So Active Terminal Guidance.
Kind of... boring, honestly. And mostly useless.
I am entirely baffled.
This ability requires a straight 0.25MSP, costs about the same as 0.25MSP of engine at 400% power, and increases accuracy by some percentage that increases with research.
Even at the first research level of +15%, straightforward math tells you that trading 0.25MSP of engine size for this ability this will improve accuracy if your engine is 1.92MSP or larger.
The higher your tech level, the smaller the engine has to be for this ability to improve accuracy when traded for engine size.
Obviously you lose some speed when you make this trade, and that makes you somewhat more vulnerable to enemy PD and AMM.
So it's probably not worth making that trade for an engine just barely big enough--a small gain to accuracy probably isn't worth the additional PD/AMM vulnerability.
But if you have a really big missile, with a really big engine, it is absolutely worth it.
If there's anything boring about this ability, it's that it is so good on big missiles that there's hardly ever a reason to not use it.