This is the fourth time I’ve see an article about this without any giving a concrete example of a real planned experiment that would depend on this level of sensitivity. (GPS cannot be used on the moon, nor does GPS require the user to have an ultra-precise clock.)
mentions precision spatial positioning and its use for spacecraft docking, but this does not require having a separate reference clock anymore than it’s needed to use GPS at high altitudes on Earth. (Again, the GPS system does not require the user to have a high-precision clock.) They also mention the dependence of the SI units on the unit of time, which really suggests the writer has no idea how measurement precision is achieved.
Figure 10 in the paper shows that after a few hours of signal acquisition positioning errors should be down to the single or low double digit meters.
In case it's unclear, this isn't a "lunar GPS system", but listening to the scatter of the Earth-based GPS signals we've got now, on the Moon.
I don't know about the rest of your comments or claims, but in general GNSS signals are used for lots of unexpected and novel things. E.g. people are detecting North Korean missile launches with GPS (the rocket plume casts a measurable "shadow" as the signals travel through it).
You asked for a "real planned experiment" and then asserted that "GPS cannot be used on the moon". This is a real, planned experiment to use GPS on the Moon.
NASA has already conducted experiments to use GPS in Earth orbits halfway to the Moon[1].
I'd think the physics of further signal propagation and the ability to receive them would be well understood.
I’m not saying your comment wasn’t relevant context, just that (a) your assertion that GPS can be used on the Moon is not currently true and (b) the future possibility doesn’t answer my original question for reasons mentioned.
NASA hasn't been to the surface of the Moon in any meaningful way since the 70s.
So, you're correct, but how is that relevant?
Obviously all discussions about "NASA plans to do X on the Moon" and "if NASA's doing X on the Moon, we've worked out that they can use Y to accomplish X" are hypothetical at this point.
So, in that context, what's the relevance of your claim that "GPS cannot be used on the Moon".
I agree that if you're going to insist on empirical evidence that no, we don't actually know if it works on the Moon.
But if that's your goalposts then I'd think you'd worry more about the hypothetical astronaut reading that GPS position or GPS time having long since asphyxiated.
After all we've got no empirical evidence that the space suits planned for use on the Moon actually work there, etc.
It's all inferred at this point, we'll see about the experiment. But if I had to bet that signal propagation works the way we expect (GPS), v.s. say some joint failing when filled with lunar regolith (spacesuit), then my money's on the GPS experiment.
My understanding that this is to enable things like a lunar GPS system, which requires the satellites themselves to have precise clocks that are synchronized with each other.
> They also mention the dependence of the SI units on the unit of time, which really suggests the writer has no idea how measurement precision is achieved.
Say more? I thought an accurate clock would be necessary to derive a reference meter and kilogram measurement.
> which requires the satellites themselves to have precise clocks that are synchronized with each other.
Yes, the satellites themselves need to keep track of their GR-induced drift relative to the other satellites. But we don’t typically describe this as needing a new timekeeping system (or new timezone, or whatever) for each satellite. We just say the GPS system makes corrections for GR, which it does. And in any case, this takes place on the satellites, not the lunar surface, which is what is mentioned in the announcement.
> I thought an accurate clock would be necessary to derive a reference meter and kilogram measurement.
So first, the fundamental SI definitions are almost never used in the field to make measurements. For the vast majority of field experiments, the measuring equipment is calibrated back at a lab using a reference system, which itself was calibrated using a chain of multiple intermediaries that eventually trace back to the fundamental SI definition. But the equipment necessary to connect to the fundamental definitions are extremely expensive and delicate, which is why it’s only done occasionally and in a few labs specialized for the purpose.
Second, even if you were measuring absolute lengths in the field with the distance light traveled in 10^X ticks of a cesium atomic clock or whatever (as opposed to the much more mundane task of measuring relative length changes with interferometry), it would all work perfectly fine if you did it within a local region of uniform gravitational field. The only reason you need to worry about the difference in clock speeds is if you are comparing event timing being done in different places where the clocks are running differently (as happens with GPS, or something like the LISA gravitational wave experiment, but which is otherwise quite unusual).
Like, don’t get me wrong, I can sort of imagine that NASA is actually planning such experiments. But then they should be mentioned in an announcement like this.
The actual White House proposal
https://www.whitehouse.gov/wp-content/uploads/2024/04/Celest...
mentions precision spatial positioning and its use for spacecraft docking, but this does not require having a separate reference clock anymore than it’s needed to use GPS at high altitudes on Earth. (Again, the GPS system does not require the user to have a high-precision clock.) They also mention the dependence of the SI units on the unit of time, which really suggests the writer has no idea how measurement precision is achieved.