How does GPS work? | Hacker News Reader

How does GPS work?

perthirtysix.com

241 pointsalfanick49 comments

It gets even more interesting if you take into account how the satellites know where they are. Around the world there are fundamental stations: https://en.wikipedia.org/wiki/Fundamental_station

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.

So gound stations need keep measuring their own positions due to continental drift! Never thought that before. Thanks.

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!

It's quite fun area, I used to work in related technologies. Some of the countries maintain their own networks, but there are also commercial networks - so called reference stations that provide RTK/Network-RTK, basically your GPS can be augmented with a model based on those reference stations (which are ground GPS/GNSS receivers with very well known position) and improve accuracy to even 1cm (based on differences from your GPS position vs positions in network compared to well known positions).

This blog post is also worth noting: https://ciechanow.ski/gps/

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 )

> This blog post is also worth noting: https://ciechanow.ski/gps/

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.

that post is great on theory, but not the implementation

for that I'd recommend this youtube series https://www.youtube.com/watch?v=i7JPjgHa7_A

Ciechanowski does a much better job explaining, I suspect the OP is just an AI ripoff.

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.

hah good morning to you too HN (it's my piece and I'm not AI)

I can tell you're AI because I've read a lot of it, and from some of the pixels.

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.

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.

It’s kind of punching down at shadows.

ITT we've never been around fundimentalist christians or other sorts of easily manipulated counter-culture types

it's more than a couple dozen people.

i'll concede the point that its far, far fewer than those who feel otherwise, and that they like to hate on each other, but the idea that this is tiny and growing smaller is just not true. the surge in homeschooling, for example, would suggest otherwise...

Science illiteracy looks less like someone vocally exposing that the world is flat, and more like someone who has never considered the topology of the earth from a perspective other than their own eyes.

Your first sentence contradicts your second.

If we did drive on a flat surface wouldn't a similar triangulation strategy work? Could be with satellites aka stationary high alt weather balloons?

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.

All of which is irrelevant to GPS users. I can't remember the last time I checked the math that my GPS was performing, to be sure it was mapping to a rough sphere.

I worked on a system to do train positioning for the NYC subway system using Ultra Wideband radio beacons using the same sort of multilateration that GPS uses to determine position, so it was basically a flat system (obviously not fully flat, elevations still existed, but the UWB radios were roughly on the same plane as the train tracks at least compared to satellites).

...but at the end of the day the ECEF coordinates we used for everything still require a roughly spherical earth, but I don't think flat earthism is a real thing for most people who talk about it. Most of it is joking/trolling. There are surely some conspiracy-minded people who believe it because they don't give any serious thought to how anything works, but the people that publicly push alternate theories (eg. GPS is balloons, not satellites) have got to just be trolls.

I’ve met thousands of people in my life with all sorts of strange beliefs, but never someone who believed the Earth is flat.

Is this a sort of person you encounter often?

Don't you know, the Google maps team is part of the conspiracy. They calculate everything assuming a flat earth, they just don't tell you that.

That is seriously the logic behind a (very intelligent, well-educated) climate change denier I know: "The stations are in cities, which are locally heating up, and the remote sensor data is being faked."

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.

This is likely the first time I wish a page requested GPS sensor permission. It would make the visualizations even more compelling.

Why would AI be needed for any of this?

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.

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)

You can go into a really deep rabbit hole thinking about those questions.. this page explains the design decisions behind LunaNet AFS navigation signal: https://insidegnss.com/the-augmented-forward-signal-afs-defi...

Some considerations:

- They don't use GPS frequencies because there is receiver on the moon that receives GPS L1 signals (LuGRE and potentially more in the future)

- Make it easy to acquire for low complexity hardware

- Use 5G forward error correction code to reuse existing hardware implementation

- Design the signal in a way so that the user can easily find start of a data frame

And those are RF level considerations... there will be more considerations needed for the data transmitted over those navigation signal that the receivers need to use to determine navigation satellite position as lunar orbit is much more complicated than Earth orbit

(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)

The last time I built my own GPS network, I used a sextant, a watch set to UTC and nautical tables to determine where the orbiting bodies were.

Harrison time pieces were indeed the cutting edge GPS technology of its day. That and moon watching.

Nowadays we have GPS and Pulsar / Quasar watching.

https://en.wikipedia.org/wiki/Very-long-baseline_interferome...

if anyone is interested in the gory details of the signal processing side, this is a great resource: http://www.aholme.co.uk/GPS/Main.htm

Page tries to load, then goes:

404 Page not found

Sorry, we couldn’t find the page you’re looking for

Yeah, they're doing some kind of nonsense to avoid scraping which also blocks any browser setup for any kind of privacy. I even tried reloading the page with ublock turned off and it still did it so I moved on to something else.

Previously slightly different url: https://news.ycombinator.com/item?id=47738343

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/

The time part reminded me of the old WWVB radio time signals.

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.

GPS works well as a time source for smart watches and other mobile devices. But indoor clocks have trouble picking up GPS signals.

Cool article. Did a very good job explaining things simply and providing good diagrams.

Periodic reminder that you can just buy a cheap IC¹ to rip GPS data signals right out of the air. I built a GPS-powered clock that sits on my desk.

¹ https://www.sparkfun.com/gps-module-gp1818mk-56-channel.html

Isn't that how all GPS receivers work? I have yet to see one wired to the satellites ;)

But yes, it's amazing how small and battery efficient GPS receivers are at this point!

Triangulation.

Saved you a click.

Those that read the article might argue that it was a false savings.

What's interesting is the Chinese GPS called BeiDou is way more advanced than the now outdated American GPS system

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)

Reply on news.ycombinator.com