[mpenet]

That. However that works just fine for ICBMs and the like…

The future is more likely to be quantum accelerometers and quantum gyroscopes, as they have no “external dependency”.

[cyberax]

More likely image-based navigation, where you just upload the entire imagery of the route and then use it to correct your inertial references.

Encrypted positioning information from low-orbit satellites is another option.

[mpenet]

That doesn't work well in some conditions, it's not new either. Some cruise missiles that have TFR (Terrain-following radar) and actually do this already.

It also is not really applicable when you are on a balistic course at *very* high altitude, course correction has to happen early in these case given the reentry speed/constraints.