Agree totally with heliduck, anyone who relys on GPS is asking for trouble, I had an GPS antenna fail inside a busy CTR in bad weather, instead of showing me tracking south as it should, it showed me tracking east, about 10 miles out to sea!
(just as well I knew where I was, and a glance at the compass confirmed it) ALWAYS cross check with the chart, nav-aids and the old MK 1 eyeball! When I took off later that day it showed me flying backwards on the screen..... Most amusing
BC.