The assumption is that in any significant event, US space assets will be unavailable either momentarily or for some extended period. Chinese Anti-SAT weapons are one indicator that this is a likely avenue of attack for any weaker actor seeking to degrade our capabilities.

So, celestial navigation is a crude but workable substitute for GPS but it has the distinct advantage of being ancient and nearly impossible to impede unless you can change the weather or the positions of the heavenly objects.

Paper from Los Alamos with a receiver-only implementation that would use:

- abnormally high signal strength

- abnormally regular transmissions

- a secondary time source to double-check time (e.g. NTP on a smartphone)

- dead-reckoning based on accelerometers/gyroscopes/compass to double-check position

http://lewisperdue.com/DieByWire/GPS-Vulnerability-LosAlamos...