Lunar orbits are not all that “unstable”; even at 100km it takes less than
10 meters/ sec of delta-v per year to maintain.
Bill
On Wed, Jul 31, 2019 at 7:07 AM David McMillan <skyefire@xxxxxxxxxxxx>
wrote:
On 7/30/2019 5:32 PM, Henry Spencer wrote:
There's no reason why a GPS-type system wouldn't work in lunar orbit,
given about the same number of satellites as the Navstar constellation
for Earth (it doesn't work without four satellites simultaneously
visible, and preferably more). If you want to use a smaller number of
satellites, you can reinvent Geostar, which requires that the ground
terminals transmit as well as receive (GPS is receive-only because of
its military origins), and can do low-bandwidth messaging too. If you
can settle for occasional position updates rather than fresh positions
every second, you *might* be able to reinvent Transit, which will give
you a fairly precise position from a single pass of a single
satellite, although it might not work quite as well on a world
rotating as slowly as the Moon.
How much would any of these constellation concepts have to rely on
ground stations for ephemeris updates? I know that GPS and Galileo rely
heavily on ground stations constantly updating the clocks on every
satellite, to the point that in the event of a Zombie Apocalypse both
constellations would degrade pretty rapidly (keep your paper maps handy,
preppers!). And given how unstable Lunar orbits tend to be, my
off-the-cuff guess is that a Lunar GPS would be very likely to need
regular position/velocity updates from some trustworthy ground
reference, in addition to clock updates. And given the distances
involved, it would seem like Lunar ground stations would work better
than ones based on Earth or in Earth orbit.
