David,
Anecdotal evidence of GPS failures is just that. GPS goes back over 10 years now, and the early units were frankly absolute c**p. I used to rent a PA28 with one of them and it barely worked. It was labelled "VFR only" which saved the flying school from ever having it checked by anybody, and it probably had thoroughly corroded electronics (parked outdoors all year, duff door seals, inside stinking of mould). This doesn't prove anything. I used to fly with a VOR which would ident but showed anything you wanted it to. I did my IMC Rating with a DME which did likewise. Doesn't prove anything, other than that avionics maintenance is expensive and doesn't get done unless it must.
I don't think MJ or myself are missing anything yet. Certainly internal interference is possible. But it ought to be repeatable if you reset the avionics to those frequencies - if it is bad enough.
If you have a decent rooftop aerial then you have much better immunity from all sources of interference (that are below the sky, which most are) than if you are using some handheld with an integral aerial which is just about hanging in there, getting just enough signal from what manages to seep in through the windows in an all-metal aircraft....
This is the problem. Probably the majority of pilots who use GPS today fly with old junk, or badly installed, or using inadequate aerials.
Even a brand new top-end Garmin handheld for £1000+ falls into that category as far as I am concerned, in an all-metal aircraft. You will be doing well to be getting half the possible satellites because most of the sky is covered up. It will still work well enough because you need only about 3 for a fix, and most of the time there are about 8 in view. Most of the time
