Sure it is a concern. Backups?

What happens when you lose GPS updating? You revert to inertial and radio updating (I can speak only for 777 and probably most Boeings). Yes, there's still a navigation solution being calculated. However your operations become greatly degraded. You can't fly in RNP4, RNP2, FANS (depending on location), or ADS-B require airspace. You can't fly any procedures that require GPS.

Since writing my first post on the subject above, on my last flight we had GPS jamming indications over Eastern Montana/Western South Dakota. I checked the POS REF page and sure enough we were back to radio updating. It didn't cause any further issues other than some safety paperwork to be filed.

Please reread my first three sentences. The concern isn’t for Boeing guys, it’s for light aircraft. Many Cessna, Pipers, etc have very limited backups, if any. Due to the ADS-B mandate, during a GPS outage their position will only be know by primary radar. All transponder, including Mode C, functions will be inoperative.

My point was I don't see it as a concern for just light aircraft.

You are right it’s a very important issue considering ADS-B usage for separation.

I think you are wrong about mode C being inoperative. When I was in a known jammed area I asked atc if they had our ADS -B and they said no but they still had mode C.

My apologies. I misread your post.

Cheers

Depends on the spectrum(s) involved, and the power at your location.

GPS signals are pretty weak, I don't know for sure but strongly suspect that mode C (and other modes) will take more power to jam than GPS. GPS is probably always going to be the weak link in terms of which systems are susceptible to jamming or other interference. That and sat based wifi

After 2020, no reason for people to keep mode c transponders in their planes, I assume many will be removed, those that remain probably won't be serviced or ever turned on.

The “backup” plan is still ground-based navigation, VOR’s. With NextGen implementation, there is a Minimum Operational Network (VOR MON) that will be maintained. For FMS that revert to DME/DME/IRU updating with the loss of GPS, there will also be aMinimum number of DME stations

Yes. They have to maintain a ground-based backup. Every single GPS satellite flies over every single one of our potential antagonists, several of which have asat capability. If so inclined, they could probably splash the entire constellation in a few days.