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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 LE

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
- tonyarkles 2y ago
  To clarify a little further, years ago I worked on a software package for high-resolution (spatial and temporal) simulation of GNSS orbits. While it seems that this would be straightforward (Kepler’s laws have been well-known for a while), it turns out that there are a number of correction factors required to deal with the non-ideal aspects of Earth orbit. For the MEO GNSS satellites, the last correction factor that I needed to implement in order to meet the system requirements ended up being Solar Radiation Pressure. Since the accuracy of a GNSS system depends on knowing the precise positions of the SVs at precise times, any error in the orbital position calculation is a direct error on the position; SRP, as it turned out, was enough to knock the real orbit off by about 1m over a… surprisingly short period of time (I don’t remember the exact period, somewhere around 30min to 6h). Down in LEO you’ve got way less predicable correction factors with significantly larger magnitudes. Even at 300km there is still a little bit of atmosphere and your satellite experiences drag. Factoring in the huge number of SVs in the Starlink constellation and it’s going to be a nightmare to model their orbits accurately enough to do decent positioning. But as I was writing that, a fun thought came to mind. All of the end-user GTs could be used as tracking stations. If the Dishys have a GPS receiver and can assume it is in a fixed static position (accelerometer to cross-check?), it can self-survey its own position on the ground probably to 1-2mm accuracy over time (higher-end consumer grade parts like the UBlox F9P can do this) and use the antenna phasing to get a good idea of the SV relative to the GT. Collect all of those high-resolution coordinates and the ranging data from all of the end-user GTs, set it up as a huge linear algebra problem, and throw it at some GPUs. It might work?
- mandevil 2y ago
  So back in the 1990s, online I talked with someone who worked in the GPS constellation. He said that they wanted multiple passes of a Navstar over their tracking locations to correct the orbital parameters after every orbital burn. Since they were in MEO, drag was a smaller deal, so burns were infrequent. In LEO drag was so much larger that regular burns are necessary. However, on reflection I don't know the precision of the Hall Effect thrusters that the Starlink satellites are using. If they are precise enough in their thrust application then the only thing you'd need to observe would be the effects of exoatmosphere drag, which are still a big deal. Steve's operational experience was only with chemical thrusters, electricals were still bleeding edge in the 1990s. Basically, because of drag MEO will always be >>> than LEO for navigation purposes. That's why all the dedicated navsats are there! It is possibly to piggy-back and get a large enough scale to compensate for the loss of accuracy. Eyeballing satellitemap.space for my location (roughly 35N), it looks like you'd get a zenith every minute or two from Starlink. Again, fine for stationary, not sure about moving, given the need for multiple zeniths to correct uncertainty. As for the laser link SpaceX is very tight-lipped about that as far as I can tell, and that is one where the precision is all going to be measured operationally. Something like LAGEOS shows that it is possible to do things incredibly precisely with light, but the question is, does Starlink have the mass, energy, and compute budget to actually do it in practice.

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

tonyarkles 2y ago
To clarify a little further, years ago I worked on a software package for high-resolution (spatial and temporal) simulation of GNSS orbits. While it seems that this would be straightforward (Kepler’s laws have been well-known for a while), it turns out that there are a number of correction factors required to deal with the non-ideal aspects of Earth orbit. For the MEO GNSS satellites, the last correction factor that I needed to implement in order to meet the system requirements ended up being Solar Radiation Pressure. Since the accuracy of a GNSS system depends on knowing the precise positions of the SVs at precise times, any error in the orbital position calculation is a direct error on the position; SRP, as it turned out, was enough to knock the real orbit off by about 1m over a… surprisingly short period of time (I don’t remember the exact period, somewhere around 30min to 6h). Down in LEO you’ve got way less predicable correction factors with significantly larger magnitudes. Even at 300km there is still a little bit of atmosphere and your satellite experiences drag. Factoring in the huge number of SVs in the Starlink constellation and it’s going to be a nightmare to model their orbits accurately enough to do decent positioning. But as I was writing that, a fun thought came to mind. All of the end-user GTs could be used as tracking stations. If the Dishys have a GPS receiver and can assume it is in a fixed static position (accelerometer to cross-check?), it can self-survey its own position on the ground probably to 1-2mm accuracy over time (higher-end consumer grade parts like the UBlox F9P can do this) and use the antenna phasing to get a good idea of the SV relative to the GT. Collect all of those high-resolution coordinates and the ranging data from all of the end-user GTs, set it up as a huge linear algebra problem, and throw it at some GPUs. It might work?
mandevil 2y ago
So back in the 1990s, online I talked with someone who worked in the GPS constellation. He said that they wanted multiple passes of a Navstar over their tracking locations to correct the orbital parameters after every orbital burn. Since they were in MEO, drag was a smaller deal, so burns were infrequent. In LEO drag was so much larger that regular burns are necessary. However, on reflection I don't know the precision of the Hall Effect thrusters that the Starlink satellites are using. If they are precise enough in their thrust application then the only thing you'd need to observe would be the effects of exoatmosphere drag, which are still a big deal. Steve's operational experience was only with chemical thrusters, electricals were still bleeding edge in the 1990s. Basically, because of drag MEO will always be >>> than LEO for navigation purposes. That's why all the dedicated navsats are there! It is possibly to piggy-back and get a large enough scale to compensate for the loss of accuracy. Eyeballing satellitemap.space for my location (roughly 35N), it looks like you'd get a zenith every minute or two from Starlink. Again, fine for stationary, not sure about moving, given the need for multiple zeniths to correct uncertainty. As for the laser link SpaceX is very tight-lipped about that as far as I can tell, and that is one where the precision is all going to be measured operationally. Something like LAGEOS shows that it is possible to do things incredibly precisely with light, but the question is, does Starlink have the mass, energy, and compute budget to actually do it in practice.