I’m not entirely sure what you are talking about, but most backup systems are Plane based.
GPS is mostly used for something called RNAV, however, RNAV can work without GPS. RNP approaches need GPS, but all airports (regardless how small, because without it would only be an airfield) have different approaches for both pilots and ground personnel to choose from. GPS always has the possibility to fail, for the simple fact that the US military can just turn if the civilian frequencies. GPS is more or less an addition and not a crucial system (except for GPWS - while most planes have radar altimeters, they can’t react fast enough in some cases)
For those interested, here’s a Video where GPS gets jammed and there’s an explanation on what they do and what happens, when it gets spoofed www.youtube.com/watch?v=4dG_Whxzdkk