[Nrsolis]

Spoofing of GPS signals isn't generally the problem the US military is most concerned about. It's availability.

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.