This is for the geeks who might be interested. I worked comm/nav avionics on F-4Es at George AFB and Ramstein AB, and I worked on the IFF systems and know some detail about this subject. First, IFF is Identification, Friend or Foe. The complete term was actually IFF/SIF, with SIF meaning Selective Identification Feature. The transponder in the F-4E was the KY-532, which worked with the KIT-1A Mode 4 computer for transmitting replies to Mode 4 interrogations. Mode 4 was an encrypted code. There was also a four digit octal code for Mode 2 , controlled by four knobs set before flight on the face of the KY-532 which was located in the CNI bay at the rear of the nose gear well. With an octal code, each of the four code digits could be selected from 0 to 7. The IFF control head in the front cockpit was the C-6280, for what that's worth to you. On the C-6280 control head there were thumbwheel code selectors for Mode 1, which was only two digits, the first which could be set to any number from 0 to 7, and the second digit set 0 to 3. Next to the right of the Mode 1 code wheels were the Mode 3/A code selector thumbwheels, which had four digits, all of which could be set to 0 to 7. Mode 3, called Mode A in the civilian world, was the SIF part of IFF/SIF, used for air traffic control. There were toggle switches to turn each mode on or off individually. There was also a selector knob to switch between Mode 4A or Mode 4B. When we used the KYK-18 keygun to insert the Mode 4 codes into the KIT-1A (and KIR-1A for the APX-76 interrogator), the code was set for today, the A code, and tomorrow, the B code. It was switched to the B code at 2400Z. That knob also had a HOLD position to prevent the KIT and KIR from dumping their codes when power was switched off after landing, so other sorties could be flown that day without having to re-key the computers. If HOLD wasn't selected, the codes automatically were dumped when electric power was cut off to the computers, to prevent the codes from being compromised if the airplane was shot down and the computers survived the crash in enemy territory. The last function on that knob, ZERO, was used to manually dump the codes to prevent their compromise in case of a forced landing, just in case the automatic code dump didn't work. There were actually two IFF interrogators in the F-4E, the APX-76 and APX-81. Both were operated by the same control head, which at this moment I can't remember its nomenclature. Hey, it's been almost 40 years since I worked on this stuff! The APX-81 interrogated two types of Soviet IFF transponders and was classified Secret back in the day. The capability was declassified a few years ago, look up "Combat Tree" to find out more. The APX-76 was used to interrogate Modes 1, 2, 3/A, and Mode 4A and 4B. The control head is visible to the left of the scope in this video. I don't know if DCS modelled the APX-81, but if they did the switches controlling it are two toggle switches on the bottom right, labeled 1 and 2 on the real airplane with OFF, PAS and ACT positions. PAS is Passive, meaning the APX-81 would not actively interrogate but would display azimuth but no range for Soviet-bloc aircraft IFF replies to Soviet interrogations. In ACT (active) the APX-81 actively interrogated the Soviet IFF transponders, which would display both range and azimuth on the F-4E's scope. In peacetime, a switch guard was installed to prevent accidentally setting the switches to Active to prevent the Soviets from finding out we had that capability by detecting the interrogations coming from U.S. fighters. DCS appears to have modeled what the radar display for Mode 4 interrogations and replies looked like pretty accurately. With Mode 4, if the interrogation code from the KIR-1A and APX-76 didn't match the code in the target aircraft's KIT-1A, or if the target wasn't equipped with Mode 4, then no IFF data was displayed (the two horizontal bars above and below the target radar return). If using the APX-76 for the other transponder modes, the far left digit, used for controlling which mode was being interrogated on the controller and has Mode 4/A showing in this video, was changed to 1, 2, 3/A, 4A, or 4/B by pushing little buttons above and below the mode. The other four digits, all set to 0 in this video, would be set to whatever code the WSO was looking for. If the code set into the interrogator control by the WSO matched the transponder code the target aircraft was squawking, then it put the same two horizontal bars above and below the target on the scope (correct mode and code) as seen in the video for the Mode 4A interrogation. If the target was transmitting the same mode but the selected codes didn't match, then the APX-76 would only display the bottom bar, not the top bar (correct mode only). While Mode 4 was used for IFF and 3/A used for air traffic control, modes 1 and 2 could be used for identifying particular airplanes in a formation or unit. For example, all of a particular squadron's F-4Es might be squawking Mode 2 code 1527, with individual airplanes in that squadron using their own individual Mode 1 codes, so flight lead might be using Mode 1 code 01, his wingman 02, the second element lead using 03, his wingman 04, etc. That way a WSO could quickly find whoever they were looking for by using the correct mode and code. Mode 4 did not have a correct mode only (single horizontal bar below the target) display, unlike the other modes. With Mode 4 it was all or nothing, either two bars or no bars displayed. One thing DCS got wrong is that the APX-76 would only interrogate other aircraft and display their transponder replies on the scope for as long as you held the button on the LRU-10 radar controller joystick, or held up the CHAL TAG spring loaded toggle switch on the APX control head. As soon as you released the button or toggle switch, the replies on the scope disappeared. The APX-76 transmitter had an automatic timeout of 8 to 10 seconds to keep the transmitter from overheating. So, if you held either switch for longer, the displayed replies would still disappear from the scope in 8 to 10 seconds. You'd have to release the switch, then activate it again to display the replies again.
@EvMstein
2 ай бұрын
A very informative piece of writing right here!
@BobGrim-g9j
2 ай бұрын
Since we’re being picky, IFF means Identification Friend or Foe.
@TheSkipjack95
2 ай бұрын
For realism (or to prepare for the wildly outlandish moment when ED adopts Galinette's IFF), you shouldnt have mode 4 on until you hop the fence. Mode 4 responds to an interrogation matching the code of the day (A or B), otherwise it stays silent. The older modes will answer any interrogator however, since they have become civilian standards and dont have the behaviour of mode 4. That makes you eminently conspicuous to anyone who asks. So when flying in friendly airspace, civilian modes (1 3 3/A, C) ON and mode 4 OFF, and at the fence reverse the selection.
@DieselThunderAviation
2 ай бұрын
That is great information, thanks! Would be nice if the IFF worked as it should across all of the DCS modules.
@billfolsom4203
2 ай бұрын
In the last century, I worked comm/nav on F-4Ds. The AN/APX-76 was the mode 4 system, somewhat unreliable in my experience. Delicate little coax connectors that hooked into the radar package and would often get pulled apart when the radar was slid out for maintenance.
@DieselThunderAviation
2 ай бұрын
Thankfully we don’t have to mess with that in DCS!
@Q3ark
2 ай бұрын
Is it possible to do a mode 3 interrogation? For example if you’re looking for a specific aircraft and I know its mode 3 transponder is on 2468?
@DieselThunderAviation
2 ай бұрын
No, the interogator on most fighters only look for Mode 4. Think the only airborne asset that can do that is the AWACS.
@Q3ark
2 ай бұрын
@@DieselThunderAviation Thanks dude, the strike eagle can do it, but we all know what’s going on with that module.
@scottwilson8105
2 ай бұрын
@@Q3arkin real life, the APX-76 interrogator in the F-4E could indeed interrogate Mode 1, Mode 2, Mode3/A and Mode 4. However, a Mode 3 code of 2468 as you used in your question would not be possible. The Mode 1, 2, and 3/A codes were octal, meaning only the digits from 0 to 7 were available. I worked on the APX-76 for 14 months doing in-shop maintenance at George AFB in the 35 CRS (1982-1983), and three years on the flightline at Ramstein AB in the 526 AMU, 1983-1986.
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