Tons of info on the web how hack GPS.
https://hackaday.com/tag/gps-spoofing/
The Military needs to use encrypted GPS signals to validate the data or go back to pre-gps navigation with look outs for other ships..
GPS tells you where you are. It doesn’t tell you where the other ship is or where it is headed (about to run into you). Encrypted GPS doesn’t solve that problem.
So in these cases of US Navy collisions:
GPS failed on both the US ship and the other ship involved.
On board radar failed on the same.
Physical sighting failed on the same.
I just find it hard to believe that’s the case.
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!
The only thing a hacked GPS would cause you to run into is the ground. These ships don’t have an auto-steering Google computer or anything. And yes, they have lookouts out all the time, at least two - fore and aft.