DW
Fair point, but I did say "in a particular spot". There is a good theoretical basis for high harmonics of certain navaid frequencies interfering with GPS. However, I have never met anyone, and have never met anyone who has ever met anyone, who can just go up in their plane that is fitted with any current-model GPS and reproduce this.
In any event, all this is an excellent reason for having a roof mounted GPS aerial. It's all down to the signal to noise ratio; if a GPS with an integral aerial is only just managing, it won't take much to push it over the edge. The total cost of a proper rooftop aerial should be about £400 incl installation, coming out on a BNC socket.
Reichman
"How did we all navigate around the world before GPS?"
The answer is that, like today, often they didn't. Even post-WW2, airliners used to get lost through gross nav errors.
I have often asked your sort of question to various old hands in sailing and flying. How did the seafarers of centuries ago manage to find Easter Island, for example, and do so at will? If you assume the horizon from say FL100, it is certain that nobody, not even a robot on a windless day, could fly a heading for that distance and end up there. These people were extremely good with the sextant and their tables. The sailors had a table on which to spread their chart and al the time in the world. The early airliners also had a full time pilot or two, a full time engine/fuel manager, and a full time navigator. They also had NDBs and even an NDB is vastly better than dead reckoning.
The trick, as far as I can tell, is that the early navigators (and this includes post-WW2 airliners) had continuous guidance along their track. Even a sextant gives you a line of latitude and is vastly better than dead reckoning and post-WW2 there were NDBs everywhere of interest which are better still. A GPS, like VOR/DME, gives you continuous guidance along a track.
Flyers that did solo transatlantic flights would have tried to use a sextant but in any case would have aimed for an easier target, e.g. the east coast of the USA, or Ireland if coming this way. You've got to do a wind corrected heading to within 5-10 degrees and you will be fine.
The difficulty, which a GPS handles very well, is that today's PPLs get barely enough training to take off, fly an easy XC flight on a perfect day, and land, yet they are expected to fly precise tracks between bits of controlled airspace.