"Roy Smith" wrote in message
...
A handheld GPS used by a hiker in the woods is working under completely
different environmental conditions than one on an airplane. The biggest
reason for a hiker's GPS to get poor signal is because of nearby terrain
or
overhead foliage blocking line of site to the sky.

By the time an airplane's view of the sky is blocked by overhead foliage,
they've probably got bigger problems than not having a good GPS signal.

For an FAA-approved device, properly installed, I think you are right
probably 99% of the time. Of course even then you could imagine cases like
what happens if the GPS antenna starts to slowly go bad? You don't want to
learn about that when it reaches a critical failure point in the final part
of an approach. Better to see the accuracy start out at 20 ft accuracy and
slip over time to 50 ft, 80 ft, 100 ft, etc. Over many flights even an
inobservant person might catch the deterioration and do something about it
when there is time.

For a non-FAA approved device, I think you are wrong. The problem here is
that the GPS software has no way to guarantee the integrity of the satellite
antenna, and very importantly it cannot guarantee the integrity of the
antenna's placement within the cockpit. If the user accidentally selects
XTrac mode without understanding the implications of that, places the
antenna out of view of most satellites within the cockpit, etc, the software
happily displays an aircraft position. And it never tells you that your
current position is only accurate to 10,000 ft horizontal!

The point is that in a non-approved device, the GPS software creates an
illusion that you are on a 2D map position, at a spacial coordinate, but
most of this software gives you no immediate way to determine if that
reading is accurate to 10 ft or 10K ft. Knowing in advance that you are
accurate to only 10K ft would probably give most pilots a reason to
investigate why, maybe resulting in a better position for the antenna, for
example. Maybe the user would find out that they had accidentally left the
unit in XTrac mode and they need to switch over to a more accurate mode.
Better to make these discoveries and tinker with such things in a calm
environment. You don't want to make the discovery that the "backup" GPS
display is worthless on the day you lose all your primary instruments in IFR
conditions.

I just don't understand the issue about pilot overload. I'm asking for two
integers, and you don't need to look at them ever if you don't want to.

--
Will