Casual Conversation

Used GPS on boats as early as the late 1980’s, back when selective availability was still a thing. For those unfamiliar with it, GPS was initially military-only, and when they allowed civilian use they were concerned that US adversaries could use it for precision attacks. So SA was included in the civilian version, which introduced a random error of up to something like 1000 yards.

The truly ironic thing is that the US Coast Guard (a branch of the very military that created SA) saw the usefulness of GPS for marine navigation, but only if SA was removed. 1000 yards could easily mean the difference between a boat running aground (or worse) or not.

So the USCG built ground stations that would receive GPS signals, calculate the SA error, and broadcast a fixed signal. That was called Differential GPS, or DGPS. Boaters could buy special DGPS receivers for years, which were as accurate as GPS without the SA error.

Eventually the military was convinced to do away with SA entirely so DGPS was retired. It was only after that happened that GPS became globally useful for car, hiking, etc. navigation.