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A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satel

A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Every NASA science mission to Mars orbit for over 20 years has had a communications relay on it, for relaying messages from rovers back to Earth, but the one in the best orbit for communications (Mars Odyssey) is also the oldest and most likely to fail. The other NASA science orbiters (MRO, ME (ESA), MAVEN) all also have relays, but the orbits all leave something to be desired for communications purposes (they are in the right orbits for answering their scientific questions, e.g. MAVEN is in a highly elliptical orbit for studying the Martian atmosphere). There was a proposed Mars Telecommunications Orbiter- a satellite whose primary mission would have been beaming back information from rovers on the surface- proposed back around 2005, but it was canceled in a budget crunch, when actual science producing satellites were prioritized. That would have been in the right orbit. Building a positioning satellite network around another body is going to be significantly harder, incidentally. Even something like TRANSIT (aka NAVSAT) (1) is going to be significantly harder on another body because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. 1: Instead of the "see multiple atomic clocks and use triangulation and the speed of light to determine distance to each of them, then our location from their known locations" which is how all modern satnav systems work, TRANSIT used a single satellite passing nicely overhead every so often. When it was right above you, you could listen to the Doppler shift and know when it reached exactly the zenith above you. If you knew it's orbit very precisely you could tell where it was in space when it reached that zenith, and therefore where you were.

skissane 2y ago
> A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Not necessarily. Starlink can be used for positioning – not with the same accuracy as GPS can, but it definitely can be used. According to [0] a research group was involved with discussions with SpaceX about officially using Starlink as a positioning system, but the discussions were terminated because Musk didn't like the idea. And then the researchers went ahead and did it anyway – even without the ability to decrypt the signal, the unencrypted synchronisation data embedded in it is sufficient to get a position fix to within 30 metres. If SpaceX was actually cooperating and using data in the encrypted signal for this purpose, it could likely be made even more accurate. I suppose Musk's point is that however accurate it is, GPS is going to be even more accurate, and having Starlink offer a "just like GPS but less accurate" service has minimal commercial value and great potential for bad PR ("look how inaccurate SpaceX's GPS alternative is!") SpaceX is proposing to build a "Marslink" constellation around Mars [1]. Likely SpaceX wouldn't have the same opposition to using it for positioning as they do for Earth Starlink, given there is no existing GPS to compete with. > because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. This is all true, but anything is better than nothing – a Martian GPS with significantly less accuracy than Earth GPS will still be superior to no Martian GPS. And it will be a step towards building the necessary research infrastructure to answer many of those questions about variations in Martian gravity and atmospheric drag. So the accuracy of the system is likely to improve over time. [0] https://www.technologyreview.com/2022/10/21/1062001/spacex-starlink-signals-reverse-engineered-gps/ https://www.technologyreview.com/2022/10/21/1062001/spacex-s... [1] https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a-version-of-starlink-for-the-red-planet https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a...
- mandevil 2y ago
  Comm sats in LEO like Starlink are never going to have orbits precisely known enough to be near GPS-quality without a lot of waiting. Because Starlink is in LEO they have to burn regularly (much more often than the MEO orbits preferred for positioning systems), and the uncertainty on each burn creates greater uncertainty on the orbit, and at 8km/s small uncertainties explode your accuracy greatly. For dedicated positioning systems they use ground based tracking to precisely update the orbital parameters after every burn, to account for that, but that is impracticable for Starlink. The only approach that you can do is a Doppler based NAVSAT approach (since the Starlink satellites don't have their own atomic clock for distance calculations), which can't really handle movement by the ground station well in the first place, and to account for the uncertainty in orbits you end up needing to wait for several to pass overhead without moving and trust that with enough passes the uncertainties cancel out. NAVSAT was never designed for real-time tracking, it was designed to zero the huge mechanical gyro's on the inertial frame for nuclear ballistic missile submarines. This approach wouldn't help a helicopter here on Earth- because it needs multiple zeniths without moving for the uncertainties to cancel out, this system would not provide much help for systems that are moving. It definitely would not provide much help for a helicopter on Mars. If you build a system for other reasons and can piggyback this off of that, sure, do it (I know that other spacecraft at least have the ability to measure Doppler shifts and so could use this system) but it isn't ever going to solve the original problem of the helicopter not able to tell what its horizontal velocity was, and it is always going to be very very rough. And of course SpaceX will want to pitch a Starlink for Mars. Quite honestly, I suspect that upgrading the DSN sites here on Earth would be far more bang for the buck, more of the 34m BWG's would go a long way, since they are already a limiting factor and if Artemis is really going to happen we're going to see dramatically more data produced that the DSN is going to need to listen for.
  - skissane 2y ago
    > For dedicated positioning systems they use ground based tracking to precisely update the orbital parameters after every burn, to account for that, but that is impracticable for Starlink. But why isn't it practical? Also, can't the inter-satellite laser links provide an additional source of information in measuring their orbital parameters? > since the Starlink satellites don't have their own atomic clock They might get them at some point. And even if the Earth constellation doesn't, maybe the Mars one will. The atomic clocks used in GPS satellites are fiendishly expensive, but the gap between them and much cheaper chip-scale atomic clocks [0] is likely to continue to narrow. [0] https://en.wikipedia.org/wiki/Chip-scale_atomic_clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock

skissane 2y ago

> A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Not necessarily. Starlink can be used for positioning – not with the same accuracy as GPS can, but it definitely can be used. According to [0] a research group was involved with discussions with SpaceX about officially using Starlink as a positioning system, but the discussions were terminated because Musk didn't like the idea. And then the researchers went ahead and did it anyway – even without the ability to decrypt the signal, the unencrypted synchronisation data embedded in it is sufficient to get a position fix to within 30 metres. If SpaceX was actually cooperating and using data in the encrypted signal for this purpose, it could likely be made even more accurate. I suppose Musk's point is that however accurate it is, GPS is going to be even more accurate, and having Starlink offer a "just like GPS but less accurate" service has minimal commercial value and great potential for bad PR ("look how inaccurate SpaceX's GPS alternative is!") SpaceX is proposing to build a "Marslink" constellation around Mars [1]. Likely SpaceX wouldn't have the same opposition to using it for positioning as they do for Earth Starlink, given there is no existing GPS to compete with. > because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. This is all true, but anything is better than nothing – a Martian GPS with significantly less accuracy than Earth GPS will still be superior to no Martian GPS. And it will be a step towards building the necessary research infrastructure to answer many of those questions about variations in Martian gravity and atmospheric drag. So the accuracy of the system is likely to improve over time. [0] https://www.technologyreview.com/2022/10/21/1062001/spacex-starlink-signals-reverse-engineered-gps/ https://www.technologyreview.com/2022/10/21/1062001/spacex-s... [1] https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a-version-of-starlink-for-the-red-planet https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a...

mandevil 2y ago
Comm sats in LEO like Starlink are never going to have orbits precisely known enough to be near GPS-quality without a lot of waiting. Because Starlink is in LEO they have to burn regularly (much more often than the MEO orbits preferred for positioning systems), and the uncertainty on each burn creates greater uncertainty on the orbit, and at 8km/s small uncertainties explode your accuracy greatly. For dedicated positioning systems they use ground based tracking to precisely update the orbital parameters after every burn, to account for that, but that is impracticable for Starlink. The only approach that you can do is a Doppler based NAVSAT approach (since the Starlink satellites don't have their own atomic clock for distance calculations), which can't really handle movement by the ground station well in the first place, and to account for the uncertainty in orbits you end up needing to wait for several to pass overhead without moving and trust that with enough passes the uncertainties cancel out. NAVSAT was never designed for real-time tracking, it was designed to zero the huge mechanical gyro's on the inertial frame for nuclear ballistic missile submarines. This approach wouldn't help a helicopter here on Earth- because it needs multiple zeniths without moving for the uncertainties to cancel out, this system would not provide much help for systems that are moving. It definitely would not provide much help for a helicopter on Mars. If you build a system for other reasons and can piggyback this off of that, sure, do it (I know that other spacecraft at least have the ability to measure Doppler shifts and so could use this system) but it isn't ever going to solve the original problem of the helicopter not able to tell what its horizontal velocity was, and it is always going to be very very rough. And of course SpaceX will want to pitch a Starlink for Mars. Quite honestly, I suspect that upgrading the DSN sites here on Earth would be far more bang for the buck, more of the 34m BWG's would go a long way, since they are already a limiting factor and if Artemis is really going to happen we're going to see dramatically more data produced that the DSN is going to need to listen for.
- skissane 2y ago
  > For dedicated positioning systems they use ground based tracking to precisely update the orbital parameters after every burn, to account for that, but that is impracticable for Starlink. But why isn't it practical? Also, can't the inter-satellite laser links provide an additional source of information in measuring their orbital parameters? > since the Starlink satellites don't have their own atomic clock They might get them at some point. And even if the Earth constellation doesn't, maybe the Mars one will. The atomic clocks used in GPS satellites are fiendishly expensive, but the gap between them and much cheaper chip-scale atomic clocks [0] is likely to continue to narrow. [0] https://en.wikipedia.org/wiki/Chip-scale_atomic_clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock