"Will" wrote:

It's nice that a certified instrument flags an unsafe condition. I would
still like to know the current level of GPS accuracy on a certified
instrument, for many reasons:

* It helps to educate me about GPS and conditions in my immediate
surroundings that might affect accuracy of the technology.

* It helps to alert me about possibly deteriorating conditions, before I get
into a situation where I needed to rely on the instrument and suddenly I
cannot.

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.