"Terry Goodrich" writes:
[About a robotic moon sample return]
Questions:

2. Could the orbital GPS system possibly help with guidance? Would it have
the range?

This is an interesting question; I suspect it could be made to work.
You would be using the satellites on the far side of the earth, using
the radiation that is aimed at the earth but missed. The beams of the
GPS transmitters are wide enough that you will (probably) be in the
primary beams of some satellites (the Earth covers +- 13 degrees from
the GPS, but the primary beam is about 18 degrees wide) and in the
sidelobes of many (these go out to about 40 degrees). Experiments
such as 'Falcon gold' have detected these with simple patch antennas
out to GEO altitudes.

http://www.navsys.com/papers/Falcon_Gold_Project.pdf
http://www.navsys.com/Papers/9901001.pdf

The moon is about 10x further away, but all the
GPS satellites are in the same direction, so a higher gain antenna
can be used, if you keep can keep it pointed at earth.

The accuracy is probably more than good enough. There is no ionosphere
to compensate for. The range (distance from Earth) has no geometry
problems, and will probably be good to a meter or so. Position on
the sky (or moon) will have geometric dilution of accuracy, since
the satellites are all in one direction. The moon is about 20x
further away than the GPS orbits. From geometry, this should lead
to estimates about 200 times worse in the cross directions, I think,
though I'm no GPS expert. But that's still only 200 meters or so.

So I guess GPS could be used. It could certainly be experimentally
tested, and if it works once it should be OK after that (at least
until they upgrade the satellites. More sophisticated GPS
satellites might have narrower beams or reduced sidelobes.)

Lou Scheffer