GPS distortion
Most cargo ships i worked on had very basic autopilots. Usually only capable of holding a heading fed from the gyro-compass with a back up to switch over to the magnetic compass should the gyro fail (happend to me once in the Singapore Strait, ... ). Some autopilots were capable of holding a GPS course which was seldom used, only very few ships at the time had ECDIS (Electronic Charts) and were capable of following a Track, like we do in aircraft. Within lets say 24nm from the coast GPS is not normaly used for primary navigation but the radar is. Plotting radar bearings on a paper charts always got us through even the most challenging waters. Also compass bearings is a great way to get a good position, simple and reliable. Out on the ocean we generally put the ship on a heading and plotted a GPS position every 4 hours. Als long as your within about 10 to 20 nm from you intended track, it will do. Besides on the ocean you dont run the risk of GPS spoofing cause you need to be fairly close to the ship. I used to keep my skills in celestial navigation up, though mostly for the fun of it. A practiced navigator can get a position by using a sextant within 2nm of the GPS position so its more than accurate enough to get across an ocean.
Sometimes US Navy convoys would jam the GPS signals, which is a non event really, some areas is the Mediterranean have bad GPS coverage, we always got through, amzazing... Bottomline its a nice experiment, but of very little value for real life provided we still have professionals on the bridge who know how to navigate.