4 ms·
How does GPS work?
- openclawclub 6mo ago[flagged]
- FartyMcFarter 6mo agoThat's a lot of "—"s. Username checks out.
- gobdovan 6mo agoPretty 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.
- loodish 6mo agoThe 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.
- delamon 6mo agoThis blog post is also worth noting: https://ciechanow.ski/gps/ https://ciechanow.ski/gps/
- sam_lowry_ 6mo agoCiechanowski does a much better job explaining, I suspect the OP is just an AI ripoff.
- shriracha 6mo agohah good morning to you too HN (it's my piece and I'm not AI)
- IAmBroom 6mo agoI can tell you're AI because I've read a lot of it, and from some of the pixels.
- StrLght 6mo agoYou 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.
- codethief 6mo agoYup, 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 https://news.ycombinator.com/item?id=47861535 )
- NooneAtAll3 6mo agothat 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 https://www.youtube.com/watch?v=i7JPjgHa7_A
- HelloUsername 6mo ago> This blog post is also worth noting: https://ciechanow.ski/gps/ 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.
- sinaatalay 6mo agoVery 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.
- jetsetman192 6mo agoWhy would AI be needed for any of this?
- sinaatalay 6mo agoIt'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.
- thakoppno 6mo agoThis is likely the first time I wish a page requested GPS sensor permission. It would make the visualizations even more compelling.
- codethief 6mo agoFor 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/ https://pmc.ncbi.nlm.nih.gov/articles/PMC5253894/
- keyle 6mo agoAlways 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.
- FartyMcFarter 6mo agoDon'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.
- IAmBroom 6mo agoThat 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."
- darepublic 6mo agoIf we did drive on a flat surface wouldn't a similar triangulation strategy work? Could be with satellites aka stationary high alt weather balloons?
- phs2501 6mo agoYes 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.
- IAmBroom 6mo agoAll 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.
- NooneAtAll3 6mo agoPage tries to load, then goes: 404 Page not found Sorry, we couldn’t find the page you’re looking for
- dexterdog 6mo agoYeah, 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.
- alexcz 6mo agoIt 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 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.
- sorz 6mo agoSo gound stations need keep measuring their own positions due to continental drift! Never thought that before. Thanks.
- deleted 6mo ago[deleted]
- lxgr 6mo agoAnd 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!
- alfanick 6mo agoIt'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).
- m-hodges 6mo agoPeriodic 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 https://www.sparkfun.com/gps-module-gp1818mk-56-channel.html
- lxgr 6mo agoIsn'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!
- cammasmith 6mo agoCool article. Did a very good job explaining things simply and providing good diagrams.
- empiricus 6mo agoAnd 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)
- empiricus 6mo ago(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)
- zikduruqe 6mo agoThe 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.
- srean 6mo agoHarrison 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_interferometry https://en.wikipedia.org/wiki/Very-long-baseline_interferome...
- minetest2048 6mo agoYou 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-defining-the-navigation-signal-standard-for-future-lunar-missions/ 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
- ChrisArchitect 6mo agoPreviously slightly different url: https://news.ycombinator.com/item?id=47738343 https://news.ycombinator.com/item?id=47738343
- toraora 6mo agoif 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 http://www.aholme.co.uk/GPS/Main.htm
- ck2 6mo agoWhat'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 https://news.ycombinator.com/item?id=47849174 It will radically increase accuracy and availability the day it's allowed (like in Europe)
- firebot 6mo agoTriangulation. Saved you a click.
- mikestew 6mo agoThose that read the article might argue that it was a false savings.
- tqi 6mo agoThe 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.
- nradov 6mo agoGPS works well as a time source for smart watches and other mobile devices. But indoor clocks have trouble picking up GPS signals.
- deleted 6mo ago[deleted]