It's never happened to me using a GPS3 with its own aerial on the dash, but as a control freak neurot I could never sit there, relying 100% on a GPS and not being able to pinpoint my position on a map. That would be highly silly and it's hard to believe people do that. A GPS is an aid to nav and a bloody brilliant one, but it's only an aid. A GPS can give up and go AWOL, a map can't (unless you're in an open cockpit!).
QDM