Earlier quoted context omitted.
GPS was trivially attackable from inception, that was not a design requirement given its purpose. The purpose of GPS was to precisely measure the planet in peacetime. A unique core capability of the US military is extremely precise INS technology, the capabilities and precision of which are closely guarded secrets and under continuous R&D. The precision of INS is dependent on the precision of your world model. GPS al…
Most of the basic attacks against GPS also worked against TRANSIT, LORAN, OMEGA, and others, which were clearly used for military navigation. I'm not seeing the design differentiation with the systems that were guiding ships and missiles.
All US military navigation systems use INS pervasively for primary navigation. You can attack GPS, LORAN, etc and it won't affect their operations. This has been axiomatic in the design of those systems for many decades. US military ships are no different, also using INS for primary navigation. As with every other US military system, they can conditionally accept fine-tuning inputs from GPS and other untrusted sources of navigation data because why not. They use a lot of open source data sources, but they don't need them or trust them.