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NASA Successfully Acquires GPS Signals on Moon
- tecleandor 2y agoOooooh. I guess that although the signal will be fainter, as they're ~21x further away than usual (240k miles vs 20k), they'll have the advantage of having less noise and practically no signal bouncing. What I don't know is: Even when receiving a good signal... how difficult would be calculating location when satellites are going to be all concentrated in a really small portion of the sky, and all of them in a proportionally small distance between them, compared to the distance of the receptor?
- TimorousBestie 2y agoGeometric dilution of precision (GDOP, or just DOP) is used to evaluate the quality of a GPS satellite configuration; on the back of an envelope any moon-based solution is going to be quite a bit worse relative to terrestrial ones.
- gcanyon 2y agoAt any location on the moon that has a clear view of the Earth, you'll have access to >= half the GPS satellites -- so 15-16+. On Earth that number is as low as 4. The logic would be different (having to pick the farthest apart to get the clearest data to work with) but I can't imagine that it would be problematic for determining location.
- LVB 2y agoI suspect the usable number is much lower and would be just those satellites mostly opposite the Earth but with some signal reaching the moon? I recall the beam width of GPS antennas being like 30 deg (?), so almost all of the signal is directed at Earth.
- h3half 2y agoThey'll be using sidelobes. This is what's done at geo altitude which is also above the GPS orbits. At geo the commercial satellite I worked with had position accuracy within about ten meters, and we always had access to 6-8 GPS satellites at a time. Obviously at the moon the signal is much fainter but my understanding is that it's essentially the same just harder to detect
- BenjiWiebe 2y agoMost receivers on earth aren't only using the GPS constellation. There's also Galileo, BeiDou, and GLONASS. Just pointing out that the typical "GPS" accuracy we're used to seeing isn't happening with only 4 satellites in view.
- nickcw 2y agoThe geometry will reduce the accuracy of the fix though as all the satellites will be in the same 8 degrees of the sky. I wish they had said in the article what the accuracy is!
- silverquiet 2y agoIs 8 degrees the angular size of the Earth from the moon? Aren't GPS satellites in relatively high orbit, so it could potentially be a larger patch of sky.
- wanderingstan 2y agoI hadn’t realized it, but you are right. - Orbit height: 20,200 km - Earths diameter: 12,760 km https://www.gps.gov/systems/gps/space/ https://www.gps.gov/systems/gps/space/ https://science.nasa.gov/earth/facts/ https://science.nasa.gov/earth/facts/
- volemo 2y agoIt’s the satellites’ size. The satellites are 20 Mm high above the ground so their spread is 53 Mm, which is 4.4 times the diameter of the Earth (12 Mm). So yes, the angular size of the satellite cloud (7.896°) is quite a bit larger than the Earth (1.785°) from the moon PoV.
- mnw21cam 2y agoAlthough since the GNSS satellites use directional antennae pointing at Earth, this experiment only picked up signals where the satellites are on the other side of Earth and close enough to its edge for some of that directional signal to leak past the edge of Earth and get to the Moon. So, the satellites that are nearly 4° away from the centre of Earth cannot be detected because they are beaming their signal nowhere near the Moon, and the detectable angular size is much less than 7.896°.
- bilsbie 2y agoI suppose you also know where the moon is, how it’s rotated and possibly your altitude on the moon. So you could treat that as a virtual satellite in the other direction.
- bobmcnamara 2y agoHow do you know those?
- maweki 2y agobesides altitude: you'd just need to know the time which the GPS signals give you. From there it's just calculating rotations.
- sohkamyung 2y agoWhile the headline says GPS, the article says signals were acquired from GPS and Galileo, which increases the number of GNSS satellites available to get a location fix.
- 7952 2y agoMore details at https://www.gpsworld.com/lugre-receiver-captures-gnss-signals-in-lunar-orbit/ https://www.gpsworld.com/lugre-receiver-captures-gnss-signal... "Despite the challenges of distance and velocity, the receiver achieved position accuracy within 1.5 km and velocity accuracy within 2 m/s. It successfully acquired signals from four GPS satellites (L1 and L5 frequencies) and one Galileo satellite (E1-E5 bands) during a one-hour observation window. Post-landing,"
- madaxe_again 2y agoAt 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.
- thatcherc 2y agoA challenge there is that there are very few stable lunar orbits! High orbits are perturbed by Earth's gravity (3-body problem) and low lunar orbits are perturbed by the lumpy distribution of mass in the Moon's interior [0]. Lunar GNSS satellites with a little bit of onboard propulsion could probably correct for some of these perturbations but once they ran out of fuel they would have a limited orbital lifetime. [0] - https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects_and_low_orbits https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effec...
- adgjlsfhk1 2y agocould the legrange points work? dealing with the 3 body orbits would be a pain, but they would give you nice separation
- mkl 2y agoYou need signals from 3+ different locations to navigate, and the two stable Lagrange points, L4 and L5, are as far from the moon as Earth is: https://en.wikipedia.org/wiki/Lagrange_point https://en.wikipedia.org/wiki/Lagrange_point.
- blebo 2y agoDon’t forget your phone on the Artemis mission so you can catch some sweet rare Pokémon!
- bayindirh 2y agoI used to get excited for these kinds of news, and from a science perspective, this is very cool. On the other hand, as we get closer to colonize other planets, or at least try to plan for this end, I get depressed more and more. We're enough burden for a single planet, and definitely too much for a solar system. Honestly, no, I don't want more humans around.
- volemo 2y agoI understand the premise that the humanity is a burden for the Earth (though I disagree — we only make the conditions worse for ourselves, “The planet is fine; the people are fucked!”), but how could we possibly be a burden to a barren wasteland like Mars?
- palata 2y ago> although I don’t agree: we only make the conditions worse for ourselves, “The planet is fine; the people are fucked Except that we are measurably living in a mass extinction. Most species are dying, except for us (at the moment). So I disagree: we make the conditions worse for all species. The climate change that we are measuring now is happening a lot faster than the one that got the dinosaurs (and most big animals) extinct. 96% of animals on Earth are cattle, living in the conditions we know. I am not sure we can say "we don't have any impact on other species".
- volemo 2y agoSure we cause a mass extinction, I’m not saying “we don’t have any impact on other species”. My point still stands, the nature doesn’t care, this isn’t the first extinction and I bet won’t be the last. However, you didn’t address my main question: how can we be a burden to a red wasteland?
- bayindirh 2y agoThe nature doesn't care yet, but it showed that it might, and it will if we don't stop abusing the ecosystem. Ah, at worst, all humanity will go extinct. Which doesn't matter much at the grand scale. It'll be exciting for the next ones, if the planet is left in a state to allow another such evolution, or somebody else likes the colors and wants to visit for a couple of revolutions around the Sol. > how can we be a burden to a red wasteland? We are not sure that it's a red wasteland. We think that life has a single foundation and will evolve from that one. Maybe it had a similar ecosystem before, and tons of bacteria are in hiatus. Maybe there's something else underground. Maybe there are other living organisms which we can't detect. I don't think that Mars has worms which might eat us for snacks and giggles, but I'm not sure that it's devoid of life completely, either. Absence of evidence is not evidence of absence.
- trebligdivad 2y agoI like this because it's such a neat unexpected idea to try.
- deleted 2y ago[deleted]
- AnonHP 2y agoIt is not clear from the article, and these are noob questions: does it mean there were no other time dilation effects to take into account? In other words, is the adjustment done within the satellite clocks enough for the signal processing near or on the moon to get the position since the moon is so far off)?
- deleted 2y ago[deleted]
- mandevil 2y agoGPS has always had to account for relativistic time dilation, it's the first human scale issue where relativistic correction became necessary. (This would be general relativity, incidentally, not special: it's the effect of the Earth's gravity on the atomic clocks in orbit.) But that's because the whole system needs absurd levels of accuracy to be useful at all: at 8 km/s of orbital velocity every source of error creates enormous error bars on the ground. I also know that NASA has experimented for years with satellites (even all the way up in GSO) using GPS signals for position-finding, so this is further out but not unprecedented work.
- Qem 2y agoSo GPS a misnomer now, given not only "global" anymore. Perhaps better change it to CPS, Cislunar Positioning System.
- jiehong 2y agoI like it! How about SPS: Satellite Positioning System
- ooterness 2y agoAbsolutely not. The GPS and Galileo spacecraft are all pointed at Earth. This demo is a special receiver that can piece together glimpses when that signal happens to slip past the edge of the Earth and reach the moon.
- queuebert 2y agoMaybe someday it can be part of a Galactic Positioning System.
- JoeAltmaier 2y agoSo when do we put up GPS satellites around Mars? It makes sense. Or just put them on the moons I guess. They're pretty far from the surface - 9K and 14K compared to GPS of 12K so maybe not bad. And less atmosphere in the way. Also less radio noise?
- deleted 2y ago[deleted]
- mandevil 2y agoThe issues would be A) that Mars upper atmosphere and internal mass distribution are not as well mapped as on Earth, so knowing your orbital accuracy is much more difficult (1)- and at 8km/s small orbital error bars become giant error bars on the surface and B) Putting a full constellation of 24-30 satellites around Mars is going to be really expensive. That's more than the sum total of all satellites to successfully orbit Mars to this day (18). 1: On Earth we account for that by using ground stations to track the satellite locations, with the ground station locations determined very very precisely using non-GPS techniques (old school surveying techniques). On Mars, that's not going to be possible until we get a lot more done, probably a later human mission would be the first time that could be done.
- bluGill 2y agoWe don't need all of mars though. One GPS receiver on your lander will give you enough information for the area you can feasibly reach - and if not just old school survey some location to park another receiver to map things out.
- JoeAltmaier 2y agoAll good points. The solutions for Mars will necessarily be unique to that environment. Still, a combination of ground stations and satellites will be inevitably used for location-finding. And, the atmosphere? What atmosphere? It's negligible compared to Earth. Got to be down the list of important variables. We can send a mission to Mars and arrive within a few meters of desired orbit, but it's going to be hard to figure out where a satellite is? My doubt-meter is hitting the pin.
- schobi 2y agoThey tried on the moon, but there does not seem to be an assumption that this is limited to the moon? So one could obtain a position anywhere in earth's orbit, up to heights of 380.000km? 1.5km accuracy is impressive then. Apart from the attenuation from distance, I would expect that the navigation sallellites point their antennas mostly downwards to earth, but you might find some that radiate outwards. I don't think you can expect to receive from half the satellites though.
- rossjudson 2y agoThis is awesome. It means we will know where we aren't when we're on the moon.
- rossjudson 2y ago...then is immediately ordered to sell them to a private equity firm, payable in $TRUMP.
- codewritinfool 2y agoI'm surprised that any useful accuracy can be obtained. Maybe an additional input to the solution would be to watch when various GPS satellites are occulted by the Earth.
- jlarocco 2y agoI'm curious what coordinate system they're using and how the math worked a little more. I guess technically they could use latitude and longitude projected all the way out to the moon, but that would be pretty hard to use.
- lnauta 2y agoSuper cool! This only works on the side facing the GNSS constellation, right? There is no signal to use on the other side.
- 7e 2y agoUse 1 km towers spaced every 100 km on the Moon's surface instead. No satellites needed.
- ant6n 2y agoThat’d be like 4000 towers
- mmooss 2y agoIf the comparison was 4000 towers on Earth compared with 32 satellites in high orbits, and if we assume the same cost as standard radio towers (?), then I'd wonder which cost more - especially over a lifetime which includes servicing, etc. For the moon, it depends on many factors that differ from Earth. The radio towers' structures probably need less material, at least.
- andrewmcwatters 2y agoBreaking: NASA discovers using Lunar GPS that taking a slingshot detour around the moon is still faster than commuting directly through I-10 during rush hour.
- qwertox 2y agoWould this be used for location services or rather timestamping? Would it be possible to use celestial navigation to obtain a more precise location?
- JumpCrisscross 2y agoDoes this make the ESA’s Pathfinder [1] redundant? Or are they measuring something materially different? [1] https://www.esa.int/ESA_Multimedia/Images/2023/06/Satnav_from_Earth_to_the_Moon https://www.esa.int/ESA_Multimedia/Images/2023/06/Satnav_fro...
- gunian 2y ago[flagged]
- mmooss 2y agoMy prior understanding was that the Artemis project included creating PNT (position, navigation, timing) in cislunar space, and that Earth's GNSS satellites wouldn't be sufficient. Is that plan now changed?
- maxglute 2y agoSmart munitions on moon when?