[RH77]

Don't forget that air density and baro-pressure change as altitude increases.

For aircraft, since baro-pressure is very unreliable at higher altitudes, the standard 29.92 mm Hg is set at 18,000 feet and the call is Flight Level 1-8-0 instead of 1-seven-thousand, niner-hundred feet or lower...

Similarly, since the air is less dense at higher altitudes, the pitot tube can no longer be reliable to report the passing air molecules. Ground speed increases with the decrease in resistance, but the airspeed indicator vs. altitude has to be compared to aircraft documentation/reference , as to not induce an overspeed and consequent aircraft overstress situation. Comparisons to GPS (if equipped), should provide the most accurate velocity.

RH77