I had the opportunity to visit one. Basically they measure their own position in relation to each other. They do that with Very-long-baseline interferometry, basically what is the time difference of quasar radio signals hitting their Radio telescopes. The things they account for is wild like local gravity field a couple of super prices atomic clocks etc. they then laser range find Satellites (all not only gps) which is a „fun“ summer student job at least at the one that I visited.
> Satellites at the GPS altitude travel at the speed of about 2.4 mi/s relative to Earth, which slows the clock down, but they’re also in weaker gravity which causes the clock to run faster. The latter effect is stronger which in total results in a gain of around 4.4647 × 10−10 seconds per second, or around 38 microseconds a day.
> Unfortunately, this is where many sources make a mistake with their interpretation of that result. It’s often erroneously claimed that if GPS didn’t correct for these relativistic effects by slowing down the clocks on satellites, the system would increase its error by around 7.2 mi per day as this is the distance that light travels in those 38 microseconds.
> Those assertions are not true. If relativistic effects weren’t accounted for and we let the clocks on satellites drift, the pseudoranges would indeed increase by that amount every day. However, as we’ve seen, an incorrect clock offset doesn’t prevent us from calculating the correct position.
(Nevertheless there are of course relativistic effects to account for, which Ciechanow proceeds to mention and which are explained in more detail in the other link I shared here: https://news.ycombinator.com/item?id=47861535 )
The author does note that:
> If you want to go much deeper, Bartosz Ciechanowski's interactive explainer on GPS is the gold standard. It covers signal modulation, orbital mechanics, and receiver architecture in far more detail than we do here.
for that I'd recommend this youtube series https://www.youtube.com/watch?v=i7JPjgHa7_A
GPS are amazing. If you understand how they work, and how they reliably know the time etc. you'd think you live in the future; and yet it's everywhere, in our pockets.
There are a couple of dozen of people that seriously think the earth is flat, and a billion people ready to mock them for it.
It’s kind of punching down at shadows.
So the trick, as always, boils down to engineering approximations, haha.
The time correction is better thought of as the fixed offset between the receiver clock and the GPS clock.
I'm guessing those visualizations wouldn't be in this post if it weren't for AI. The interesting question is what happens when ed-tech ships this pattern at scale. Exciting future.
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If/when that goes away, I wonder if it will be cheaper to use a gps chip to make "self setting" clocks, or if everything will just be wifi connected.
¹ https://www.sparkfun.com/gps-module-gp1818mk-56-channel.html
But yes, it's amazing how small and battery efficient GPS receivers are at this point!
Saved you a click.
Except BeiDou is banned on American devices unlike Russian GLONASS
Even though GPS is a read-only service and could not affect civilian devices and it's already built-in to most phone/watch chipsets
Biden admin tried to change policy but ran out of time
https://news.ycombinator.com/item?id=47849174
It will radically increase accuracy and availability the day it's allowed (like in Europe)
