Firefly's Moon-Orbiting 'Blue Ghost' Lunar Lander Tracked Earth-Orbiting GPS-Type Satellites (behindtheblack.com) 38
Long-time Slashdot reader schwit1 shared this observation from space/science news blogger Robert Zimmerman:
Having now reached lunar orbit in preparation for its landing on March 2, 2025, an engineering test instrument on Firefly's Blue Ghost lunar lander has now proven that even from that distance spacecraft can use the multiple GPS-type satellites in Earth orbit to track their position.
[From NASA.gov]: The Lunar GNSS Receiver Experiment (LuGRE) acquired and tracked Global Navigation Satellite System (GNSS) signals for the first time in lunar orbit – a new record! This achievement, peaking at 246,000 miles, suggests that Earth-based Global Navigation Satellite System constellations can be used for navigation in transit to, around, and potentially on the Moon. It also demonstrates the power of using multiple GNSS constellations together, such as GPS and Galileo, to perform navigation.
After lunar landing, LuGRE will operate for 14 days and attempt to break another record – first reception of GNSS signals on the lunar surface.
This test is a very big deal. It tells us that operations on the Moon, at least those on the near side, will likely not require a GPS-type infrastructure in lunar orbit, thus allowing a lot of difficult missions to proceed sooner while saving a lot of money and time.
[From NASA.gov]: The Lunar GNSS Receiver Experiment (LuGRE) acquired and tracked Global Navigation Satellite System (GNSS) signals for the first time in lunar orbit – a new record! This achievement, peaking at 246,000 miles, suggests that Earth-based Global Navigation Satellite System constellations can be used for navigation in transit to, around, and potentially on the Moon. It also demonstrates the power of using multiple GNSS constellations together, such as GPS and Galileo, to perform navigation.
After lunar landing, LuGRE will operate for 14 days and attempt to break another record – first reception of GNSS signals on the lunar surface.
This test is a very big deal. It tells us that operations on the Moon, at least those on the near side, will likely not require a GPS-type infrastructure in lunar orbit, thus allowing a lot of difficult missions to proceed sooner while saving a lot of money and time.
To what accuracy? 1km? (Score:3)
Accurate to what? 1km? 5km? You might be able to triangulate with similar accuracy simply based on signal strength of terrestrial AM and FM radio signals.It's another story if they can reliably resolve position within 100 meters. That will put you within a month's crawling distance of your science objective
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https://www.asi.it/en/2025/02/... [www.asi.it] says:
Despite the significant distance and high speed, the position was calculated with very high accuracy, with an error margin of about 1.5 kilometers for position and about 2 meters per second for velocity.
The distance from Earth means there is not much angular separation between signals, which increases the "dilution of precision" and thus leads to higher uncertainty/error in the solution (colloquially called "bad geometry").
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Indeed.
also, what you want is the position relative to the lunar surface, not relative to the Earth, so the errors in the ephemeris of the moon, and the geoid model of the lunar surface, are both now additions to the imprecision.
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No, there is no bad geometry.
Because that is not how it works.
The satellite tells you *exactly* where it is above the earth surface.
And with that you know *exactly* where the satellite is in 3D space.
And that means, you only need three to calculate *exactly* where you are.
You had a point if you compared it with Astro navigation on the earth/water surface, looking at a star using a Sextant and measure its angle above the horizon. Where you would need two or three measurements in short timely succession and w
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There absolutely is dilution of precision and it absolutely is affected by geometry.
https://en.wikipedia.org/wiki/... [wikipedia.org]
Also, if you have only three satellites, you will not get a solution in this kind of application. You could get a solution if you have some other constraint on the variables, such as being on the surface of the Earth.
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The simple GNSS equations are "snapshot" solutions that work at a single point in time, and so must also solve for time as an unknown (in addition to however many spatial dimensions) -- or take it as a given, which you will not be able to do on a vehicle that you launched to the moon. Even on the Earth, your clock is almost never accurate enough to replace the fourth satellite as a constraint.
Having more satellites is interesting/relevant for increased accuracy and precision, and also for [everythingrf.com] detecting the pre
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Or you have three satellites: then you know exactly where you are.
(emphasis mine) You keep using that word. I do not think it means what you think it means.
We're talking about precision, or the lack thereof, and how that increases with distance from the satellites. There is no "exactly" in this context.
The picture (on wikipedia) and explanation is in theory correct. But in practice wrong: as you have no measurement errors with GPS.
"No measurement errors"?!? What world do you live in? See also: Entrope's post above.
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GPS is not about measuring anything.
It is about telling you: here we have 3 or 4 or 5 (or more) reference points. And those reference points are exactly where we claim they are. And from those reference points: you can exactly CALCULATE with simple geometry, where you are.
No idea what there is so complicated to grasp about that.
Geographic survey works like this since 5 or 6 thousand years. This is your plot of land? Considering this three points we set up there: your corner stones are EXACTLY here and here
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And those reference points are exactly where we claim they are.
They are _NOT_ "exactly" where they claim to be. That's one big point of discrepancy between our understanding of this issue.
And from those reference points: you can exactly CALCULATE with simple geometry, where you are.
Again, no. Here's a writeup from gps.gov on its accuracy: https://www.gps.gov/systems/gp... [gps.gov]
Basically, the best we're getting on earth is between millimeter and centimeter precision. That's not "exact", it's within a margin of error.
Geographic survey works like this since 5 or 6 thousand years. This is your plot of land? Considering this three points we set up there: your corner stones are EXACTLY here and here and here and here -> pointing to the map. And that makes your land xyz square feet/meters/mushrooms big. And is: exactly here -> pointing at the corner stones.
Crazy that you used this analogy. Land moves, measurably. Those points don't stay where they are relative to the other points or much of anything else, tho
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Basically, the best we're getting on earth is between millimeter and centimeter precision. That's not "exact", it's within a margin of error.
This is exactly.
Because that is the precision the technology gives you. Otherwise you can nitpick what ever you want: you would a way pinpoint your location using plank-space and plank-time.
What the funk is wrong with you? Since when is a mm accuracy not exactly where you are?
Right. Do you think time granularity stops at microseconds?
If your clock can only count micros
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There's so much wrong in these statements! A clock on earth and an identical one out by the moon will end up drifting apart, even if they're atomic clocks, because time dilation is real.
Yes. But there is nothing wrong. The clocks adjust themselves.
Boing.
That is how GPS works around earth. And a GPS receiver at moon, only needs the time/-ing difference of the signals it receives. Its own time frame is irrelevant.
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You only need two satellites and the surface of the earth.
No, that's not true. I'll try to explain why.
Each satellite transmits (very roughly) "My precise position is (here), and the time by my clock is (timestamp)". (In fact the satellites send a bit more - including each other's positions - but that needn't concern us here.)
With one satellite, this tells you nothing about your position: you know the timestamp the satellite thought it was when it sent its transmission, but you have no idea how long that transmission took to arrive. (Unless you already happen to h
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You only need two satellites and the surface of the earth. As we are talking about moon: you know how far away you are from the moon. So: same way to solve the problem.
And you know how far you are from the moon because you measure it by GPS!
Unfortunately, that's circular logic. Your position relative to the moon is what you want to measure. Saying "if you know your position, you can measure your position" tells you nothing.
Your spacecraft could, of course, carry a laser altimeter. But laser altimeters are much more complex than GPS receivers, and that only tells you how far you away from the surface of the moon. But the moon is lumpy at large scale and rough at small sc
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And you know how far you are from the moon because you measure it by GPS!
No, I know it because I know with what speed I approached it.
And I know because I look down onto its surface, and cam measure my distance.
But the moon is lumpy at large scale and rough at small scale. You need an accurate surface model of the moon... and you need to know where you are on the surface.
Technically true, but irrelevant if you are 1000km away, or more.
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And you know how far you are from the moon because you measure it by GPS!
No, I know it because I know with what speed I approached it.
Good lord, I hadn't quite realized how thoroughly you don't understand the problem.
Bye.
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I understand the problem pretty well. :P
But seem never to have done some geometry in school
Hint: I know I am close to the moon, or at least between the satellites and the moon.
So the "other solutions" to the equations: are all places where I am not possibly can be at the moment.
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You give a decent description of how GPS works. (in short: your position is the intersection of three spheres of radii determined by the time delay between the GPS sending the signal and you receiving the signal.)
But you didn't include the error analysis, so your description says nothing about whether the precision is worse at higher distances.
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It's not based on signal strength. It's based on clocks. Each satellite sends a signal, basically saying what time it is. The GPS device logs when the signal from each satellite told stating it's 6pm (for example) arrived and location (I'm over Tokyo, etc.), and based on the arrival time difference of each signal and using precise knowledge of where each satellite is located be at that particular time (6pm) you can triangulate your distance from each satellite and thus determine your own location. Note: D
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Yes. In particular, the big difference from AM and FM broadcast is that these signals are designed to make it easy to align them with the time they were generated, and also for multiple signals to be processed while sharing the same spectral band.
GPS L5 and Galileo E5a signals can be tracked to about 1e-8 seconds (1% of a "chip" time), and L1/E1 to about 1e-7 seconds (because they have a lower chip rate). AM signals are much too low in frequency to have that much bandwidth, and FM signals don't have the sam
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Accurate to what? 1km? 5km? You might be able to triangulate with similar accuracy simply based on signal strength of terrestrial AM and FM radio signals.It's another story if they can reliably resolve position within 100 meters. That will put you within a month's crawling distance of your science objective
Uh, hang on a minute. Are we not also going to utilize GPS for timing here?
Using GPS is quite key for sustaining STRATUM-1 timing. Positioning is nice, but signal timing might be even more useful.
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Can they connect to Starlink satellites for communication or is the latency too much?
This. Everyone is focused on positional accuracy gains with GPS which is definitely useful, but using GPS as a timing source for signaling is probably a lot more useful.
Idunno...maybe? (Score:5, Informative)
Gps gives you roughly a 50000km baseline. P code is 10 MHz, and if you can time a bit transition to 1/100th of a bit for 1 nsec timing, that's roughly .3 m / 50e6 rads of angle precision with a 350e6 lever arm out at the moon.
So that's about 2 meters of cross-range with a damn good receiver if you can magically hear the best possible baseline?
In practive the sidelobes from the gps transmitters occur about 45 degrees off axis, so you get about half the baseline and maybe 4 meter precision, best case? Maybe 20-40 meters if you allow for slop in all the usual places?
I guess that's good enough for most applications. Then again, the DSN or whatever predecessor of the DSN that was used for Apollo got the tracking to similar accuracy for landing radar hand-off using all of three ground stations.
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Even 50m precision is still very, very useful when coming in to land. It's another sensor that can validate data from things like altitude radar, and also help discard unlikely results from things like optical sensors. The recent Japanese precision automated landing used cameras to look at the lunar horizon and match it to 3D renderings of what they expected to see, for example.
Then when you get close to the surface you optically find the exact landing spot.
Just great (Score:4, Funny)
Discard the second fix (Score:4, Informative)
In college we explored replication the GPS algorithm, your final calculation would always give you two position fixes -- one in space, the other within the earth's atmosphere. Your algorithm would discard the orbital result and return the second answer.
Scratch that, reverse it, and you've got your answer for a lunar fix /s
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The one in space would be roughly equidistant from the GPS satellite as the GPS satellite was from Earth, wouldn't it? Wouldn't be much use.
But if you have ANY point of reference, you can make a navigation service, and a bunch of standardised and time-reference satellites around Earth is a great start... do the same on a few other local bodies with enough information and you basically have "Space GPS" if you can receive it.
Going forward, it makes far more sense to triangulate (trilaterate, technically) you
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True - but the sats have directional antennas, so they 'beam' their signal towards earth not away from it.
My guess is this system uses the sats that aren't pointed directly away from the moon at the time. Given the distance, I guess that actually means quite a few sats that have just come "out from behind the earth", but haven't yet orbited around the earth far enough that their signal is too directional to use. It's pretty clever.
Side note, the original design for GPS was to send a signal so weakly that on
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Schrodinger would keep both.
we forget everything... use the stars (Score:2)
how does a ICBM navigate ?
STARS you can do this mechanically if you really want
The SR-71 high-speed reconnaissance aircraft was one example of an aircraft that used a combination of automated celestial and inertial navigation.
Earth? Sin? StarÃ? (Score:2)
I hope they will also be trying more "astronomical" ways of positioning.
After all, we should have a rather precise idea of their relative position nowadays.
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Ditto IMHO. Based on my extensive experience reading golden age fiction and manga about lunar exploration, while using GNSS sats is nice to have it would be a bad day if your supplies' landing was suddenly trashed due to political issues among any of the countries that run or can spoof, hack or destroy them. No good reason not to use stars, at least to automatically check position by them, and anyone not putting markers on the lunar surface or orbit as one of the first missions is asking for it.. then again
Captain Mal says (Score:2)
I swear by my pretty floral bonnet, I will end you.