writes:

You have no clue what the jamming susceptibility of modern GPS is or what
features exist (current and planned) to thwart it.

Actually I do, as I've been following GPS since long before the average person
became aware of its existence. I'm afraid jamming is a serious potential
problem, for a number of reasons related directly to the technology of GPS and
to satellite communications in general. Spoofing is a serious issue, too,
which is why the DoD started encrypting its P code years ago. Unfortunately,
encryption is not a realistic option for civil aviation users, because of the
logistics of key distribution, and because it would make the signal unusable
to other user communities.

In reality, jamming effects a small area and is a real concern only to the
military which would expect jamming in the area of enemy targets.

Anyone can jam a GPS signal, and a small area is more than sufficient--if it
happens to be centered on New York City, for example. Spoofing requires more
sophistication, but hardly anything unattainable for bad guys.

A solar flare large enough to "knock them all out at once" would also take
out a lot of other stuff making the lack of GPS a minor issue.

If GPS is the only navigation option, it's a major issue even if other systems
are affected as well. VORs, at least, would still be available.