[jandrewrogers]

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.