This thread reminded me of an unassuming edit I made on Wikipedia:
Since the advent of Software Defined Radio, GPS simulator applications have been made available to the general public. This has made GPS spoofing much more accessible, meaning it can be performed at limited expense and with a modicum of technical knowledge.
I cited this Defcon talk as a reference, and it’s worth your time if you find this issue interesting. The gist of it is that creating a signal with false information is no longer on the cutting edge of military technology, but something pretty much anyone can do. With a bit of rudimentary programming, you could create a position stream based on AIS data from a target ship and beam it aboard with a directional antenna, walking it off its intended track and pretty much taking it where you want.
Given how much trust we put in a gps derived position, I’m afraid that bad thing will happen before effective countermeasures are put in place.