3 ms·

Very interesting article. It also illustrates how dangerous unit conversions are especially combined with thousands separators: 20000km (GNSS satellite altitude

Very interesting article. It also illustrates how dangerous unit conversions are especially combined with thousands separators: 20000km (GNSS satellite altitude) is NOT 12.4 miles. Does anyone know if it would be feasible nowadays to just use starlink (or other LEO satellites) as a GNSS constellation? Even without precise onboard-clocks, would it not be possible to just bounce clock signals from earth as long as latency is known?

loxias 2y ago
> as long as latency is known? Ah, but how do you know latency, without accurate clocks? :) Accurate clocks facilitates measuring latency, which is used to calculate distance. :)
- lxgr 2y ago
  True, but the accurate clock doesn’t need to be aboard the satellites as long as they have continuous (low-jitter) connectivity to the timing source, and Iridium and newer Starlink satellites do.
- chipsa 2y ago
  How does NTP work? GPS is stratum 0. Starlink could be effectively stratum 1.
adgjlsfhk1 2y ago
starlink is in a low enough orbit that it isn't ideal (atmospheric drag will make the position harder to accurately predict)
- lxgr 2y ago
  I’d be surprised if that were an issue. Starlink probably could learn the position of every satellite very accurately based on latency and signal strength and angle data from all the ground stations alone if they wanted to, if they don’t already.
  - adgjlsfhk1 2y ago
    you would need nanosecond accurate clocks in the satellites and groundstations
lxgr 2y ago
Yes, this is actively being explored, see for example this for Iridium: https://investor.iridium.com/2021-05-24-Iridium-Makes-Strategic-Investment-in-DDK-Positioning,-Provider-of-Enhanced-GNSS-Accuracy-Solutions https://investor.iridium.com/2021-05-24-Iridium-Makes-Strate...
grecy 2y ago
Elon tweeted a while back they could do positioning, but it’s just not a high priority right now

loxias 2y ago

> as long as latency is known? Ah, but how do you know latency, without accurate clocks? :) Accurate clocks facilitates measuring latency, which is used to calculate distance. :)

lxgr 2y ago
True, but the accurate clock doesn’t need to be aboard the satellites as long as they have continuous (low-jitter) connectivity to the timing source, and Iridium and newer Starlink satellites do.
chipsa 2y ago
How does NTP work? GPS is stratum 0. Starlink could be effectively stratum 1.

adgjlsfhk1 2y ago

starlink is in a low enough orbit that it isn't ideal (atmospheric drag will make the position harder to accurately predict)

lxgr 2y ago
I’d be surprised if that were an issue. Starlink probably could learn the position of every satellite very accurately based on latency and signal strength and angle data from all the ground stations alone if they wanted to, if they don’t already.
- adgjlsfhk1 2y ago
  you would need nanosecond accurate clocks in the satellites and groundstations

lxgr 2y ago

Yes, this is actively being explored, see for example this for Iridium: https://investor.iridium.com/2021-05-24-Iridium-Makes-Strategic-Investment-in-DDK-Positioning,-Provider-of-Enhanced-GNSS-Accuracy-Solutions https://investor.iridium.com/2021-05-24-Iridium-Makes-Strate...

grecy 2y ago

Elon tweeted a while back they could do positioning, but it’s just not a high priority right now