Time is the only issue in this matter.
If you roughly know where the bad guys will be coming from, and if they will be low-level or high-level intruders, you can design a decent system (sorry, I will not give numbers)
If you need to expand the scope of your watch, still with a short notice capability, it will be hugely costly... in fact it used to be done up to ten years ago in our world, and is still done in some specific parts of the World.
Once again, it will depend of the perceived threat.
But airliners being hijacked after their take-off, from an airport close to the intended target.... big challenge (it's a basic story of minutes, nautical miles and knots)