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.
And this precision data can then in turn be used to map localized terrain movements due to volcanic activity, mining etc. using high precision GPS. I think some of these can detect (in the long term) movements of as little as a few millimeters!
Yup, it was also posted in the other thread on GPS the other day and it is quite a bit better than OP's article, particularly because it doesn't give a false account of the involved relativistic effects:
> 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 )
> 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.
You don't need to belittle someone else's work. It's a series of articles, and author has 2 more articles that aren't related to articles Ciechanowski wrote at all.
Always makes me laugh when you get some dimwit that claims the Earth is flat, but then uses Google maps in his car. Magic!
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.
Yes but the math (which happens in the receiver, so can be replicated by a user with an open source receiver) would be very different. You actually wind up with a 3D position relative to the Earth's center, which then needs to be mathematically mapped to lat/lon - that's what the WGS84 datum is for.
The supply of actual people that think the earth is flat and aren’t trolling far exceeds the supply of people that want to mock a group that largely doesn’t really exist.
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.
And then you try to actually build a GPS network, and ask yourself: what kind of antennas should we use? what should be the freq? how much power? how will the receiver detect the precise nanosecond when it receives an incredible weak signal? (in current GPS the signal is bellow thermal noise)
(also you receive the signal from all satellites at the same time, on the same freq, and some random reflections. and then you need to extract independent streams of bits for each satellite, each with its own nanosecond timestamp for receive time)
Pretty cool. Would be nice to have the equation system as well in a recap, and the math not collapsed by default. Also had to look up other resources to understad that time correction refers to correcting a relatively short window of time, as it was not clear that receiver clock is actually accurate enough for short periods (milliseconds) to treat as affine.
So the trick, as always, boils down to engineering approximations, haha.
The receiver frequency is generally assumed to be accurate. In practice the time quantization is the far bigger error and masks any frequency fault. That time quantization is why most receivers report an accuracy of 3m or 10ns.
The time correction is better thought of as the fixed offset between the receiver clock and the GPS clock.
Very cool to see these browser-native interactive 3D visualizations! Gives this such a different energy than a regular blog post would have had.
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.
It's not that AI is necessary, but it's that one may not choose to (or have the skills to) spend a whole weekend hand-coding a 3D interactive visual. But one might spin up Claude Code and build whatever the explanation actually calls for in 15 minutes.
For anyone interested in a more detailed account of (general-)relativistic effects in GPS and other positioning systems, I really liked this article: https://pmc.ncbi.nlm.nih.gov/articles/PMC5253894/
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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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!