I am not anti GPS.
And for certain situations ie above MSA the risk is low enough that its acceptable.
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 suppose you could have a tone on a fequency which triggered when the local GPS strayed away outside tolerance from a fixed reference. Which would then flag in the cockpit.
bose the art of landing an aircraft when you can't feel anything from your knee down.