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Comment by eminence32 | Hacker News Reader

There's a lot of really great info in here. One random things I learned from this:

> As that angle increases, the signal from a satellite travels more sideways and its larger portion gets affected by the atmosphere. To account for this, GPS receivers ignore ranges measured from satellites at very low elevation angles. ... atmospheric effects are primary source of GPS inaccuracies.

(I know GPS has inaccuracies, but I didn't really know what caused them, but if I had to guess, the atmosphere wouldn't have been on my list of guesses for the top causes)

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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...

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.

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.

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.

This is also true for celestial navigation with a sextant and the light refracting in the atmosphere: the "Altitude Correction Tables" give the combined correction for refraction, semidiameter, and parallax under standard atmosphere conditions.

* https://reginasailing.com/wp-content/uploads/2020/04/Altitud...

* https://thenauticalalmanac.com/Altitude_Correction_Tables.pd...

* https://thenauticalalmanac.com/Altitude_Correction_Tables_fo...

Ionospheric distortions are the largest source of errors in single-frequency solutions! And it's why the WAAS birds transmit a correction model, which all modern (post-2004 or so) receivers can apply.

Multi-frequency receivers can derive the corrections directly because the distortions affect the different frequencies in predictable ways, and they can work back to "ionosphere-free" pseudoranges, and base the rest of the solution on those.

To your quoted comment, nicer receivers also tend to have a configurable "horizon mask" aka "elevation mask", so you can tune this rejection behavior. I could swear I've heard of some that let you configure the mask height _per azimuth_ but I can't find an explicit reference right now.

Elevation masking is tricky because if you crank it up too high, you force yourself into poor-DOP geometries. But if you relax it too low, not only do you get heaping piles of ionospheric distortion, you also invite ground-clutter multipath. I think it's primarily used by stationary timing receivers, because they know their position is fixed, they're less susceptible to GDOP.

Author doesn't mention refraction from varying atmospheric density introducing a non-straight path. Maybe that is negligible for air? It's extremely important for sonar ranging. Lots of things affect water density.

Atmospheric density isn't as relevant as electron density:

https://gssc.esa.int/navipedia/index.php/Ionospheric_Delay

For as long as this blog is, one thing that's missing is a discussion of multipath errors. Multipath errors are when the GPS signal reflects off of buildings or mountains, giving the illusion that the satellite is further away than it is. This is why it can sometimes be hard to get a precise location in cities.

Not only that. You can use a ground-based fixed station to listen to GPS signals and work out how much they have been affected by the atmosphere. This then gets fed back into the weather prediction models used by many weather forecasting services.

Even better, use a receiver in low-earth orbit. See also: COSMIC, GeoOptics, Spire, and PlanetiQ.

There’s an interesting chicken-and-egg problem there. You don’t know what angle the signal is coming from (unless you have some kind of sophisticated multi receiver setup) - so first you need to estimate your position, then figure out whether the satellite is low in the sky, then you can determine whether to trust the timing of the signal from that satellite.

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