They would have to “hack” the receiver on the ship. If someone “hacked/altered a set of GPS signals it’s going to read out the same error for everyone. As far as I know all US warships carry standard GPS units as a backup should there be a problem with the “secure” GPS system. If the two disagree there is an alarm interface and Radars don’t lie!
Interesting, did not know about the second GPS to compare.
If the Navy have something like that, they should be able to detect any hacking of the unencrypted signal that would alternate the civilian ship coarse.
I wonder if the other ships are using GLONASS for navigation and that is the system being hack, so the Navy does not know it because they watching the US system only.
No, they can mess with the signal. Much easier and has been done since GPS was invented.