But below that there are so many external factors which effect the quality of of the position that the risk is unacceptable currently especially as there are no warnings when it does go out.
I think you'll find that GPS is probably the best nav system with regards to warning the pilot when there is no GPS signal!...you get flags, messages, etc...Is yours a certified installation or is it a hand held mounted on the windscreen?
Actually the GPS jamming trials didn't even affect my 430W or Aera, even though I flew slap bang through the area. Mind you I have it properly mounted and IFR certified.