have anybody somedetails about it?
Other parts of Rivet Top were Rivet Gym stations used for COMINT between GCI and MiGs.

Bunny on acig.org
ndeed, if you read "Clashes" you'll see that this capability and bad COMSEC almost certainly led the NVN to believe that the SRO-2 was no longer secure. Apparently, the Tree-equipped a/c were able to pick up squawking MiGs at 50-60nm, even in ground clutter, and on several occasions Tree contacts were called out on the radio with these ranges given (with the crews getting their butts chewed back at base). The N. Vietnamese (and the Soviets) were listening in all the time, so this sudden doubling of radar range undoubtedly made them perk up. Shortly thereafter, it was noticed that the VPAF had begun to use their transponders much less frequently, only turning them on at turn points and at other critical moments in intercepts, rather than using them continuously as had previously been the case.
Since the APX-81 used the dipole array on the APQ-109 antenna (normally used by the APX-76 IFF interrogator), you would only get a reponse from the bad guys's IFF when the antenna was pointed at it. The APX-81 video was inserted on to the b-sweep and lit it up whenever the antenna was pointed in the right direction. Now, if you did this while your own radar was in stby then normal returns didn't interfere with the APX video. Of course you didn't get range info, but a quick radar look with the antenna on the target gave enough data to sneak up on the bad guy without setting off his RWR.

The Soviet tactical doctrine was that fighters were to be under ground control. This entailed the ground controller knowing where the fighter was. To do this, the fighter's IFF unit needed to be kept on at all times.

In the mid-1960s, someone working for the USAF found a way to interrogate Soviet IFF units. The resulting QRC-248 was first used onboard EC-121s operating in the Vietnam conflict. This capability was top secret and initially, the EC-121 crews were only allowed to monitor when the Soviet IFF sets were interrogated by ground radar, a so-called "passive" mode. Eventually, they were allowed to actually interrogate the IFF sets themselves, in an "active" mode. This capability was first used in 1967 and resulted in a dramatic improvement in US air-air results. The EC-121s could issue MiG warnings to specific aircraft, with position information.

Eventually, a APX-80 Combat Tree system was installed in F-4s operating in Vietnam. This allowed the fighters themselves to interrogate their foe's IFF units. The F-4 crews could then see on a cockpit display where the opposing MiGs were.

The North Vietnamese caught on and started turning their IFF units off. However, now the ground controllers had a hard time controlling them, diminishing their effectiveness.

I don't know if it was the same as the "Combat Tree" set, but the F-4Es I worked on had a Soviet IFF interrogator called the APX-81 system. There was a receiver-transmitter, the RT-961, located inside Door 19 behind the rear cockpit, and it shared a control head with the APX-76 IFF/SIF interrogator system, the APX-76 being also located in Door 19 but consisting of the RT-868, SA-1568, and SN-416B. The control head was located in the rear cockpit on the lower part of the rear main panel, just above the WSO's left knee. The control head was labeled "APX-80", which I believe may have just been a sort of average of the designations APX-76 and APX-81. I don't believe there ever actually was an APX-80 system. The APX-81 specific switches were labled something similar to "Type 1" and "Type 2" (can't recall the exact verbage just now), and both switches were three positioned: OFF, PASSIVE, and ACTIVE. Both APX-76 and APX-81 displayed their data on the radar scopes; there wasn't an independent display. Both APX-76 and APX-81 shared the "T" shaped dipole antennas mounted on the radar dish, four on the F-4E and I believe 8 on the F-4D. The APX-81 didn't have a destruct charge when I worked on it, and I was assigned to flightline maintenance so never got to see the inside of the RT-961, so I don't know if there was provisions for a destruct charge inside.

To get to the point, there might not have been any really obvious cockpit differences for a Combat Tree F-4D, I've never seen any cockpit photos or diagrams calling it out as a separate system. My guess is it also used the APX-80 control head, if it wasn't the APX-81 system I worked on.

Scott Wilson


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