To be honest, it's pretty hard to believe that they used solely the IFF to identify their target... There is a lot of civilian airways in that area, and the one flown by Tu-154 is actually not the closest one.
Generally CW radars are more susceptible of targets flying same radial speed, that target range.
My guess: Locked on the drone using MHI + IADS. IFF checked, ok. Received fire command. Fired Missile. IFF checked, ok. Target lost somehow. Relocked on target (passenger plane) with the closest radial speed (still MHI). IFF checked, ok. (locked on plane, IFF answer received from the drone) Hit.
Yeah, that's seems to be the most plausible explanation. However, there is still one problem, that was revealed by Piston79: all sources claim that the target drone was launched at 60 km range (and certainly wasn't flying away from the battery), and that would dictate the low flight profile/low speed for the missile. In this case it is extremely unlikely for the missile to travel for 360 km.
Arkhangelsk: And in jamming mode range is calculated - system knows 5N62 elevation and you set rough target altitude. Rough range is calculated from these values (you can see it on a P-14 screen or range indicator).
Yes, their boresights must be at least parallel, or better IFF antenna should tilt its boresight to and from radar antenna's boresight according to interrogated target range, so their boresights cross each other at the target. But it is not easy to do and have it working correctly under battle conditions. So this is probably the reason why they used main antenna for IFF.
Loc: Hungary, Europe
If you learned how to operate the KRUG, than you wouldn't be lost in the OSA (SA-8)... ... difference is mainly system (short) range, self contained target acquisition radar, and the possibility to guide two missiles against one target parallel.
Me being a complete KRUG system fan, I would love to see Gecko implemented some day. Too bad Hungarian air defense brigade didn't get a bunch of those during the cold war. Tacticals systems, dealing with low-level targets, sneaking through the terrain, it's real fun.
Loc: Hungary, Europe
One of the largest air battle of Operation Allied Force, happened on the 37th day of the campaign.
On NATO side the attackers were:
F-15C CAP armed with 6xAIM-120 2xAIM-9M
F-16CJ Weasel (in a can ) armed with 2xHARM 2xAIM120A 2xAIM-9M
EA-6B Prowler ECM Support
RQ-1 Predator Intel Support
F-117A armed with 2xLGB
B-2A armed with several JDAMS
On Serbian side, 3 Neva systems were on duty during that night. (others were relocating)
They launched all-together 7 V-601P 5V27 (Goa Mod.1) missiles against 4 targets.
Some sources confirms that a second F-117A was damaged during this battle. "An F-117A of the 49th FW was damaged during strike mission by a nearby explosion of an SA-3 SAM, ...causing loss of part of the tail section, but the aircraft was able to return safely to Spangdahlem air base, Germany. (source: Air Forces Monthly, July 1999, p. 75)"
Weasels launched several HARM's during the battle, but the defenders had one extra trick, the IRZ...