I doubt there for long range detection (and IFF just doesn't make sense), considering the PAK-FA doesn't fulfill any of the 3 requirements necessary for that (Large size/high power in processing /high power in output), my best guess is that it's mostly for EW/ECM.
The NATO datalink system operates in L band so even a single one dimensional array could determine the horizontal direction of an emission from such a system... if you get a datalink signal from a direction there is no return from a very large powerful nose mounted X band radar then it is probably safe to presume there is a stealth aircraft there and its approximate direction.
With that in mind you could then use that AESA array to scan for targets near the source of the emissions determining range and speed... certainly enough to direct other sensors on other aircraft (irst and L band and X band radars) to that area and indeed to get a scan from longer wave ground based systems.
It doesn't have to generate a total solution on its own... as part of a team and a network just indicating a presence passively is enough to start the persecution of the target.
2.) there won't be 1-on-1 fighter scenario, they'll likely be a Su-35/T-50 task group sharing information with each other. The PAK-FA will also be a beneficiary of stealth.
MiG-31s can link radar signals to form an enormous synthetic radar... I would expect the Irbis and the radars fitted to PAK FA would be at least able to do the same.
While it's true the L-band AESA by it's self probably won't be powerful enough due to size restrictions, it was never meant to be used by itself.
Power is relative.. the larger elements in an L band radar should allow plenty of power to be used, while the fact that the F-22 is not optimised to evade L band radar should make it rather more effective no matter what the power is used.
What Cromagnon says is very true, but lets not make it black and white with Russia being the innocent defender. Russia is looking for a serious attack capability and it is getting that with the PAK-FA.(the VDV would also make for a baddass imperial force btw)
Cromagnon is perfectly correct... what attack capability would Russia want?
When was the last time it invaded a country?
In the 25 years since the end of the Cold War the only territory they have annexed was the very recent situation in the Crimea, which was Russian territory anyway.
There was plenty of opportunity.... they could have invaded several 'stan states that were former Soviet republics, and Georgia and Ukraine could both easily have been brought back into the fold.
The simple facts are that Russia is sick of carrying those countries and Putin is not interested in them... Russia is already big enough.
the PAK FA is multirole because all Russian AF fighters were multirole and would take on air to ground duties when enemy air power was less of a threat.
The exceptions were the PVO interceptors like the Tu-128, but even the current MiG-31 can carry anti radiation missiles and bombs... they don't really have an only for fighting fighter like the F-15C.
Talking about Who has better RCS is same nonsense talks of how much RHAe armor tanks have. That is top secret and there are problably less people that actually know such values than you have fingers on your hand. Trying to evaluade or speculate about RCS is far more complex than armor values for tanks.
And more importantly the better armour or RCS you have costs more money, yet your enemy is not stupid... if their anti tank weapons don't work from the front they will only attack from the sides or rear. If the RCS is too small then they will use IR guided weapons like Verba and Morfei and develop BVR missiles with those sensors and guidance systems.