Here's a question I never thought I would ever ask:
Can anyone point me to the definition of ground speed?
You probably won't believe it (I still don't do), but I just discussed two different definitions of ground speed.
Here they are:
Ground speed is:
(a) the speed over ground
(b) the horizontal component of the absolute velocity.
The difference is that at altitude, (a) will be smaller than (b) .
Suppose you fly at FL320 (~10 km) at a horizontal speed of 700 Kts (def (b)).
If the local radius of the earth is 6371 km, you fly a curve at 6381 km from the earth's center. The speed over ground in that case is 700 * 6371/6381 = 698.9 Kts (def. (a)). A whopping difference of 1.1 Kts
So why bother? Well, for standardization / certification this very theoretical difference matters. If you need to validate a ground speed accuracy in the order of 3 m/s you can't afford to loose too much of it in the definition.
Any pointers to an official definition are greatly appreciated.