[mfsch]

Not sure what techniques exactly you’re referring to, but the commonly used techniques for GPS enhancement (e.g. RTK [1]) are not simple distance measurements between GPS receivers and base stations but rather both stations measuring the satellite signal and using the difference of the signal in both locations to improve position estimates.

I imagine it’s possible to set up a ground-based network, but you would need a high density to cover large surfaces (you want to see at least four stations from every position). I also imagine that it would be difficult to get accurate vertical positions if the stations are all in the same horizontal plane.

[1]: https://en.wikipedia.org/wiki/Real-time_kinematic_positionin...

[Gwypaas]

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

[1]: https://en.m.wikipedia.org/wiki/LORAN