Ah, thanks for the replies. I hadn't thought it thru enough had I? I was thinking along the lines of the 'speed measurement' factor giving the answers, but of course the wind is also moving.
We've got lots (and lots) of wind-turbines around here and they us something I think is "l@ser-radar" to work out the windspeed to adapt the windmill to the wind condition.
could the same not work on an aircraft?
Cheers
G