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Comment by JackMcMack | Hacker News Reader

Something not mentioned: the "new" L5 signal at 1176Mhz, combined with the existing L1 signal at 1575Mhz, allows the receiver to estimate the atmospheric effects and reduce the uncertainty, allowing for a much better position fix. Think centimeters instead of meters.

One more thing I've wondered: the system depends on the sattelites knowing and broadcasting their exact position, but how do you determine this position? From ground stations, sure, but how exactly? What's the margin of error on that?

And to add to this, how do you bootstrap this?

Galileo had an outage from 2019-07-11 to 2019-07-18 [0]. I've not read much about the details what caused the outage, or why it took an entire week to get back up & running.

[0] https://www.gsc-europa.eu/news/galileo-initial-services-have...

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The knowledge of a satellite's orbit is taken by using the prior parameters of the orbit, predicting where the satellite will be, and pointing a combination of telescopes (for precise angular measurements) and radars (for precise distance measurements) at this location, and measuring the error between where the satellite is and where it is expected to be. A set of these observations are then used to update the "known" orbit.

This known orbit is then provided back to the satellite so that it can be broadcast. If this system of updates stopped working, the quality of GPS position estimates would degrade pretty quickly (think weeks, not years).

This also means that if a GPS satellite were to need to maneuver for some reason -- either periodically boosting back into its assigned orbit or for debris avoidance -- the normal system of updates will catch this and users will never have to know or care that the satellite moved.

You're spot-on, the bootstrapping is exactly why the outage took so long to recover: https://berthub.eu/articles/posts/galileo-accident/

That was a very interesting read, thanks for the link!

From the article:

The outage in the ephemeris provisioning happened because simultaneously:

* The backup system was not available

* New equipment was being deployed and mishandled during an upgrade exercise

* There was an anomaly in the Galileo system reference time system

* Which was then also in a non-normal configuration

So they had to do a cold boot, which is by design slow because it focuses on high accuracy/certainty. Disappointing to read that the collaboration between the involved companies is downright bad in case of emergencies such as this. And the communication is also terrible, there's no public/official report of what exactly went wrong, why it took so long to recover, and what lessons were learned. It sounds to me that GPS being under military control is an advantage over Galileo.

Look up GPS operation control segment (OCX). Currently it's mostly Airforce and JPL, transitioning to Space Force. Lots of details published.

It's basically just a huge square root extended Kalman filter tracking all GPS satellite states.

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