MJ
"1. Military Jamming which can be done without NOTAM and will effect both roof and handheld."
Agreed
"2. Illegal Jamming some little baw bag has bought a jammer on holiday or has made one for the crack and has turned it on. Roof mounts should be better due to the airframe shielding them from it."
Agreed. Unless somebody floats it off on a baloon.
"3. Terrian shielding. Not Much of a problem in the SE but wales and Highlands in the glens it is an issue. The roof mounts will be better but it will still cause problems as they will only be able to pick up the straight up contacts. Which as we know from the GPS sphere theory gives you the worst fix. You might say so what. But if you decide to go up the wrong glen your maybe looking at trying to turn in a climbing situation as the sides come in."
This one featured in various stuff emanating recently from the CAA and has since been copied by some GA magazines. But it doesn't stand up. Look at how low you would need to be relative to nearby hills in order to create significant shielding. I haven't drawn a diagram but very roughly, you would need to be flying say 3000ft (half a mile!) away from (but parallel to) a 5000ft high rock face, while flying below 1000ft agl, and with another rockface half a mile on the other side, to create a cone with an included angle of 45 degrees. Put it another way, in the bottom of the Grand Canyon.
Many would say that somebody flying thus is an idiot, but certainly he ought to be navigating visually
"4. Wx. Anything which contains large amounts of water will block the signal."
Perhaps a massive downpour might but I have not seen any reception loss in heavy rain, or under/next to really massive CBs, a few miles in diameter (in N Spain). I think this is because, typically, one is receiving 6-8 satellites and they tend to be spread around in the sky at various azimuth angles. I've been looking at this on Oziexplorer which has a satellite display and it is clear enough.
"5. EMC clutter as the previous poster mentioned (hadn't thought of this before but radius squared rule means that once your above 500ft I would be very doudtfull that it would effect either)"
A rooftop aerial would help a lot.
"6. EMC hot spots as indicated on charts (bit obvious this one) "
Yes, a powerful signal could saturate the receiver and it might take a while (minutes) to recover. Again, a rooftop aerial would help a lot from ground based sources, but I have flown close enough to hilltop radars (a few miles away but at a similar level) to hear their RF in the headsets (with the headsets not plugged in!) and the GPS was not affected.
"7. The yanks start mucking about with the SA on the civi signal."
Re-introduction of SA would have no impact on en-route navigation. 200m or so is still good enough. It would muck up useful GPS altitude indication though.
"8. Yanks go to war and decide to turn the civi off."
Agreed, but their economy depends so much on GPS that if they turn it off, the situation is likely to be so dire that most likely no GA will be flying anywhere. They have never turned it off, and despite stories spread by some instructors it was never even degraded, beyond the original SA. The military have developed jamming and that, I expect, would be used to deal with specific threats. It can also be applied to the civilian signal alone.
"9. Some internal problem with the unit."
As likely as anything. Probably the most likely scenario, in a particular installation on a particular flight.
"10. Fault in the wiring to either the unit or the remote reciever.
Have seen this one once with a GA and many times in boats. For VFR club flyers at the weekend I think it will be the most likely."
Sure, as for 9.
The smart thing would be to embrace GPS officially but teach a technique whereby it is used together with checking the displayed features on the map. This implies a moving-map GPS. This could be done at PPL level. Obviously the proper way is to use GPS and ground navaids together but that isn't going to interest the sort of pilots some here think will just rely on GPS completely.