There are two types of IRST in War Thunder, which, for simplicity, I will be referring to as generation 1 and generation 2.
Here is the IRST system of the FlaRakPz, as you can see, it does not show distance (0 km reading is a bug, it should say nothing there), nor does it show relative speed to or from you.
Here is the IRST system of the FlaRakRad, as you can see, it does show distance and it shows relative speed to or from you.
Now how do these two systems operate?
Most, if not all Generation 1 IRST systems operate the same. They detect the heat signature of the target and move the turret to keep the heat signature in the center (or in the same place) as it appears on the sensor.
They offer nothing else.
When it comes to Generation 2 IRST systems, things get a bit more interesting.
Infrared sensors by themselves cannot detect range to target. And as you need range to calculate speed, they cannot do that either.
Hence, come in electro-optical methods. The IRST performs the function that it always has since generation 1.
But, it is supplemented by a laser beam in the electromagnetic region.
These laser beams operate like laser range finders and give the distance to the target.
To knowledgeable players of Russian and Warsaw pact air forces with IRST systems like on the MiG-23, MiG-29 and Su-27, they would know that the MiG-23 only provides direction of target, while the other two give a lot more information such as speed and distance.
My intention for making this thread is to discuss whether or not, Second generation IRST systems should trigger LWR (laser warning receivers) on vehicles/aircraft that have them such as helicopters.