With the fuzzy GPS signal you had DGPS correcting it by broadcasting the current change from a known point. [0] Before GPS LORAN [1] and it's versions was used

With the fuzzy GPS signal you had DGPS correcting it by broadcasting the current change from a known point. [0] Before GPS LORAN [1] and it's versions was used relying on ground stations [0]: https://en.m.wikipedia.org/wiki/Differential_GPS https://en.m.wikipedia.org/wiki/Differential_GPS [1]: https://en.m.wikipedia.org/wiki/LORAN https://en.m.wikipedia.org/wiki/LORAN