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Moon Communications Earth NASA

Can Astronauts Use GPS To Navigate On the Moon? (ieee.org) 99

Posted by BeauHD from the lunar-positioning-system dept.
schwit1 shares a report from IEEE Spectrum: Here on Earth, our lives have been transformed by the Global Positioning System, fleets of satellites operated by the United States and other countries that are used in myriad ways to help people navigate. Down here, GPS is capable of pinpointing locations with accuracy measured in centimeters. Could it help astronauts on lunar voyages? Kar-Ming Cheung and Charles Lee of NASA's Jet Propulsion Laboratory in California did the math, and concluded that the answer is yes: Signals from existing global navigation satellites near the Earth could be used to guide astronauts in lunar orbit, 385,000 km away. The researchers presented their newest findings at the IEEE Aerospace Conference in Montana this month.

Cheung and Lee plotted the orbits of navigation satellites from the United States's Global Positioning System and two of its counterparts, Europe's Galileo and Russia's GLONASS system -- 81 satellites in all. Most of them have directional antennas transmitting toward Earth's surface, but their signals also radiate into space. Those signals, say the researchers, are strong enough to be read by spacecraft with fairly compact receivers near the moon. Cheung, Lee and their team calculated that a spacecraft in lunar orbit would be able to "see" between five and 13 satellites' signals at any given time -- enough to accurately determine its position in space to within 200 to 300 meters. In computer simulations, they were able to implement various methods for improving the accuracy substantially from there.
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Can Astronauts Use GPS To Navigate On the Moon? More

Can Astronauts Use GPS To Navigate On the Moon?

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  • Lol, sure, if... (Score:2, Interesting)

    by meerling ( 1487879 )
    If you can pick up the signal of at least 3 of the GPS satellites, and you're GPS device is set to compensate for being outside their orbits, and for the relativistic shifts due to speed and direction differences due to the moon, or your orbit of the moon if you haven't landed yet, then sure, it'll work fine.

    Sometimes even NASA forgets to take those relative speed differences into account and have to deal with it after the fact.
    (Once they realize what was wrong in the first place.)
    Don't forget, the GPS sate
    • Forgot to mention, if you're on the darkside, you're probably screwed since I doubt you'd be able to receive signal from enough satellites.

    • If you can pick up the signal of at least 3 of the GPS satellites, and you're GPS device is set to compensate for being outside their orbits, and for the relativistic shifts due to speed and direction differences due to the moon, or your orbit of the moon if you haven't landed yet, then sure, it'll work fine.

      Sometimes even NASA forgets to take those relative speed differences into account and have to deal with it after the fact.
      (Once they realize what was wrong in the first place.)
      Don't forget, the GPS satellites are geosynchronous with Earth, NOT Luna.

      I thought there were some speed and height limitations on GPS where it wouldn't work anymore.

      Maybe it's just the civilian side.

      • I thought there were some speed and height limitations on GPS where it wouldn't work anymore.

        Maybe it's just the civilian side.

        I think that's on the receivers, to make sure the answer they give is plausible. With 4 satellites there are actually two solutions in space and time, but one of them will be outside the satellites' orbit and would vary wildly if it's the wrong solution (which it will be for terrestrial users). The satellites don't care (or know) where you are or how fast you're travelling.

        • Yes, you're correct. But also, most civilian receivers are/were nerfed to stop naughty people using them in missile guidance systems.
          No problem for well-financed "terrorists" backed by a large nation-state, of course.
          (If you can build a device capable of > 1000 Knots and >18Km then figuring out the GPS is probably the least of your worries...)

          "In GPS technology, the phrasing “COCOM Limits” is also used to refer to a limit placed to GPS tracking devices that should disable tracking when th

          • But also, most civilian receivers are/were nerfed to stop naughty people using them in missile guidance systems.
            No, civilian GPS are not "nerfed".

            In earlier times, they simply sent a wrong signal, which made you displaced by a few hundert meters. Military GPS knew the displacement and could calculate the correct coordinates. The displacement was planet wide a fixed offset, so local governments came to the idea to set up a reference point. E.g. you take a mountain which is high enough to cover about 100km (o

            • That is nonsense. An ICBM enters the atmosphere under so much speed that it is surrounded by ionized gases, receiving GPS signals is impossible in such circumstances. And the ability to hit a target always was already in the 10m range long before GPS even existed.

              The realities of the physics did not in any way interfere with Congress's ability to pass a law. Which they did. The law exists. Civilian GPS receivers comply with it. Is it a useless and silly restriction? Yes. When has that ever stopped Congress?

          • Re: (Score:2)

            by Agripa ( 139780 )

            There are other restriction as well like civilian receivers may not use a antenna phasing for jam resistance which is too bad since it would help considerably with multipath distortion in cities.

        • Re: (Score:2)

          by GoRK ( 10018 )

          The reason is not technical, but is instead a legal mandate that receivers should not work above certain altitudes and speeds to prevent their use in weapons systems. These COCOM limits are frequently a problem for amateur rocketry and high altitude balloon launches and other civilian aerospace users. Although it's "law," receivers often implement their enforcement (if any) in nonstandard ways. It is easy to find receivers that are not limited, and there are open source GNSS hardware and software receivers

          • The reason is not technical, but is instead a legal mandate that receivers should not work above certain altitudes and speeds to prevent their use in weapons systems.

            So, only applies to equipment manufactured by US-interacting corporations.

            Waiting for Huawei to release an unrestricted GPS receiver/ decoder ASIC for use world-wide, with a drop-in replacement for more popular restricted ASICs. They have a significant amount to gain, and nothing to lose. Sell them at a lower price than a Motorola (or other re

      • Maybe it's just the civilian side.

        The "civilian side" (AKA Selective Availability) has been turned off for almost 20 years.

        • Ahh, yes. I remember as well when this was disabled: it was too troublesome to enable that higher availability mode for military equipment, supplying and maintaining the decryption keys, so it was left enabled for everyont.

        • They're not talking about SA, they're talking about COCOM restrictions on GPS receivers. GPS receivers are supposed to refuse to operate at high altitudes and speeds as part of arms export regulations.

      • There are (or used to be) limits in the domestic GPS receivers to stop other nations using them for ballistic missile guidance, just search for COCOM limits on GPS.

      • I thought there were some speed and height limitations on GPS where it wouldn't work anymore.
        No.

        The problem in this case is that only satellites that are "behind" the earth but not covered by the earth can be "seen" from the moon as they direct their signals to earth and by missing earth and hitting the moon you can calculate your position. But I doubt it will have cm accuracy.

        • Re: (Score:2)

          by rnturn ( 11092 )

          Receiving the signals around the limb of the Earth would likely exacerbate the ranging distortions that the ionosphere imposes on the position solutions---worse than an Earthbound receiver would see as the signals traverse even more ionosphere. I understanding that the positioning accuracy that the paper presented was/is around 300m---which is about what the original design spec assumed would be available using the C/A code tracking. SA came about when the original user equipment was found to be performing

    • Re: (Score:2, Insightful)

      by digitig ( 1056110 )

      If you can pick up the signal of at least 3 of the GPS satellites, and you're GPS device is set to compensate for being outside their orbits, and for the relativistic shifts due to speed and direction differences due to the moon, or your orbit of the moon if you haven't landed yet, then sure, it'll work fine.

      About half the constellation should be visible from the moon at any time, so seeing 4 (3 is not enough) satellites should be no issue. And I doubt NASA would be planning on buying an off-the-shelf receiver from Amazon. The biggest issue I can see is that the dilution of precision would be poor because the satellites would all be in pretty much the same direction.

      • Actually, the only satellites that are useable are the ones on the far side of the Earth. The half youre talking about have their backs toward the moon. GPS satellites beam their signals with highly directional antennas down toward the surface. Spacecraft have to use the sats on the far side of the planet which are pointing their antennas their way but are visible over the limb of the planet. And yes, the PDOP would be terrible.

        • Actually, the only satellites that are useable are the ones on the far side of the Earth. The half youre talking about have their backs toward the moon. GPS satellites beam their signals with highly directional antennas down toward the surface. Spacecraft have to use the sats on the far side of the planet which are pointing their antennas their way but are visible over the limb of the planet.

          Surely they're not so directional that the earth's attenuation is less significant than the sidelobes? The transmitters would need a massive aperture for that to be the case.

    • *NOT* GeoSync (Score:5, Informative)

      by DrYak ( 748999 ) on Thursday March 19, 2020 @03:42AM (#59847744) Homepage

      Don't forget, the GPS satellites are geosynchronous with Earth, NOT Luna.

      None of the satelite positioning system (GPS, Glonass, Galileo, Beidu, etc.) is on a geosynchronous orbit.
      That's why you need to have an ephemeris table (either fetched from the internet or downstreamed at snail pace from the sats themselves).
      That's why the actual visible sattelite at any fixed position change over time.

      That's also why they work at any position on the earth surface, including the poles (Well for the networks that have enough coverage, at least).

      • The question I have, though, is whether the GPS satellites' antennae emit in the proper direction to be detectable from the Moon. If they're designed to focus their transmission down, then at best all you'd get is sidelobe signals, which would be significantly weaker (antenna designers try to minimize sidelobe signals, because that lets them put more signal downrange in the desired direction). Additionally, the math is going to be more complicated; you can usually get a fairly wide spread when reading from

        • The question I have, though, is whether the GPS satellites' antennae emit in the proper direction to be detectable from the Moon. If they're designed to focus their transmission down,

          That's already addressed even in the summary:

          Most of them have directional antennas transmitting toward Earth's surface, but their signals also radiate into space. Those signals, say the researchers, are strong enough to be read by spacecraft with fairly compact receivers near the moon.

          I wouldn't be expecting positioning satellite to be extremely focused, because (unlike, say some type of communication) a positioning receiver ideally isn't only listening to the satellite most straight above (though it migh

          • ...indeed, because you don't have a large spread of satellite across the whole sky, all of them pretty close. Instead all the 13 visible satellites at any time will all be pretty close to each other and all be globally ~380'000km away.

            On the gripping hand, putting a couple of GPS-series satellites into Earth-Moon L4,5 positions, pointing at the Moon, would considerably improve all aspects of the location.

            Actually, putting one at L4 (or L5) and one at L2 would probably help even more, but the L2 one wouldn'

            • On the gripping hand, putting a couple of GPS-series satellites into Earth-Moon L4,5 positions, pointing at the Moon, would considerably improve all aspects of the location.

              Again, positioning system satellites aren't geostationary, you DO NOT need to have them at fixed position in the sky. Thus you do not need them at Lagrange point. You could just have a couple of extra satellite orbiting the Moon and increasing the precision.

              Actually, putting one at L4 (or L5) and one at L2 would probably help even more, but the L2 one wouldn't have an obvious directional preference, so you'd need a pretty wide-angle antenna on the feed, with consequences for range.

              The only interest of putting precision-enhancing positioning satellite at lagrange points in the future is if there are some future misson going there [wikipedia.org] and you could piggy back a GPS emitter on them.
              Now the thing is, if you're considering piggy backing,

              • Again, positioning system satellites aren't geostationary, you DO NOT need to have them at fixed position in the sky.

                I know that. You're confusing me with someone who misunderstands that point. I've dispensed corrective messages to others in this thread on that point.

                A satellite at one of the Lagrange points would need station-keeping fuel and an ephemeris equation in the receivers in any case ; it's position on the sky would always be varying, just not as much as the position of a terrestrial GPS satel

    • Me reading headline: "What?!? Of course not!"

      Me reading article: "Whoa dude that's some impressive work..."

      They took all the things you mention into account and still think it's possible.

      • They took all the things you mention into account and still think it's possible.

        Unpossible. The world's subject matter experts in every field are exclusive to Slashdot comment boxes.

      • Spacecraft have been using GPS from orbit outside the constellation for a while now. This is no different, except for the distance involved.
        • Citation required.

          Specifically, which spacecraft, whose orbital position is not already known to sufficient precision from the ephemeris calculated from their actual launch parameters (not the calculated parameters, because not all cores of a booster stage fire).

          Spacecraft do have their ephemerides updated, regularly, but TTBOMK that it the result of either horizon-crossing of the radio detectability ( a very cheap on/ off + clock measurement) or optical/ radio positioning in the sky (needs "setting circl

    • Another change from terrestrial GPS receivers is that you wouldn't need to account for the differing speed of light through the atmosphere.

      It's possible (for someone who knows a lot more than me) to build your own GPS receiver from first principals, which avoids the limits placed on commercial receivers (>1900km/s and/or >18,000m). Here's an example [aholme.co.uk] which uses a custom FPGA board, coupled to a Raspberry Pi. If a sufficiently dedicated and knowledgable person can build one at home, I suspect a satelli

      • Another change from terrestrial GPS receivers is that you wouldn't need to account for the differing speed of light through the atmosphere.

        Agreed, as long as you are including the ionosphere in your atmosphere. Which most people do accidentally, but in this case, it is significant - a large (the largest?) part of the jitter in a received GPS signal is in delay of nanoseconds in transmission through the 70km+(?) ionosphere, giving much of the ~10m raw uncertainty in a GPS location. But if you nail a GPS rece

    • NASA managed to achieve ten meters of precision with Apollo radio navigation in the 1960s, using much more primitive equipment than GPS satellites and modern receivers and computers. The idea that they couldn't make a lunar GPS receiver today is quite ridiculous.

    • Re: (Score:2)

      by rnturn ( 11092 )

      ``Don't forget, the GPS satellites are geosynchronous with Earth, NOT Luna.''

      Uh. No they're not. Their semi-synchronous (12 hour orbits). Gawd. I hope NASA didn't forget to take that into account.

      • I never quite understood how semi-synchronous orbits are not a subcategory of geosynchronous orbits. After two orbital periods, the satellite is in the same place in the sky just like as after just one period.
        • For a geosynchronous satellite, if you point your receiver at it, it receives (or sends) for [maths: day_length * 1.0000] of a day.

          For a semi-geosynchronous satellite, in the same position twice daily, if your receiver's beam width is (say) 10 degrees (about your fist-width at arm's length), then your receiver/ transmitter will connect for [maths: day_length * 2(twice-daily) * (10/360) * (beam size)]
          or [maths : day_length * 0.50505...]

          Is 5% of a day nearly equivalent to 100% of a day? If you still think

    • The GPS satellites are NOT geosynchronous. Perhaps you do not know what geosynchronous means? They DO orbit the Earth. If that is what you mean when you say geosynchronous, then you are correct even though that is not what the term means.

      Geosynchronous means not only that the satellite orbits the earth, but also that its period of rotation about the Earth is such that its position over the surface of the earth does not change. The other word that is often used is geostationary. A geosynchronous/geos

      • Geostationary is not the same as geosynchronous.

        Geostationary means that the period is 24 hours and the orbital inclination is 0 degrees (aligned with the equator). These orbits keep the satellite constantly over the same point above the equator. This is where you find most communication satellites.

        Geosynchronous means only that the orbital period is 24 hours. Since these orbits can be inclined, they do not always keep their satellites over the same spot on the surface of the Earth.

        Geostationary orbits a

      • To us a GPS receiver in space, outside the Geostationary orbital radius, I expect that new algorithms will be needed in the receiver to accommodate being "so far out there". Who knows what a consumer GPS receiver would give for a soution? Something bizarre, I suppose.

        That does rather propose an amusing experiment.

        Hmmm. Do you remember the geometric concept of "duals" [polygons, polyhedra, curves, surfaces where (measure-point to centre) * (measure-point to dual-point) = constant (IIRC) ? So, I think that f

    • Don't forget, the GPS satellites are geosynchronous with Earth, NOT Luna.

      Alternatively, do forget that, because it's completely untrue.

      In fact, Even if some GPS satellites were geosynchronous, you'd still need at least two on the sky visible from your location which are off the celestial equator.

      Unless my memory is wildly wrong, all satillites of all the GPS constellations are on orbits inclined about 60 degrees to the celestial equator (the exact number varies for constellation).

    • what one would call 'theoretical science' since no one is going to the moon since ... last time , so since Musk has actual plans a better question would be "can astronauts use iPhones to facebook and sext on mars ?" and gps yea, sure ...
  • Land on the moon then go "Siri, Navigate to Starbucks!"

    • It's only a matter of time before we have the technology to power our way from planet to plant without necessarily relying on orbital mechanics and gravity boosts. Mars will be at X spot in two months, point the ship and go.

      This might require many navigational beacons placed strategically throughout the solar system

      I wonder if any enterprising scientist at Jet Propulsion Labs or NASA have given this any kind of theoretical thought.

      • > It's only a matter of time before we have the technology to power our way from planet to plant without necessarily relying on orbital mechanics and gravity boosts.

        The alchemists of the ancient world also doubtless believed that transmuting lead to gold was only a matter of time. We've learned enough to understand the mechanics, but the energy costs remain outrageous and uneconomical.

        • Re: (Score:2)

          by sycodon ( 149926 )

          And the world is flat, Sea monsters be there, etc.

          Plasma Drives [wikipedia.org], Ion Drives [wikipedia.org], etc. When you can lift a gigawatt reactor into space and make it the heart of the ship, then you suddenly have lots of energy.

          BTW, you can turn lead into gold now [thoughtco.com].

        • We actually have the technology, since decades.

          E.g. a rocket engine based on nuclear decay, fueled with water ... Someone already answered to you with the plasma engine ... and yes the vasimir engine is more than a decade old. A nuclear decay based engine is probably the most primitive engine thinkable ... only "NIMBY" reasons and the question how to refuel it (it likely has not enough power to launch with water from earth) are preventing us building one.

      • Re: (Score:2)

        by cusco ( 717999 )

        We already have myriad navigational points throughout the solar system, they're called planets and we know very precisely where they are at any particular moment. No beacon is necessary as long as the sun is still shining on them.

      • I wonder if any enterprising scientist at Jet Propulsion Labs or NASA have given this any kind of theoretical thought.

        The necessary code was published away back in the early 1990s. It is public domain - government un-classified work.

        "Point the ship, light blue touch-paper and go" is possible. If you want to waste money. The reason that complex route are chosen are for either faster delivery (for a given payload and launch vehicle) or cheaper deliver (given payload and flexible arrival time.

        it is not a pro

    • Asking siri to navigate to starbucks is how you got to the moon in the first place.
  • and can substantially reduce the risk of landing upsets, because you'll land it in the flat spot.

  • Choose starting point: this crater
    Choose destination: that crater over there

  • 100 to 200 meters is fine for determining orbital positions, but isn't great if you're trying to land in a particular location.

    Maybe there's a case for sending out automated landers with GPS receivers around any planned landing site. They can acquire enough GPS location fixes over a few months to measure their exact location then provide simple RF beacons for accurate local navigation within less than a meter.

    • 100 to 200 meters is fine for determining orbital positions, but isn't great if you're trying to land in a particular location.

      Maybe there's a case for sending out automated landers with GPS receivers around any planned landing site. They can acquire enough GPS location fixes over a few months to measure their exact location then provide simple RF beacons for accurate local navigation within less than a meter.

      Or, they could just output some signals that can be used for landing guides. There's no need for the beacons to know where they are relative to earth.

    • Yes, I used to do GPS differential correction on data all the time back when I was surveying. We used to have to do it manually after collection with the old Trimble units. Now, they can use ground stations in real time, although it's not as accurate as doing it after the fact.

      Real time DGPS from a couple moon ground stations could probably cut this error by a factor of 2-5.

      Sam

    • There are two simple ways:
      a) use ground based radar and tell the probe where it is - that is classic
      b) use astro navigation, drawback is you need a camera or some antenna to lock on pulsars etc. - also classic

      I guess the idea to use GPS is based on the fact that GPS receivers are a commodity now ... every (smart)phone has one.

  • Otherwise we would be there now.

    • Otherwise we would be there now.

      Based on what. Why would we be there? What would we be doing there? You go to places to do a thing. You need to have a reason to go somewhere and plan to get there.

      It's like me saying it was all talk that I was going to drive to Germany 4 weeks ago and I'm never going because I wasn't there, completely ignoring that my car was in the workshop.

      Your logic ... isn't logic.

      • Re: (Score:2)

        by MrKaos ( 858439 )

        It's like me saying it was all talk that I was going to drive to Germany 4 weeks ago and I'm never going because I wasn't there,

        I isn't "we" as much as one isn't many. Singular and plural nouns have a function. We is referring to the human race. You are not the human race.

        Your logic ... isn't logic.

        You criticize my logic with such an obvious flaw in your own logic.

        completely ignoring that my car was in the workshop.

        If your car was in the workshop for longer than it took to build the car in the first place you would question sincerity of the multiple excuses given for the delays.

        Otherwise we would be there now.

        Based on what. Why would we be there? What would we be doing there? You go to places to do a thing. You need to have a reason to go somewhere and plan to get there.

        We have had ample industrial capacity to go back to the moon for decades. I personally believe there are reasons to go back to th

  • Just put up a GPS around the Moon (Score:3)

    by bobs666 ( 146801 ) on Thursday March 19, 2020 @07:13AM (#59848166)
    I don't think trying to use the Earths GPS will do what you want. So send up an array of Satellites to the Moon and Mars. As well as any other Body big enough and interesting to need location data.
    • With 60 or so starlink sats dumping out of a single launch it does make you wonder how cheaply another global (or lunar etc) positioning system could be established.

      Reddit has some interesting writeups related to this

      Starlink solution to GPS denied environments [reddit.com]

      What is the cost of an atomic clock? [reddit.com]

      • it does make you wonder how cheaply another global (or lunar etc) positioning system could be established

        Starlinks are in much lower orbits than GPS satellites, and have a correspondingly shorter lifetime.

        The original (and replacement) PGS satellites were deployed 2 or 3 to a launch. The more recent replacements 3 or 4 to a launch. You might get a dozen to a Starlink-a-like launch, and still need 5 or so launches.

        to put them into lunar orbits ... you'd be lucky to get 2 per launch (at my guess). you've go

  • Since NASA's moon rocket is years behind and way over budget, Musk will use one of his own rockets to place 60 GPS Satellites around the moon before landing there.

  • "position in space to within 200 to 300 meters"

    I'd hate to miss the LM on the other side of a hill by that margin while the oxygen supply of my backpack is running out...

    • Well, the computer says we've landed, within a margin of error of a few hundred meters. I assume that we didn't crash into the surface since we're still alive, so...watch that first step, OK?

    • the oxygen supply of my backpack is running out.

      If you have been so stupid as to put yourself in that sort of position (by not following the "thirds" rule), than you deserve your fatal re-education, and any offspring you have also deserve to die (or minimally, be publicly sterilised) for your error. you're a dangerous failure, and the species is better with you and your genes dead.

  • Are the signals from the GPS satellites omnidirectional? I don't know where to look at the specs but I am pretty sure they optimize their energy over the disc of the Earth from their point of view. From the pictures those are directional antennas.

    So if that is right that means that if you are on the surface of the moon you could have line-of-sight to a GPS satellite that you can't get a signal from. You are more likely to get a signal from a much further satellite that appears closer to the Earth's

    • The GPS sat antennas are quite directional. You are correct in that receivers located outside of the GPS constellation (such as near the Moon) would have to use signals from satellites on the far side of the Earth near the limb of the planet to still be in their beam pattern. A spacecraft can use a much larger and more sensitive antenna than we use on handheld units on Earth to help with this a little.
  • Just because you can receive a GPS SV signal doesn't mean your position solution will be much good. It's the geometry. The VDOP would be favorable because you'd be using SVs that are essentially directly overhead. But for accurate, overall positioning you want to use SVs that are spread out at lower elevations (relative to where you are) in addition to those appearing overhead. You simply won't get that when near the moon.
    • The VDOP does not get better with a bunch of overhead sats. The DOP depends on how spread out the satellites are, and if theyre in unfavorable geometric configurations (such as straight lines). Any small angular grouping of sats will result in poor DOP, VDOP included.

      • Re: (Score:2)

        by rnturn ( 11092 )

        The GPS-based landing system prototypes we were building in the '80s depended on using multiple overhead SVs. (Testing was tricky because the constellation wasn't even fully populated yet.) You want more to be seeing more SVs that are more overhead---you'll have larger PDOP (from larger HDOP) but you don't care about that so much. Lowering VDOP is your goal when knowing your altitude is important. True, your overall positioning accuracy is better when tracking a widely spread selection of SVs but the config

    • But for accurate, overall positioning you want to use SVs that are spread out at lower elevations (relative to where you are) in addition to those appearing overhead.

      Which is why I suggested putting a GPS-class satellite at L4 and L5, to give at least one signal from a substantially different distance to the rest.

      Putting one at L2 would help considerably too. More than one at L1

  • GNSS in LEO, GEO and Beyond (Score:3)

    by BobC ( 101861 ) on Thursday March 19, 2020 @11:55AM (#59849358)

    A while back I participated in a study into using GNSS (GPS, BeiDou, Galileo, etc.) with SmallSats in LEO. We got a basic simulator running that showed the signal processing changes needed (mainly increased Doppler compensation), and were done.

    Then we got curious and started raising the orbit. We could get good fixes all the way out to GEO, but once above the GNSS constellations (generally located in MEO) most of the antennas are pointed away, so we had to use some tricks to get more "satellites in view", such as using signals bounced from oceans.

    We then increased antenna gain so we could use these fainter signals, only to find that we became able to see some fixed terrestrial beacons! This let us push the simulation all the way out to lunar orbit, including the lunar Lagrange points.

    To further improve the signal, we simulated GNSS satellites at the lunar Lagrange points (all but L3). This could work! But it would likely prove far more practical to have just one at L1, then a separate lunar constellation.

  • "But I was just following my GPS, and it TOLD me to turn into this crater! What was I suppose to do, NOT listen to it? That's the last time I ever use Apple Maps..."

  • The Apollo missions got to the moon and back just fine with gyroscopes and the DKSY flight computer/ground station tracking long before there was a such thing as GPS.

    This is a solution looking for a problem.

  • Turn on your GPS. Detects you're on the moon. Comes up:
    Congratulation, you're on the moon. Please enter in your code to unlock Moon Mode.

    Let the jokes begin.

  • The G is for "Global". The moon is a globe.

    Of course it'll work. Duh.

    It's science!

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