In real life it uses a missile approach warner to detect the distinctive spectral signature of a missile launch - the "flash" of the initial launch and then the boost motor igniting to get the thing out and flying. It then knows that a missile is approaching from a rough bearing. It might use a mm wave radar to clarify the launch as well as the bearing - not sure. A radar would seem to be essential to my mind to determine the range to the missile as it scoots in. I think it then deploys countermeasures to hard kill the missile with a premature detonation at a safe stand off range from the host platform. That's the rough idea.
In game terms I'd simply increase the save by 1 or 2? How it copes with multiple attacks (can it be "swamped" or saturated) and how much "ammo" it carries is another matter.
Does that help?
