GPS has relatively few satellites... but if in doubt, I think Starlink with its absurdly large fleet of satellites can be used as an alternative. Maybe not at a precision level good enough for precision farming, but good enough to roughly guide troops to where they need to go.

Modern cruise missiles and stuff like Storm Shadow, Taurus etc. already rely on sensor fusion - GPS, inertial navigation, radar, optics and probably a ton more to always keep track of the position.

Assuming Starlink is a no-show for whatever reason, airliners and ships will have to revert back to stationary navigation aids aka your good old light towers and radio beacons.