Ok i will list the things I can think off that could cause signal loss.
Sorry chaps, but I think you are missing the point. All the wonderful things listed
could perhaps cause signal loss, but conflict with numerous anecdotal evidence of GPS reliability in bush regions from pilots who do rely on it (rightly or wrongly).
The one factor that could fit all of the spurious behaviours described on this thread is interference from
within the aircraft .
It's far more plausible than the idea of interference from the ground, which might come and go in seconds because of the square law of attenuation, or so-called jamming trials, which would have small boats beached all around our coastline if they are half as effective as some people seem to think.
The EMC (electromagnetic compatibility) issue is the reason that airlines ban electronic devices during all or part of the flight, rulings that are themselves based largely in superstition as no-one can know if a laptop with an unspecified electrical defect (permitted in flight) is a worse EMC hazard than a cell-phone (banned) that's working properly.
EMC problems would be hard or impossible for engineers to diagnose without installing specialised recording equipment, but persistent and unexplained GPS outages of the type related in this thread must have a tangible explanation that could be found without scrapping the equipment or indeed the airframe!
The 'rising floor of noise' is a point well made and definitely a problem for the future, although perhaps easily addressed in antenna designs that favour the sky where the satellites are supposed to be. But internal EMC problems won't be fixed until the technicians (and the pilots) figure out that an interference source 30cm away inside the plane is 10 to the power of 8 (don't know how to write that on Prune) or 100 million times more powerful than one 3 km away on the ground!
David