NASA Successfully Acquires GPS Signals on Moon
155 comments
·March 5, 20257952
querbu
More details:
+ 44dBm GPS transmit power
-210dB path loss
+ 15dB rx antenna gain
That's slightly worse (5dB) than signal strength of a handheld device/ phone (~15dB typical handheld loss, urban environment). There is still ~15dB margin. They are cheating a little with the 15dB narrow antenna gain, which requires accurate pointing. Nice result and there is room for improvement. Note: -210dB = 10^-21
JumpCrisscross
> They are cheating a little with the 15dB narrow antenna gain, which requires accurate pointing
Stars. Finding Earth is a necessarily solved problem for E-L communication.
kridsdale1
Only for 2 weeks per month!
slow_typist
That number doesn’t tell much without knowing the background noise on the L-bands out there. It would also be interesting to know whether they achieved this with commercial Gnss receivers.
querbu
Since the high gain antenna is pointed at the entire earth, the noise temperature is similar earth temperature. Semi-custom gnss receiver, they collect mostly correlator outputs, and 2.5 seconds of raw IQ
zokier
> It would also be interesting to know whether they achieved this with commercial Gnss receivers
They used Qascom QN400 receiver.
H8crilA
Are those reference numbers for L1 signals, or for L5 signals? I remember that L5 uses much longer chip sequences, and thus can deliver much higher processing gains.
querbu
L5 can achieve 10-15 dB in ideal circumstances, with tuned loops. Not more. And those aren't ideal circumstances - the GPS signals are received mostly when satellites are grazing the earth (from the moon POV).
throw0101d
> 2 m/s
7.2 kph; 4.5 mph.
jiehong
> 7.2kph
You mean 7.2 km/h.
Thanks for the conversion, though!
hunter2_
While kph is uncommon and ought not to have come about, the first sentence here [0] acknowledges it, so I don't think it's fair to say that people who use it don't mean what they say.
queuebert
7.2 km h^-1 if you want to be fully SI correct
madaxe_again
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.
thatcherc
A 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_effec...
staplung
Technically, satellite positioning only needs 1 satellite. GPS requires several but one of its forerunners was Transit[1] which I believe only needed a signal from a single satellite at a time. It worked by measuring the doppler shift of the signal coming from the satellite. Of course that only works if the orbit can eventually cover all (or much of) the surface and for all I know there is no such frozen orbit for the moon. Also, it would still presumably require extensive surface-based tracking and correction.
BWStearns
I wonder if lunar space elevators might be the fix here. If I understand correctly, such an elevator would not be as subject to the perturbations since the tension would keep it's orbit stable (is it still an orbit if it's tethered?).
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.
0_____0
Oh wow, good reading in that link.
I had no idea the moon was that lumpy. The wiki entry says that despite the mascons there are 4 known stable orbital inclinations?
adgjlsfhk1
could the legrange points work? dealing with the 3 body orbits would be a pain, but they would give you nice separation
ekianjo
Is the 3 body problem mostly meaningful above a certain mass? If you had small satellites could you deal with it?
t43562
Lunar Pathfinder.
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.hammock
1.5km is not bad. Chris Columbus could resolve latitude to about 100-200km, and longitude only by dead reckoning.
I do wonder though with computers and cameras and celestial navigation, why that is not used vs GPS on the moon
patmorgan23
Are there any existing systems for that? Would you be able to resolve to a similar level of accuracy with computer vision looking at the stars?
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)
mlyle
I'm curious what the assumptions are. Looking forward to reading the paper.
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.
mhb
Wouldn't the next step be plopping some down on the moon?
mannykannot
I was wondering that too, and that led to the question whether, without an atmosphere, all radio communications at any wavelength would be strictly line-of-sight. It turns out, however, that the moon has something of an ionosphere, though I don't know whether it would support over-the-horizon radio (or, for that matter, whether it tends to interfere with the accuracy of GPS, as it does on Earth.)
mmooss
> I suppose the next inevitable step is LPS
That has been planned for awhile as part of Artemis.
null
tecleandor
Oooooh. 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?
nickcw
The 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
Is 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.
volemo
It’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.
wanderingstan
I hadn’t realized it, but you are right.
- Orbit height: 20,200 km
- Earths diameter: 12,760 km
gcanyon
At 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.
BenjiWiebe
Most 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.
LVB
I 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
They'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
TimorousBestie
Geometric 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.
bilsbie
I 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
How do you know those?
maweki
besides altitude: you'd just need to know the time which the GPS signals give you. From there it's just calculating rotations.
sohkamyung
While 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.
AnonHP
It 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)?
mandevil
GPS 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.
null
schobi
They 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.
lnauta
Super cool! This only works on the side facing the GNSS constellation, right? There is no signal to use on the other side.
trebligdivad
I like this because it's such a neat unexpected idea to try.
JumpCrisscross
Does 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_fro...
mmooss
My 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?
JoeAltmaier
So 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?
mandevil
The 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
We 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
All 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.
mandevil
Atmospheric drag on satellites- especially how it changes with solar output levels- is a hard thing to model accurately, and a major contributor to orbital uncertainty here on Earth. The Martian atmosphere is two orders of magnitude thinner, but it is far less than two orders of magnitude understood. And the level of our understanding matters for our ability to correct for it's perturbations.
NASA is very good at sending spacecraft through regular space and hitting precise windows (MCO units issues aside), it's in orbit that things get more complicated, because now there are just a lot more potential interactions to deal with. We can use LOS on planetary occultations to give you some data, but it's still a lot of work to get from there to mascon maps, upper atmospheric data, etc.
staplung
> So when do we put up GPS satellites around Mars?
A system like GPS? Probably never. It would be fantastically expensive and solve a problem that no one has. In any case, the moons would be a poor choice for signal transmitters: 1. landers are harder than satellites 2. two moons is not enough for a system like GPS 3. three-body problems mean that we can't really know the future configurations of the system with high precision on anything but the very short scale.
In any case, it costs something like $700 million per year to operate the GPS system here on Earth.
7952
Navigation just seems comparatively easier than earth. You are much more likely to have a clear view of the terrain or sky. And the terrain is much less likely to change than the earth so computer vision should be easier.
JoeAltmaier
Already several orbiters around Mars now. To include a GPS radio in each would have been negligible further cost.
And even three orbiters would give you a better fix than none.
I think the naysayers are reaching, to argue against GPS transmitters around Mars. It seems inevitable.
JumpCrisscross
> when do we put up GPS satellites around Mars? It makes sense
Once we have tens of Starships of annual transport between Earth and Mars such that putting about a dozen satellites in Mars orbits every decade or so [1] is cost effective.
Using Elon math that’s the 2030s. Ignoring his mortality-driven forecasts, probably the 2050s.
They competition would be balloons, which can be made from indigenous polyethylene [2], floated above a settlement with a loud radio. You’d have range and direction home, which should be good enough for decades, potentially into the 2100s when, on a very optimistic schedule, inter-settlement transfer begins to become common.
null
null
jlarocco
I'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.
rossjudson
This is awesome. It means we will know where we aren't when we're on the moon.
More details at 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,"