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>> Military solutions will fall back to inertial, celestial or optical guidance, and people using GPS for navigation will at worst show up where they need to be somewhat later than planned.

> The US Navy re-started celestial navigation a few years ago:

Also, the SR-71, B-1, and B-2 and other aircraft have automated celestial navigation systems to provide corrections to their INSs:

https://www.thedrive.com/the-war-zone/17207/sr-71s-r2-d2-cou...

Honestly, for military applications, GPS seems to be mainly useful for bad weather and providing navigation to the smallest units (and I'd think a small unit would be able fall back to a map and compass).

ICBMs also use celestial navigation systems for same reasons.

This kind of weapon has a backup INS

https://en.wikipedia.org/wiki/Joint_Direct_Attack_Munition

The INS doesn't need to be terribly high performance because it only needs to work for the time it takes a bomb to fall from the sky.

These military instructional videos are so great. They explain a lot of concepts very clearly. Nowadays videos seem to be more casual, with more focus on practice than understanding the theory.