How does the SA-2E actually track a target? And how does it track the missiles it's guiding?

The more I've thought about this, the more confused I've become.

Using the simplest, obvious ways to track a target and missile, I calculated would lead to 140m inaccuracy at ranges of 40km, yet SAM Simulator is much more accurate than this. Missile's often get within 20m against a slow straight flying target like the F-86 target drone.

So either SAM Simulator isn't realistic, or the SA-2 is using a clever tracking system.

If it's using a clever guidance system, then how does it work? I can think of three different ways that would improve accuracy from 140m of the simple tracking technique, but I'm just guessing. I don't know how it actually tracks the target and missiles.

Does anyone know what tracking algorithm the SA-2E uses?