NASA Uses GPS On the Moon For the First Time

An anonymous reader quotes a report from Popular Science: On March 2, Firefly Aerospace's Blue Ghost made history, becoming the first commercial lunar lander to successfully touchdown on the moon's surface. The groundbreaking lander is wasting no time in getting to work. According to NASA, the joint public-private mission has already successfully demonstrated the ability to use Earth-based GPS signals on the lunar surface, marking a major step ahead of future Artemis missions. Accurate and reliable navigation will be vital for future astronauts as they travel across the moon, but traditional GPS tools aren't much good when you're around 225,000 miles from Earth. One solution could be transmitting data from the Global Navigation Satellite System (GNSS) to the lunar surface in order to autonomously measure time, velocity, and position. That's what mission engineers from NASA and the Italian Space Agency hoped to demonstrate through the Lunar GNSS Receiver Experiment (LuGRE), one of the 10 projects packed aboard Blue Ghost. [...]

"On Earth we can use GNSS signals to navigate in everything from smartphones to airplanes," Kevin Coggins, deputy associate administrator for NASA's SCaN (Space Communications and Navigation) Program, said in a statement. "Now, LuGRE shows us that we can successfully acquire and track GNSS signals at the Moon." LuGRE relied on two GNSS constellations, GPS and Galileo, which triangulate positioning based on dozens of medium Earth orbit satellites that provide real-time tracking data. It performed its navigational fix at approximately 2 a.m. EST on March 3, while about 225,000 miles from Earth. Blue Ghost's LuGRE system will continue collecting information over the next two weeks almost continuously while the lander's other tools begin their own experiments.

Hey Siri, Where am I?

[rossdee]

You are on the moon.

Re:

[laughingskeptic]

Only if Siri knows the Moon's orbit well enough, otherwise the position is just a lat,lon with an altitude of 362 million meters.

I'm surprised

[Ol Olsoc]

That there was any question of it working or not. It would work a little weirdly, since the receiving station (blue ghost) would receive its data from outside the GPS sats orbits, but the principal is the same. Geolocation via Time Delay of Arrival with highly accurate clocks, and known satellite positions. Possibly they were worried about signal strength?

All that said, this is pretty cool.

Re:

[Barny]

Possibly the time shift too. The longer the distance from your triangulation points, and the more chances of relativity affecting their signals, the more accurate they need to be.

Re:

[PPH]

Yes. But that's a receiver problem. It may be beyond the capabilities of commercial GPS units. But the algorithms can be tweaked.

Re:I'm surprised

[RockDoctor]

The (radial) difference in gravitational gradient is most severe near the surface of the Earth, so between GNSS orbit and Earth you'd probably get considerably more relativistic error than between GNSS orbit and the Moon. How much more ... beyond my calculations. but the gradient is in that direction.

The problem with using it would most likely be that the proportional difference between Receiver and Satellite01 versus Satellite 02 would be smaller. From the Earth's surface, IIRC GNJSS is at about 4~5 earth radii up, so you'd easily get a 20% difference. From Moon to GNSS orbits, you'd get (working numbers from memory), about 380,000/320,000 difference, or 12% difference.

Not as bad as I'd thought. You'd get considerably bigger error bars on position, so it's advantage over dead reckoning wouldn't be so much. Which might be more of a problem if you're shipping people to work (outdoors) on the Moon who can't navigate by the stars using the Mk-1 human eyeball and brain. But really, who's going to be doing that.

(Bear in mind : I work making geological maps underground. Mapping my way into wherever I'm working is not even as distant as second nature - it's first instinct. It gives me a headache to think of being unable to work out where you are unless you arrived there unconscious. Though I've met people so handicapped and pity them their disability.)

Re:

[VaccinesCauseAdults]

Interesting. Do you use GPS or GNSS systems underground? I assume the signal is generally unavailable. Are there similar local systems using high intensity (or perhaps frequency) used for triangulation and navigation underground?

Re:

[SteelCamel]

The maths is a bit beyond me, but I would expect accuracy to be best when the satellites are widely spaced. It's basically triangulation, and when triangulating on the ground you want to choose landmarks as far apart as possible to get the best accuracy. In the worst case, if the landmarks are so close you can't separate them, you don't get a position fix.

On Earth, the satellites are widely spread across the sky, covering anything from 100 to 180 degrees depending on terrain. On the Moon, they're all in a v

Re:

[Orgasmatron]

Signal strength is a huge issue, considering that by the time the signal reaches the Earth's surface, it is already well below the noise floor, and the moon is much more distant than the earth's surface.

The broadcasts are actually Pseudo-Random Noise (PRN) blocks which trade time for energy, sending far less than one bit per symbol. Things like WiFi work in the other direction, using QAM or TCM to trade energy for time, letting them send more bits per symbol.

There was an extensive write-up on the technical

I'm surprised this works at all

[v1]

GPS works on triangulation, and gets much more accurate measurements from satellites that are positioned such that the satellites and the receiver on earth form triangles with large angles. Small angles (such as those from satellites almost directly overhead) provide very low precision. If all the satellites you can hear are right above you, the distance between the satellites is small compared to your distance from them, and the vertex at your location is a very small angle.

But when you're on the moon, ALL the satellite you can receive are at almost the same arc from you, so the angles at the receiver are ALL very small. (all the satellites are "directly above you" or at least in the same location in the sky no matter where you go) That should produce very poor precision information. Maybe it's not as big of a deal on the moon where you're not looking for inches-precision, and getting within say a mile is okay? We're so used to getting turn-by-turn accuracy right to someone's door here. On the moon I suppose they might be happy to just know which crater they landed in?

Re:

[drinkypoo]

Down side, all angles are narrow

Up side, you can see more sats at once

Re: I'm surprised this works at all

[dj245]

In the previous article [slashdot.org], several people estimated between 4 and 50m accuracy on this basis. Seems to be in the ballpark of what they are getting. Whether or not that is useful enough I will leave to those who know better.

Re:

[Owza]

I wonder what happened to this (2022) https://scitechdaily.com/satel... [scitechdaily.com]

Re:

[omnichad]

***checks GPS***

"Yep, still on the moon."

Re:

[RockDoctor]

But when you're on the moon, ALL the satellite you can receive are at almost the same arc from you,

If my memory is correct (if!), then the GNSS constellation is about 120,000 km in diameter, and you're viewing it form about 380,000 km range. So they'll cover a 17~18 wide patch on the sky.

Far from ideal ; but not unusable.

Adding one or two clocks in peri-lunar orbit would help a lot. Putting 4 or more in Earth orbit at ... 600,000 km forn Earth would probably help a lot.

Oh, hang on - they're only looking

Re:

[necro81]

If my memory is correct (if!), then the GNSS constellation is about 120,000 km in diameter, and you're viewing it form about 380,000 km range. So they'll cover a 17~18 wide patch on the sky.

Alas, your memory is not correct. GPS satellites are in medium Earth orbit, or about 20,000 km altitude. (Galileo is similar, 23000 km.) So from the Moon the constellation only spans about 5-6 degrees of the sky.

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[laughingskeptic]

GPS works on trilateration which is similar to but not the same as triangulation. It measures the time that it takes signals to propagate from three or more sources with known positions and then converts those times to distance using the speed of light. The calculated position is then the point of intersection of the spheres. trilateralization does not suffer from the "small angle" issue. It still works if all three satellites are on top of each other at different altitudes -- there will only be one solu

Re:

[thegarbz]

What you describe doesn't make sense. Any uncertainty in the trilateration introduces error which is proportional to the distance from the source of the signal. Even in the scenario where the satellites are on top of each other, the further away you are from them the more any positional error increases as the perfectly straight line drawn slightly askew forms an error based on the angle.

Some very fancy mathematics involved

[0xG]

Since time passes at a different rate on the moon from earth, relativity and all that.
Of course, not much good on the far side of the moon.

Re:

[laughingskeptic]

1 second elapsed on Earth looks like:
1000000000 + 0.44 nanoseconds at geosynchronous orbit
1000000000 + 0.66 nanoseconds on the Moon.

The relativity induced error on the Moon would be less than on Earth and this error would be more biased towards the vertical axis of the position computation on the Moon than on Earth.

Re:

[VaccinesCauseAdults]

The standard GPS calculations include corrections for both Special Relativity and General Relativity. The magnitudes are different and the signs are opposite at Earth surface. While the differences are small per second, the different clock rates between satellites and receiver have to be taken into account. I wonder what the difference would be between taking an off-the-shelf consumer GPS unit and letting it do standard Earth-based calculations versus modifying a GPS to do the full lunar calculations.

Re:

[RogueWarrior65]

Yes, but no atmospheric distortion or noise from human-activity-generated RF should help a lot. Plus, I'm sure they are using some big ass antennas (comparatively) and amplification.

ATMOS

[zawarski]

This is your final destination.