Hey dlb511
You and the rest of us might be interested in the information at this site.
http://www8.garmin.com/aboutGPS/waas.html
And some of the other factors at play with GPS. It's good, but not perfect. Also, keep in mind that some of the enhancements can be turned off at the "flick of a switch" in the event of national security, a.k.a. 9-11!
But just imagine, position accuracy virtually any place on this planet to within +/- 3 ft- incredible!!
PS- I've used my hand-held on an commercial flight and have known where, what altitude and speed we are traveling. This all before 9-11!
Good luck!