Actually, its nearer 10 m accuracy with a 12 channel receiver. You can have a look at
http://users.erols.com/dlwilson/gpsacc.htm for the results of a series of tests of common GPS receivers. To cut a long story short, the accuracy of the GPS III+ is RMS 4.2m, mean error of 3.6m, within 3.4m 50% of the time, and within 7.5m 95% of the time with an average of 6.8 satellites in view. (A 1° error tracking a VOR at 40 nm will put you 1293m off track as a comparison)
Positional errors are very rare (by this I mean the GPS tells you you're 10 miles from where you should be)...each satellite transmits a health status message, so the receiver will reject error signals. If there are too few satellites in view, then the position error will increase, but your GPS unit will let you know, even the cheap ones like the eTrek.
There's nothing wrong with using GPS as a primary means to Navigation, I've used it to fly IFR in the US. But it IS wise (and good airmanship) to back up you navigation using other methods as well, its called 'situational awareness'.
Anyway, I reckon this post is a wind up
Cheers
EA
