Most GPS signal loss is not caused by flying upside down, or flying inside some 100000ft deep canyon in the Alps
It is caused by interference/jamming.
In extreme cases, the rooftop antenna is all that will work, and so far it has always worked except for ~1-2 mins off the coast of Italy in 2004, low level (1000ft above the sea) when there was obviously powerful GPS jamming which took out all 3 GPSs I had.
I can wipe out my Garmin 496 (antenna stuck on top of dash) by tuning (merely tuning) one of my radios to a specific NAV frequency. It's a totally baffling one; not a subharmonic of 1575MHz, and the paired DME possibility is eliminated by turning off the DME. But the rooftop antenna is unaffected.