I'm not sure there IS an alternative. Ultimately, knowing where you are is in comparison to some known location (even inertial navigation is based on knowing a

I'm not sure there IS an alternative. Ultimately, knowing where you are is in comparison to some known location (even inertial navigation is based on knowing a starting location and tracking movement away from that). GPS "just" provides a set of globally-visible waypoints rather than having to map those that are only locally-visible ahead of time. In theory, I suppose you could use the movement of astronomical bodies to do the same, but making a portable device that can automatically detect (without necessarily LoS the sky) the relative positions of sufficient markers seems like a tough ask.