How did this thread get to 5 pages ?
Two observers on the ground (ie dirt, either geoid, ellipsoid or prolate spheroid), measured distance apart. Aircraft flies directly over the two of them. From observer zenith (straight-up) to observer zenith measure the time taken. A bit of arithmetic and hey presto ground speed ! If aviation has managed to come up with an alternative definition I'll be unsuitably impressed (ps for PA this is the non-relativistic version).