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Comment by tliltocatl | Hacker News Reader

High lunar orbits is unstable because of Earth influence (i. e. long-term orbiting requires lots of fuel for extensive orbit maintenance). Most low orbits are unstable as well because of uneven mass distribution, but there are some exceptions: https://en.wikipedia.org/wiki/Frozen_orbit Low orbits would also require either more satellites for complete coverage or synchronizing launch window, which is PITA enough as it is now.

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But would they need to be in the orbit? Since nothing is on the moon, couldn't you just create a GPS array on the ground?

For a GNSS-type system to work, you need four transmitters in sight. This would imply a moderately dense ground network on the moon for a receiver at orbital height -- and as this article points out, we're not doing great at building even a spare ground network of right-side-up transmitters right now -- but would need increasing density for a descending receiver. We don't really have challenges right now with accurately determining orbital parameters of probes above the moon, it's the final few miles where GNSS-level accuracy would be tempting; but that's also the domain where seeing four ground-based transmitters is basically impossible.

If you were trying to land on a specific site though, couldn't you drop down a dozen or so transmitters at the potential landing site? The assign them a location, and have them transmit the relative location to anything landing near them?

How would you drop them? Don't think a radio transmitter would survive a hard landing, so you will need a proper lander (with altimeter) for each transmitter. And at that point you can just strap your actual payload instead of the transmitter.

The point would be you could drop them a bit randomly, once they land, find out where they are, and tell them where they are, to then talk back to the payload as it's landing.

It's the moon, even a hard landing isn't that hard.

>nothing is on the moon

It's not exactly nothing, the moon itself with its mountains is there and blocking the radio signal and it means propagation isn't unlike that of VHF/UHF on Earth. You still only have no visibility beyond the horizon. In order for it to be visible and useful during landing you would need the base station to be reasonably close. I guess that would make it more akin to ILS/VOR/DME than GPS. That obviously wouldn't be feasible until we have a permanent base there (perhaps an unmanned one).

the moon is made of cheese everyone knows this

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