Re: GPS accuracy
Hmm, that would depend on where the display in the passenger cabin is getting its feed from. If it's coming from the barometric altimeter, your Garmin may have been more accurate.
Assuming that the plane was right and the Garmin wrong for a moment, there's another factor here. The signal view through the plane's window will only get a limited subset of the sat constellation, as the metal body of the aircraft will act as a rather efficient obstruction to others, which is a bit of a downer for good triangulation. For a fix in 3D, you need a minimum of four(?) sats locked and the fix improves with more. A GPS on a balloon with a good signal view of the entire sky should get a far more accurate 3D fix than the rather one-sided bias from the plane's window.