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At any location on the moon that has a clear view of the Earth, you'll have access to >= half the GPS satellites -- so 15-16+. On Earth that number is as low as 4. The logic would be different (having to pick the farthest apart to get the clearest data to work with) but I can't imagine that it would be problematic for determining location.

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Most receivers on earth aren't only using the GPS constellation. There's also Galileo, BeiDou, and GLONASS.

Just pointing out that the typical "GPS" accuracy we're used to seeing isn't happening with only 4 satellites in view.

I suspect the usable number is much lower and would be just those satellites mostly opposite the Earth but with some signal reaching the moon? I recall the beam width of GPS antennas being like 30 deg (?), so almost all of the signal is directed at Earth.

They'll be using sidelobes. This is what's done at geo altitude which is also above the GPS orbits.

At geo the commercial satellite I worked with had position accuracy within about ten meters, and we always had access to 6-8 GPS satellites at a time. Obviously at the moon the signal is much fainter but my understanding is that it's essentially the same just harder to detect

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