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More satellites is mostly better. Multi-constellation receivers for example can obtain higher precision by using some combination of different GNSS. There is a

More satellites is mostly better. Multi-constellation receivers for example can obtain higher precision by using some combination of different GNSS. There is a minimum error you can practically achieve this way that has a fair amount to do with the wavelength, but it does help. In most cases, though, faster/more reliable fix will probably be a bigger advantage than precision. When you have a partial sky view, more satellites means a better chance of enough satellites falling in that window for a fix.

zettabomb 3y ago
I'm curious, hoe do multi-constellation receivers use them all at once? Are the signals just all the same thing? On some level I know they're just atomic clocks, and the governing equations should work if all of them are synchronized, but I can't help but think that feeding data different constellations in all at once wouldn't produce a good result. Or perhaps you can answer somewhat the opposite question - if the data IS all the same, then why are there multiple constellations and standards? Apart from Beidou, which I believe can actually serve as a radio repeater of some kind, aren't they all doin the same thing?
- jcrawfordor 3y ago
  The calculation of a fix from GNSS is actually rather complex and the potential complexity is almost unlimited. The most accurate GNSS fixes are obtained from offline solvers that analyze long-duration (e.g. 24 hour) recordings of the GNSS data streams and use computer models to determine the most likely solution accounting for many types of error. This is all to say that there isn't really one approach to calculating a fix from GNSS, and receivers embedded in devices like phones use pretty simplified methods so that they can run in real-time and with a short time to first fix. Most GNSS are largely similar, though, and there are certainly enough papers describing how to perform fixing across satellites of independent constellations so I'm sure it's been implemented in at least higher-end devices. Still, I suspect inexpensive consumer receivers are probably calculating a fix per constellation and then combining those. Fixing across constellations requires combining ephemera, time base, etc. data that varies between them, but all of these are known with high precision, you just have to pull them together. Since most GNSS are mostly the same, new GNSS constellations are usually launched either to improve coverage in a region, or these days with several global constellations, more often for sovereignty reasons. If you have weapons systems that rely on GNSS, it's better to have your own constellation. GPS for example no longer has a wartime selective availability capability (that would deny use by anyone other than US forces) but it did in the past and, if WW3 broke out, it would probably be implemented again. For much this reason different GNSS constellations tend to operate in different bands (not always, though) so that a country can selectively jam adversarial GNSS systems without affecting their own. GNSS has its origins in the military and is still largely a military capability today, with Galileo being the odd one out that focuses more on commercial applications (Galileo's selective availability capability is used to charge a subscription for higher-precision uses). And some GNSS do have "accessory" capabilities, probably the most prominent being Galileo's two-way search and rescue messaging capability that will compliment COSPAS-SARSAT. But that's always been the way with satellites. For example, the GPS constellation carries detonation sensing equipment for nuclear counter-proliferation. It just saves money to combine payloads. One of the reasons to not put GNSS constellations in separate bands is that it simplifies the construction of multi-constellation receivers. Of course, it also makes it more difficult to selectively jam. Some combination of these interests mean that GPS, GLONASS, and Galileo are very close siblings in the radio spectrum. It gets surprisingly political.
  - bilegeek 3y ago
    > It gets surprisingly political. https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)#Tension_with_the_United_States https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)...
  - zettabomb 3y ago
    >The most accurate GNSS fixes are obtained from offline solvers that analyze long-duration (e.g. 24 hour) recordings of the GNSS data streams and use computer models to determine the most likely solution accounting for many types of error. I actually have a receiver that does exactly this! It is a Trimble Resolution T, available for very cheap (I believe they're from old cell towers), and it's intended exclusively as a time reference. You first run a 24 hour data collection, and it precisely determines your location before storing it. From then on, the receiver only calculates the time. Very cool stuff, I'm planning on building a GPS-disciplined rubidium clock and stratum 1 NTP server with it. >Still, I suspect inexpensive consumer receivers are probably calculating a fix per constellation and then combining those. This sounds like a reasonable solution now that you mention it, for an inexpensive device. Part of my confusion is based in the fact that the constellations actually use different reference datums [0]. Wouldn't the different constellations give meaningfully different fixes even if the solution was "perfect"? I mentioned in another comment that I pretty much forgot that this could be a political thing. That's a very good point and I don't know why I didn't think of it. Especially with selective availability, which as far as I understand, the US government could just turn on again at a moments notice. [0] https://gssc.esa.int/navipedia/index.php/Reference_Frames_in_GNSS https://gssc.esa.int/navipedia/index.php/Reference_Frames_in...
- deleted 3y ago
  [deleted]
- krisoft 3y ago
  > I can't help but think that feeding data different constellations in all at once wouldn't produce a good result Obviously if you just mix up the bytes willy nilly then nothing good will come out of it. How familiar are you with the terminology of mathematical optimisation? If you are then the intuition is that each satelite provides a different error term in one big function you are trying to optimise. The “knowns” you plug in to the terms are your observations (pseudorange, phase, etc) and the orbital parameters of the satelite broadcast by the satelites. The unknowns you are trying to solve are the x,y,z,t coordinates of the receiver you have. You are trying to find the unknowns which minimise the sum of the error terms. The error terms of the various constellations might have slightly different ”shapes”, but you can still run your optimisation them the same. As you add more satelite the number of unknowns remain the same (4), but you are adding more and more equations to your overdetermined system. When things are not pathological that leads to a better, more precise fit. > aren't they all doin the same thing Even if they were, sometimes it is valuable to have your own toy. If Russia is having a war in some region of the world and the US decides to turn off the GPS signals there is not much Russia can do about that. It is a US satelite and the US adjusts it as they see it fit. If Russia has their own satelites and the US would like to deny them this the US have to attack Russian assets directly. That is a very different ball game.
  - zettabomb 3y ago
    I think I know enough about optimization problems for what I need, but GNSS seems like a step up. I guess my though is that optimization clearly works fine within a single constellation because all of your information is definitely good, but across constellations are you guaranteed that they will necessarily agree? Or maybe more concretely - are my position and time in a theoretical "perfect" fix the same on GPS vs Galileo vs GLONASS etc exactly the same?If not, is splitting the difference closer to ground truth? I think that depends on what reference datums they use, and they're not the same [0]. Regarding the political aspect - you're right, and it's easy to forget about. I could say something cynical about that but I'll leave it. [0] https://gssc.esa.int/navipedia/index.php/Reference_Frames_in_GNSS https://gssc.esa.int/navipedia/index.php/Reference_Frames_in...

zettabomb 3y ago

I'm curious, hoe do multi-constellation receivers use them all at once? Are the signals just all the same thing? On some level I know they're just atomic clocks, and the governing equations should work if all of them are synchronized, but I can't help but think that feeding data different constellations in all at once wouldn't produce a good result. Or perhaps you can answer somewhat the opposite question - if the data IS all the same, then why are there multiple constellations and standards? Apart from Beidou, which I believe can actually serve as a radio repeater of some kind, aren't they all doin the same thing?

jcrawfordor 3y ago
The calculation of a fix from GNSS is actually rather complex and the potential complexity is almost unlimited. The most accurate GNSS fixes are obtained from offline solvers that analyze long-duration (e.g. 24 hour) recordings of the GNSS data streams and use computer models to determine the most likely solution accounting for many types of error. This is all to say that there isn't really one approach to calculating a fix from GNSS, and receivers embedded in devices like phones use pretty simplified methods so that they can run in real-time and with a short time to first fix. Most GNSS are largely similar, though, and there are certainly enough papers describing how to perform fixing across satellites of independent constellations so I'm sure it's been implemented in at least higher-end devices. Still, I suspect inexpensive consumer receivers are probably calculating a fix per constellation and then combining those. Fixing across constellations requires combining ephemera, time base, etc. data that varies between them, but all of these are known with high precision, you just have to pull them together. Since most GNSS are mostly the same, new GNSS constellations are usually launched either to improve coverage in a region, or these days with several global constellations, more often for sovereignty reasons. If you have weapons systems that rely on GNSS, it's better to have your own constellation. GPS for example no longer has a wartime selective availability capability (that would deny use by anyone other than US forces) but it did in the past and, if WW3 broke out, it would probably be implemented again. For much this reason different GNSS constellations tend to operate in different bands (not always, though) so that a country can selectively jam adversarial GNSS systems without affecting their own. GNSS has its origins in the military and is still largely a military capability today, with Galileo being the odd one out that focuses more on commercial applications (Galileo's selective availability capability is used to charge a subscription for higher-precision uses). And some GNSS do have "accessory" capabilities, probably the most prominent being Galileo's two-way search and rescue messaging capability that will compliment COSPAS-SARSAT. But that's always been the way with satellites. For example, the GPS constellation carries detonation sensing equipment for nuclear counter-proliferation. It just saves money to combine payloads. One of the reasons to not put GNSS constellations in separate bands is that it simplifies the construction of multi-constellation receivers. Of course, it also makes it more difficult to selectively jam. Some combination of these interests mean that GPS, GLONASS, and Galileo are very close siblings in the radio spectrum. It gets surprisingly political.
- bilegeek 3y ago
  > It gets surprisingly political. https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)#Tension_with_the_United_States https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)...
- zettabomb 3y ago
  >The most accurate GNSS fixes are obtained from offline solvers that analyze long-duration (e.g. 24 hour) recordings of the GNSS data streams and use computer models to determine the most likely solution accounting for many types of error. I actually have a receiver that does exactly this! It is a Trimble Resolution T, available for very cheap (I believe they're from old cell towers), and it's intended exclusively as a time reference. You first run a 24 hour data collection, and it precisely determines your location before storing it. From then on, the receiver only calculates the time. Very cool stuff, I'm planning on building a GPS-disciplined rubidium clock and stratum 1 NTP server with it. >Still, I suspect inexpensive consumer receivers are probably calculating a fix per constellation and then combining those. This sounds like a reasonable solution now that you mention it, for an inexpensive device. Part of my confusion is based in the fact that the constellations actually use different reference datums [0]. Wouldn't the different constellations give meaningfully different fixes even if the solution was "perfect"? I mentioned in another comment that I pretty much forgot that this could be a political thing. That's a very good point and I don't know why I didn't think of it. Especially with selective availability, which as far as I understand, the US government could just turn on again at a moments notice. [0] https://gssc.esa.int/navipedia/index.php/Reference_Frames_in_GNSS https://gssc.esa.int/navipedia/index.php/Reference_Frames_in...
deleted 3y ago
[deleted]
krisoft 3y ago
> I can't help but think that feeding data different constellations in all at once wouldn't produce a good result Obviously if you just mix up the bytes willy nilly then nothing good will come out of it. How familiar are you with the terminology of mathematical optimisation? If you are then the intuition is that each satelite provides a different error term in one big function you are trying to optimise. The “knowns” you plug in to the terms are your observations (pseudorange, phase, etc) and the orbital parameters of the satelite broadcast by the satelites. The unknowns you are trying to solve are the x,y,z,t coordinates of the receiver you have. You are trying to find the unknowns which minimise the sum of the error terms. The error terms of the various constellations might have slightly different ”shapes”, but you can still run your optimisation them the same. As you add more satelite the number of unknowns remain the same (4), but you are adding more and more equations to your overdetermined system. When things are not pathological that leads to a better, more precise fit. > aren't they all doin the same thing Even if they were, sometimes it is valuable to have your own toy. If Russia is having a war in some region of the world and the US decides to turn off the GPS signals there is not much Russia can do about that. It is a US satelite and the US adjusts it as they see it fit. If Russia has their own satelites and the US would like to deny them this the US have to attack Russian assets directly. That is a very different ball game.
- zettabomb 3y ago
  I think I know enough about optimization problems for what I need, but GNSS seems like a step up. I guess my though is that optimization clearly works fine within a single constellation because all of your information is definitely good, but across constellations are you guaranteed that they will necessarily agree? Or maybe more concretely - are my position and time in a theoretical "perfect" fix the same on GPS vs Galileo vs GLONASS etc exactly the same?If not, is splitting the difference closer to ground truth? I think that depends on what reference datums they use, and they're not the same [0]. Regarding the political aspect - you're right, and it's easy to forget about. I could say something cynical about that but I'll leave it. [0] https://gssc.esa.int/navipedia/index.php/Reference_Frames_in_GNSS https://gssc.esa.int/navipedia/index.php/Reference_Frames_in...