You need signals from 3+ different locations to navigate, and the two stable Lagrange points, L4 and L5, are as far from the moon as Earth is: https://en.wikipedia.org/wiki/Lagrange_point.
But-- GPS already produces an okayish fix. Improving it with another signal from somewhere else would make a big difference.
> and the two stable Lagrange points, L4 and L5
We have plenty of spacecraft hanging out around L1, etc. It's possible to orbit it without too much issue. Having one broadcast a navigation signal synchronized with GPS would not be too bad.
> are as far from the moon as Earth is
The issue isn't that they're far away-- it's that they're all in pretty much the same direction. There's very small uncertainties in orbits and measured path length, but if they're all in the same direction you get very poor lateral position.
This is the same effect you can get if you can only see a little tiny bit of the sky with GPS. You might have enough satellites to navigate, but since they're all close to the same direction the navigation solution is much worse.
> and the two stable Lagrange points, L4 and L5
We have plenty of spacecraft hanging out around L1, etc. It's possible to orbit it without too much issue. Having one broadcast a navigation signal synchronized with GPS would not be too bad.
> are as far from the moon as Earth is
The issue isn't that they're far away-- it's that they're all in pretty much the same direction. There's very small uncertainties in orbits and measured path length, but if they're all in the same direction you get very poor lateral position.
This is the same effect you can get if you can only see a little tiny bit of the sky with GPS. You might have enough satellites to navigate, but since they're all close to the same direction the navigation solution is much worse.