At that resolution the utility is somewhat questionable - I suppose the next inevitable step is LPS, and having a fleet of selenostationary satellites performing the same function locally.
[0] - https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effec...
> 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.
I had no idea the moon was that lumpy. The wiki entry says that despite the mascons there are 4 known stable orbital inclinations?
https://web.archive.org/web/20210307002503/https://science.n...
And they're very lumpy.
> The mascons' gravitational anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. "If you were standing at the edge of one of the maria, a plumb bob would hang about a third of a degree off vertical, pointing toward the mascon," Konopliv says. Moreover, an astronaut in full spacesuit and life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's center.
Another option might be a LORAN style system put up on towers. With lower gravity and no atmosphere I imagine we could stick transmitters up very high without super complex construction, maybe even just a giant carbon fiber tube with a transmitter at the top.
https://www.sstl.co.uk/what-we-do/lunar-mission-services
QUOTE: A constellation of interconnected lunar orbiters will enable surface missions operating on the far side of the Moon, without direct to Earth line of sight, to keep constant contact with Earth. It will also provide lunar navigation signals to support critical mission phases such as precision landing of scientific equipment and the operation of rovers. In addition to communication services, the Lunar Pathfinder spacecraft has been selected by ESA and NASA to host a number of experimental payloads:
An ESA GNSS receiver capable of detecting weak signals coming from the Earth GNSS infrastructure (GPS and Galileo), demonstrating its potential role into Lunar navigation
A NASA retro-reflector to demonstrate laser ranging capabilities
An ESA radiation monitor to study orbital radiation conditions
Acting both as technology and service demonstrator, Lunar Pathfinder is the opportunity for scientific and commercial mission developers to support the development, test and standardisation of Lunar communication infrastructure, and for emerging off-planet telcos to acquire experience of lunar asset operations and off-planet service delivery.
Lunar Pathfinder is due to operate in an Elliptical Lunar Frozen Orbit (ELFO) for an operational lifetime of 8 years. The spacecraft can operate 2 simultaneous channels of communication with lunar assets: 1 in S-band and 1 in UHF. Performance, such as coverage and data-rate, depend both on the relative position of the user asset to Pathfinder at the moment of the connection, as well as the capabilities of the communication module onboard the user asset. Once safely retrieved onboard Lunar Pathfinder, communications are relayed back to Earth ground stations in X-band.I do wonder though with computers and cameras and celestial navigation, why that is not used vs GPS on the moon
Ussally enhancing an existing technology that is widely deployed and understood to fit a new situation is better than inventing something wholly new (though not always)
> Sextants can be read accurately to within 0.1 arcminutes, so the observer's position can be determined within (theoretically) 0.1 nautical miles (185.2 meters, or about 203 yards). Most ocean navigators, measuring from a moving platform under fair conditions, can achieve a practical accuracy of approximately 1.5 nautical miles (2.8 km)
Some napkin math, assuming using a Sextant to achieve similar accuracy of 0.1 arcminutes on the Moon, because Moon is about 3.7 times smaller than Earth, that 0.1 arcminutes is around 50 meters on the Moon. One can expect extremely clear sky and certainly not riding waves on the Moon, so the practical accuracy should be close.
Celestial navigation works on the Earth's surface (or the Moon's surface), because being able to determine the orientation of the local horizon (or zenith) is equivalent to determining your latitude and longitude. But that doesn't work for a spacecraft that doesn't have a horizon reference.
Of course, if you're orbiting the moon and you can accurately observe the directions to landmarks such as mountains and craters, you can fix your position relative to them. But that's not really what you'd call "celestial navigation".
I assume you would need orbital ephemeris info much like we need for GPS satellites.
But, given measurements of angles to multiple planets, how well could you estimate your position? Would there a lot of error for your normal vector to the earth's ecliptic plane?
[0] https://theaviationgeekclub.com/the-sr-71-astroinertial-navi...
a kilometer-or-so is about what you get on Earth without a lot of sophisticated corrections, averaging, and kinematics. So, if they're not doing all that stuff, they could be doing quite well. (on the other hand, one of the bigger correction terms-- the ionospheric delay -- they don't have to deal with-- but they have to deal with all of their measurements being in "one direction"). If e.g. they don't know about the moon's relative motion, that's a big disadvantage.
If, on the other hand, they get the kilometer after a -loooot- of averaging, that's quite bad.
I don't know how big of a fleet you need to make this worthwhile, though. Just one satellite in a different direction would collapse that big error ellipse to a much shorter arc.
However, building out a ground system using towers on the moon would work. With the low gravity of the moon and no wind, you could build pretty tall structures with beacons for positioning. You'd need line of sight so it would be costly to build out this system across the entire surface.
A 1km tall tower built every 58km would give you similar positioning as GPS (three intersecting circles). But also, the moon is not terribly interesting so the few long trips that would put you far from base wouldn't require instant positioning. A star tracker and an hour of watching GNSS would work just fine. If there ever was a significant population on the moon, they'd have the lunar equivalent of busses (traveling over marked paths) or light rail to get them in-between facilities.
Building a network of a very tall tower every 58km around a globe of the Moon seems very uneconomic.
That has been planned for awhile as part of Artemis.