• Emergency avoidance system to prevent an aircraft being taken off its light plan, avoid buildings etc and, in extreme cases, allow controllers on the ground to auto-land it
And what if as in the case of 9/11 there are multiple threats, just how many ground control systems will there be,
ie "well we had 3 but there where 4 threats, sorry about the BA aircraft, but the US ones got priority, Although it's on-board building avoidance system worked right up to the point where we shot it down, then sadly it failed"
or even,
"thank god the pilots where there to land the 4th aircraft safely"
