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 )
for that I'd recommend this youtube series https://www.youtube.com/watch?v=i7JPjgHa7_A
The correction factor is about 38 microseconds per day — small enough to ignore in everyday life but catastrophic for GPS accuracy if unaccounted for. No other engineering system relies on relativistic corrections in its day-to-day operation quite like this.
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.
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.
So the trick, as always, boils down to engineering approximations, haha.
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