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Good evening,
GPS altitude is less reliable than GPS position
because you derive it from the Pythagorean Theorem (or something similar) where
the unkown altitude is the vertical leg (~23,800 mi) of a triangle and
position is the horizontal leg. Due to the large size of the vertical leg,
small percentage errors will be very large in magnitude. Also, and this
applies particularly to flight near the ground, the update rate for GPS is (or
was last time I checked) about 1 second. A jet making an approach at 140
kts is going about 225 ft/s. With a 700 fpm descent rate that would
correspond to this approach speed on a 3 deg glide slope (VS in fpm is TAS in
kts * 10 and divided by 2 as my instructor used to teach) even if the accuracy
were perfect you would have already landed 11 ft high or low by the time you
realized the error.
For anything I'd be likely to fly in real life
(most of my professional simulator time has been in airliners, space shuttles
and now spacecraft) the numbers (Cessna 150 eg) would be....
60 kts approach speed (~95 ft/s)
300 fpm descent rate (~5 ft/s)
Still enough to throw you off. Accuracy has
improved dramatically since the elimination of SA, but the time lag is still a
bugger.
Hope this enlightens. (Hope it's
right)
Nickolas Hein
Morgantown WV
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