John Weiss wrote:

I've monitored the flights I've been on using hand-held GPS

I think you're wrong. Consumer GPS units are not designed to
move at 500+ knots.

You are wrong.

My Garmin Geko has no problems giving altitude, rate of ascent or
descent and speed when moving at 500+ mph.

I've also had an older Garmin Nuvi (a mapping gps for use in cars) and I
think the highest speed that it registered was 950 km/hr (almost 600
mph). Using a newer TomTop 1400 I remember flying into LAX last year
and watching it continuously re-calculating the route based on the
sparse roads in the desert southwest I was flying over.

Also, GPS altitude is inherently less accurate than its XY position.

GPS altitude is based on ellipsiodal model of the earth, so the exact
verticle position of where sea level is on any given point on the planet
will deviate from the elliptical model.

When you have a line-of-sight to a significant portion of the sky from
the window of a plane that's 6 or 7 miles in the air, your hand-held
consumer GPS can make contact with upwards of 12 gps satellites so the
altitude accuracy will be, at worst, 50 to 75 feet. Lat/Long accuracy
will be 8 to 10 feet - something my Geko can do from a small open field
at ground level.