Originally Posted by
FlightDetent
Mystery solved, exactly why I asked- it is GROUNDSPEED. Winds blowing with +20 knots as early as 2000 ft above surface are quite an everyday occurence. If the aircraft track is parallel to the wind direction, ... voila!
Most of the commercial and enthusiast flight tracking websites only capture groundspeed (because it's part of the ADS-B squitter, and easier to decode), but of course that only tells half the story.
In Europe, at least, most commercial jets have Mode S EHS, which means that they also send a load of other parameters in response to ATC interrogation, but they are a bit trickier to capture.