5 ms·

I'd like to note that it is feasible to reuse current OneWeb satellite as ad-hoc GNSS, just the accuracy will be much worse than current service from Galileo or

I'd like to note that it is feasible to reuse current OneWeb satellite as ad-hoc GNSS, just the accuracy will be much worse than current service from Galileo or GPS

gchadwick 6y ago
This is the point I'd really like to see some more detail on or discussion on from relevant experts. I'm sure you can build some kind of GNSS with the capabilities of OneWeb satellites, can you build something with similar capabilities to existing GNSS (GPS was original designed in the 70s so was designed with receivers with limited computational capability, with modern capabilities can you build a system that requires lots more satellites for a good fix, where those satellites don't need the same capabilities as a GPS satellite for instance)? If not do you need small tweaks to OneWeb's current platform or a total overhaul to build a GNSS? I can well believe the UK government has been sold on a thing that can't work, but equally I can see reasons it may be possible to use OneWorld's existing platform for a GNSS service.
- dr_orpheus 6y ago
  One of the biggest drivers of cost on GNSS satellites is the need for an extremely accurate clock. Position is calculated based on a series of ranges to the satellites with a "known" position. That range is calculated is based on a time-of-flight so you need a very accurate time source. The whole Galileo project cost is 10 billion [0] and this is "cheap" compared to the US GPS system. I can't find great numbers for it, but for the cost of building two new GPS satellites this year is $1.8 billion [1] and operating cost for the existing constellation I believe is on the order of 100s of millions. I know someone could probably do it for cheaper, but I guess my point is putting an atomic clock in space is expensive. But I also wanted to come back to the cost of operations and "known" satellite position. Because if you need your GPS satellites to have good enough position and time knowledge you need something on the ground with a better than extremely accurate clock. See this picture of the backup clock arriving at the GPS operations center [2] Edit: Also see my comment a couple comments down on my assessment of possible accuracy of the current one-web satellites to get an idea of accuracy. [0] https://en.wikipedia.org/wiki/Galileo_(satellite_navigation) https://en.wikipedia.org/wiki/Galileo_(satellite_navigation) [1] https://www.defense.gov/Newsroom/Releases/Release/Article/2079489/dod-releases-fiscal-year-2021-budget-proposal/ https://www.defense.gov/Newsroom/Releases/Release/Article/20... [2] https://www.schriever.af.mil/News/Article-Display/Article/276171/new-alternate-master-clock-arrives-at-schriever/ https://www.schriever.af.mil/News/Article-Display/Article/27...
ChuckMcM 6y ago
Exactly correct, the rub is how much phase error (inaccuracy) are you willing to tolerate. Now if you you had planned to include a localization option in the satellites, that would be an interesting thing.
SergeAx 6y ago
Can an error be compensated by the number of satellites?
- phreeza 6y ago
  Not an expert by any means, but from my experience with physics experiments I would guess the error goes down as 1/sqrt(n). So more satellites help, but with diminishing returns.
- dr_orpheus 6y ago
  Theoretically, yes. But you are already limited by the position knowledge of the OneWeb satellites themselves. Currently they rely on on-board GPS receivers for their position knowledge which is quoted as 10m (1σ) [0]. Timing knowledge is quoted as 50 ns (1σ), so when calculating time-of-flight from the satellite this will add another 15m of error. So you won't be able to get better than that number, and it will likely be significantly worse than that and you could get errors up in to the 100m range. With regular handlheld GPS receivers or smartphones you can usually get an accuracy of a couple of meters under open sky [1/2]. And if you get precision GPS receiver units which can use dual-band and differential GPS you can get down in to the centimeter accuracy range [3]. So there is a very big orders of magnitude difference with what you could do with the OneWeb satellites right now. [0] https://onewebsatellites.com/arrow/ https://onewebsatellites.com/arrow/ [1] https://activelivingresearch.org/positional-accuracy-six-portable-gps-receivers https://activelivingresearch.org/positional-accuracy-six-por... [2] https://www.gps.gov/systems/gps/performance/accuracy/ https://www.gps.gov/systems/gps/performance/accuracy/ [3] https://r12.trimble.com/ https://r12.trimble.com/
  - SergeAx 6y ago
    This is an enormously detailed answer, thank you so much!
gpm 6y ago
Why is this the case? Is triangulating your position from LEO harder somehow? Or is it something like that satellites don't have accurate enough clocks? Or something else?
- Joeri 6y ago
  Navigation satellites carry atomic clocks, oneweb satellites do not.
- ivalm 6y ago
  Geostationary satellites have very well known position and use atomic clocks to ensure they are synchronized. So when you get a signal for 3 geostationary sats you can have both extreme precision in their position and in time delay between your and sat locations. For LEO sats the sat position is more complicated and they don’t have atomic clocks to synchronize. Now, their position IS somewhat known and they do have clocks so they can work somewhat, just not as well (alternatively since there will be a ton of Leo sats maybe you can do statistical analysis to improve?)
  - Denvercoder9 6y ago
    GNSS constellations do not use geostationary satellites.
    - jhayward 6y ago
      > GNSS constellations do not use geostationary satellites. This is not quite accurate. The main body of GNSS constellations are usually MEO cut in to several orbital planes, but GPS and Beidou, (Global systems) as well as QZSS and IRNSS (regional systems) all have GEO satellites broadcasting a NAV signal payload.
  - manquer 6y ago
    GNSS systems typically use MEO ~20,000 KM.
  - Rebelgecko 6y ago

I'd like to note that it is feasible to reuse current OneWeb satellite as ad-hoc GNSS, just the accuracy will be much worse than current service from Galileo or GPS

gchadwick 6y ago

This is the point I'd really like to see some more detail on or discussion on from relevant experts. I'm sure you can build some kind of GNSS with the capabilities of OneWeb satellites, can you build something with similar capabilities to existing GNSS (GPS was original designed in the 70s so was designed with receivers with limited computational capability, with modern capabilities can you build a system that requires lots more satellites for a good fix, where those satellites don't need the same capabilities as a GPS satellite for instance)? If not do you need small tweaks to OneWeb's current platform or a total overhaul to build a GNSS? I can well believe the UK government has been sold on a thing that can't work, but equally I can see reasons it may be possible to use OneWorld's existing platform for a GNSS service.

dr_orpheus 6y ago
One of the biggest drivers of cost on GNSS satellites is the need for an extremely accurate clock. Position is calculated based on a series of ranges to the satellites with a "known" position. That range is calculated is based on a time-of-flight so you need a very accurate time source. The whole Galileo project cost is 10 billion [0] and this is "cheap" compared to the US GPS system. I can't find great numbers for it, but for the cost of building two new GPS satellites this year is $1.8 billion [1] and operating cost for the existing constellation I believe is on the order of 100s of millions. I know someone could probably do it for cheaper, but I guess my point is putting an atomic clock in space is expensive. But I also wanted to come back to the cost of operations and "known" satellite position. Because if you need your GPS satellites to have good enough position and time knowledge you need something on the ground with a better than extremely accurate clock. See this picture of the backup clock arriving at the GPS operations center [2] Edit: Also see my comment a couple comments down on my assessment of possible accuracy of the current one-web satellites to get an idea of accuracy. [0] https://en.wikipedia.org/wiki/Galileo_(satellite_navigation) https://en.wikipedia.org/wiki/Galileo_(satellite_navigation) [1] https://www.defense.gov/Newsroom/Releases/Release/Article/2079489/dod-releases-fiscal-year-2021-budget-proposal/ https://www.defense.gov/Newsroom/Releases/Release/Article/20... [2] https://www.schriever.af.mil/News/Article-Display/Article/276171/new-alternate-master-clock-arrives-at-schriever/ https://www.schriever.af.mil/News/Article-Display/Article/27...

ChuckMcM 6y ago

Exactly correct, the rub is how much phase error (inaccuracy) are you willing to tolerate. Now if you you had planned to include a localization option in the satellites, that would be an interesting thing.

SergeAx 6y ago

Can an error be compensated by the number of satellites?

phreeza 6y ago
Not an expert by any means, but from my experience with physics experiments I would guess the error goes down as 1/sqrt(n). So more satellites help, but with diminishing returns.
dr_orpheus 6y ago
Theoretically, yes. But you are already limited by the position knowledge of the OneWeb satellites themselves. Currently they rely on on-board GPS receivers for their position knowledge which is quoted as 10m (1σ) [0]. Timing knowledge is quoted as 50 ns (1σ), so when calculating time-of-flight from the satellite this will add another 15m of error. So you won't be able to get better than that number, and it will likely be significantly worse than that and you could get errors up in to the 100m range. With regular handlheld GPS receivers or smartphones you can usually get an accuracy of a couple of meters under open sky [1/2]. And if you get precision GPS receiver units which can use dual-band and differential GPS you can get down in to the centimeter accuracy range [3]. So there is a very big orders of magnitude difference with what you could do with the OneWeb satellites right now. [0] https://onewebsatellites.com/arrow/ https://onewebsatellites.com/arrow/ [1] https://activelivingresearch.org/positional-accuracy-six-portable-gps-receivers https://activelivingresearch.org/positional-accuracy-six-por... [2] https://www.gps.gov/systems/gps/performance/accuracy/ https://www.gps.gov/systems/gps/performance/accuracy/ [3] https://r12.trimble.com/ https://r12.trimble.com/
- SergeAx 6y ago
  This is an enormously detailed answer, thank you so much!

gpm 6y ago

Why is this the case? Is triangulating your position from LEO harder somehow? Or is it something like that satellites don't have accurate enough clocks? Or something else?

Joeri 6y ago
Navigation satellites carry atomic clocks, oneweb satellites do not.
ivalm 6y ago
Geostationary satellites have very well known position and use atomic clocks to ensure they are synchronized. So when you get a signal for 3 geostationary sats you can have both extreme precision in their position and in time delay between your and sat locations. For LEO sats the sat position is more complicated and they don’t have atomic clocks to synchronize. Now, their position IS somewhat known and they do have clocks so they can work somewhat, just not as well (alternatively since there will be a ton of Leo sats maybe you can do statistical analysis to improve?)
- Denvercoder9 6y ago
  GNSS constellations do not use geostationary satellites.
  - jhayward 6y ago
    > GNSS constellations do not use geostationary satellites. This is not quite accurate. The main body of GNSS constellations are usually MEO cut in to several orbital planes, but GPS and Beidou, (Global systems) as well as QZSS and IRNSS (regional systems) all have GEO satellites broadcasting a NAV signal payload.
- manquer 6y ago
  GNSS systems typically use MEO ~20,000 KM.
- Rebelgecko 6y ago