Separation is about predicting where aircraft will be at points in time. That's about groundspeed (and track).
I'm sure a bunch of qualified ATCers will pile on to me if I'm mistaken.
(On the weekend I watched a very graceful bird flying 'backwards'. Heading west into a stiff breeze but tracking east along a waterfront, looking for juicy morsels. Airspeed 25 kms per hour, heading 270. Groundspeed 5 kms per hour, track 090. How does knowing the bird's IAS is 25 kms per hour help avoid a collision with it?)
(PS: The bird was neither an African nor European swallow.)
Last edited by Clinton McKenzie; 20th Apr 2022 at 03:52.
Reason: To add the PS.