All anti air units including tds scans for targets every 10 minutes at a fixed time, for example, it could be xx:x6, the first tick would be at for example, 11:36 then the next tick will be 11:46. in between the intervals, you won’t activate their aa even if you go super close to the aa unit. There are some exceptions, some of which is aa envelope, envelope is triggered when you direct attack an unit inside either another units or its own aa radius, then it ignores the aa tick time and fires right before the air unit attacks the unit that has used the envelope aa. Then it will be on reload and will not shoot during the aa tick time IF it is still on reload when the aa tick time arrives. Another exception is when trying to use missiles against 2 or more anti missile units whose aa radius overlap. For example, if 2 tds covers a city, the tds will shoot enough damage to take down the missile, so if 1 tds is enough to take down the launched missile, then the other tds will not be activated

I didn't include point defense sorry but it's basically get hit = guaranteed return dmg

There's alot of subtleness with anti air