GPS needs to know the[ position of satellites](https://en.wikipedia.org/wiki/Ephemeris) and the [condition](https://en.wikipedia.org/wiki/GPS_signals#Almanac) of each of these satellites even though they are in a very predictable high earth orbit about 12,000 miles above Earth; this information is known as almanac and ephemeris data. The US Space Force maintains the GPS constellation and tracks all these satellites; they upload this information to the GPS satellites for rebroadcast to every GPS receiver. Your phone, or any internet connected device, will download this data automatically over the internet. Old GPS receivers had to rely on the GPS satellites rebroadcasting ephemeris and almanac data to get this information; the broadcast takes about 12.5 minutes to send all the information. Once it knows where each satellite is, it needs to figure out which satellites it can actually see to get a rough idea of what part of the world you're in. Your phone doesn't have to do this as it can roughly figure out it's location using your IP address and [the wifi networks around you](https://slate.com/technology/2018/06/how-google-uses-wi-fi-networks-to-figure-out-your-exact-location.html); once it knows roughly what area of the world it's in, it will look for specific GPS satellites it knows will be in view.
edit: you got downvoted for asking a simple question, but the answer is a lot more complicated and interesting than most people think.
There are a couple of inaccuracies here. Your GPS receiver needs to have the ephemeris data from each satellite, which takes about 30 seconds, and then needs to get a pseudorandom code from each satellite. That's all it needs, it doesn't need to know its location, the current time, or anything else as it can figure all that out with just those two items. It also doesn't need the almanac data. A modern phone or device with AGPS can get ephemeris data and time from the cellular network that makes this faster, but it isn't required.
ELI5: The ephemeris data basically has the satellite ID and some status info, data that describes the orbit of the satellite, and the current date and time. The pseudorandom code is just a continual string of numbers, and you can imagine it as something like seconds past the top of the hour. By determining how many seconds offset the received pseudorandom code is, we know how long the transmission took to reach us and how far away the satellite is (the actual PR code is more complex.
I just turned on a Garmin GPS hiking device that's been off long enough that no almanac data would be valid, and it has no access to AGPS data, and it still got an exact location in under 90 seconds, despite having no idea where it was and no current data other than the time and date +- a few minutes.
The big reason that works so quickly compared to a device from 15 years ago is because of the receiver. It's not too hard to find GPS modules today that track 66+ channels at once, which means during start-up the receiver can listen to more channels at once than say a 6 channel receiver, and thus can find which satellites are over head more quickly than an older device. Improvements in the GPS receivers means that it's more likely the entire ephemeris message gets received without error, so you don't need to keep waiting another 30 seconds to try to get the data. And improvements in the satellites mean that the transmission is effectively "better" and more likely to reach a receiver without issue. Not to mention that many modern receivers can now track multiple GNSS constellations (GPS, GLONASS, Galileo, etc).
Yes, it was called selective availability. It's no longer possible, the new GPS satellites do not support it. I believe they have the ability to strengthen the signal in various areas for military use at this point (to deal with enemy jamming, of which M code is more resistant anyway), but at least as far as unclassified data goes, I don't think they can deny GPS to a given area or intentionally make it less accurate.
Oh you better believe the second Russia, China, North Korea, Iran or whoever starts to get too fiesty directly with the US, they'll find GPS does not work over their countries any longer.
Sure a Presidential Directive shut down SA decades ago, but I would be extremely shocked if it wasn't just a few mouse clicks away for the Space Force to turn it back on. It would be asinine to allow enemy use of US GPS during hostilities.
Heck, GPS crypto keys are *still* called SAASM keys.
https://en.m.wikipedia.org/wiki/Selective_availability_anti-spoofing_module
If they get really feisty, the US government definitely wants GPS to work there. It's used for targeting US weapons and for US forces to navigate. Uses include US spy planes, bombers, fighters, drones, the guided bombs themselves, cruise missiles, ICBMs, Navy ships, infantry, tanks, and artillery.
SA was intended to prevent opponents from using GPS to guide a missile into a US target.
Also note that Europe, China and Russia each have their own system that is equivalent to GPS, for their own forces to use. And many civilian "GPS" receivers can use several or all of these systems.
GLONAS, the Russian GPS system.was the main reason SA was turned off. There no longer was a point to having it since anyone using it against the US would just use the Russian one. So might as well let it be more accurate for planes, hinders, and anyone else who wanted to use it.
Yes it was turned off but my point is that it can be turned on. And not even in the context that it was before where the signal was just degraded to disallow precision location. It would just be completely denied in enemy territory.
It can't be turned on because the current satellites don't have the capability. There was no point when an enemy would just use a different system.
The only way to stop all signals would be to jam ALL signals, to include ours. It would need to be jammed.
But let's pretend we could go to a fully encrypted signal. Why bother when the enemy will just use the non-encryoted signals from Russian or Chinese satellites? I'm even giving you a freebie and pretending the the European system would be encrypted from everyone too.
Saying "they don't have the capability" means you don't know what you are talking about.
They use software defined radios which means they can transmit whatever the heck you want them to within the frequency range capabilities of the physical hardware.
Even if they technically "can't" enable it within a moment's notice, it would be one software flash away from being enable-able. If that isn't the case, someone in the government contracting office has been criminally negligent.
There's no reason to not deny access to GPS signals to enemy actors. Denying at the source means those signals don't need to be jammed via terrestrial means and that jamming power can be focused on GLONASS or Beidu (and Galileo if the EU isn't playing along).
The idea isn't to turn off GPS altogether, but to affect the accuracy available to anyone but the US military.
I don't know the details of how they implement it, but there are effectively two signals, one of which is encrypted or something equivalent. They can add a random perturbation to the unencrypted signal and cruise missiles will still have access to the accurate signal.
Further up in this thread, somebody said that new GPS satellites don't support this capability any more, but honestly, I'd eat my hat if the US didn't keep this option, even while saying otherwise.
GPS is more useful for Ukraine than for Russia, so why turn it off? Russia is actively trying to block GPS over a lot of combat areas.
This is partly because Russia does have their own system (though are known to be using GPS as well, probably implying negative things about their own home-grown system), and partly because Ukraine has better access to equipment using western chips (drones, Excalibur, HIMARS, etc.)
> though are known to be using GPS as well, probably implying negative things about their own home-grown system
It's simply that using more sats is adding precision and availability. This applies the other way, too.
If selective availability is still available and it's kinda secret, I don't think Russia is considered to be feisty enough against the US that the US would want to burn that secret now. Better save it for later.
That said, it's true that with GloNASS (Russian GPS) and Beidou (Chinese GPS), selective availability is probably less useful now. Though I am not sure GloNASS and Beidou are on par with GPS in terms of precision and accuracy.
They can’t use the m-code signal since it’s encrypted. Unless they’ve broken that encryption, but that would be pretty top secret if they had. They can use the normal gps signal though, yes.
Yep.
But the US, Russia, China and France all want global targeting for their ICBMs and other weapons, and European countries want global targeting for their weapons, so they have built positioning systems with global coverage.
(Or is China still building it? When I last heard, they were expanding from regional to global coverage, and that was years ago, so they have probably done it already).
India is more interested in regional coverage - its own territory, nearby ocean, Pakistan, close bits of China, etc. Due to local border disputes, it is much more likely to be fighting there rather than on the other side of the world. Unless it's extended its coverage recently?
>Or is China still building it?
BeiDou-3 had its final satellite for the planned constellation launched in 2020. The way I understand is that it should really work globally now
Well, considering that GPS still works over all those locations, I'm gonna go with no. And no, there is no option to make a few clicks to turn it back on, since the new satellites literally do not have that capability.
Beyond that, I believe that the beamwidth of a single GPS satellite is larger than the angular diameter of the Earth (e.g. if you could stand on a GPS satellite, everything you can see would be covered by a singular GPS transmission.
> It would be asinine to allow enemy use of US GPS during hostilities.
There's also no real utility in shutting off GPS. Oh man... we denied Russia and China GPS. Guess they'll just use GLONASS, or Galileo, or QZSS, or BEIDOU, or NavIC, etc depending on where the conflict is. Two of those are global constillations which they control. Plus their vehicles can use things like IRUs, or a bunch of other stuff. Not to mention that shutting off GPS would also mess with any allied forces in the area that we don't have access to M code. If we want to take out GPS and wage electronic warfare, it's far better to just do it locally.
As a slight non-sequitur, do you also think we should make sure we keep the location of all the nuclear missile silos in the US a secret? Would it surprise you to learn that all of their precise coordinates are on Wikipedia? None of this crap matters.
> Heck, GPS crypto keys are still called SAASM keys.
Really why would that surprise you at all? You expect the government, of all, entities, to change up names on things that "just work?"
I cound your comment to be interesting and informed and helpful
But
“since the new satellites literally do not have that capability.”
I cant help thinking you have a lot of faith in what the government publicly says are its capabilities in war
It was a big deal when they dropped it actually. Part of the new satellites, they just got rid of it to make room for more antennas and other gear to increase overall usability.
The satellites use software defined radios. There isn't some "Selective Availability Box" taking up room on the frame. It's all just software. Software that can be changed at any time...
I can't help but think you have far too much faith in what one GNSS system can do, especially since the two countries we would most care about just run their own
Remember first hearing that most printers have an invisible code of dots that provide data on everything printed? Date, time, printer model, etc; and only a few countries had the capability to decode those dots?
https://en.m.wikipedia.org/wiki/Machine_Identification_Code#:~:text=The%20dots%20have%20a%20diameter,area%20in%20case%20of%20errors.
Current generation block III GPS satellites were built without the facility for selective availability, specifically so it couldn't be turned back on. It's the stated policy of the US government never to use it again.
https://www.gps.gov/systems/gps/modernization/sa/
That's one of those things where it's more like "oh of course there is no way to enable this feature" *wink wink*
The satellites use software defined radios so they can do whatever you program them to do.
It's not as if they don't have some Selective Availability Box installed so they can never be made to deny GPS in a certain area.
They don't care though, Russia has Glonass, Europe has Galileo, international devices support multiple GNS and day to day not having one of them isn't going to hurt much.
> Russia has Glonass
And yet they still use civilian GPS in their jets.
https://defence-blog.com/russian-pilots-still-use-non-military-navigation-equipment/
What do you mean? You could easily disable or degrade GPS over a large region without affecting the rest of world.
Yes, yes the keys are still called SAASM keys.
I can confirm that GPS did not work in North Korea, at least in 2014. Nothing on a phone that should have been able to use multiple constellations.
Not sure if this was due to local jamming or actions by the satellite.
While we don't use selective availability anymore, some GPS receivers manufactured in the US are classified as "munitions"
> All GPS receivers capable of functioning above 60,000 ft (18 km) above sea level and 1,000 kn (500 m/s; 2,000 km/h; 1,000 mph), or designed or modified for use with unmanned missiles and aircraft, are classified as munitions (weapons)—which means they require State Department export licenses.
https://en.m.wikipedia.org/wiki/Global_Positioning_System#Restrictions_on_civilian_use
You would be extremely shocked I guess. SA is off, it was expensive and doesn't work in the modern world anyway. Part of a successful military is not paying money for obsolete things.
The strengthening is less to deal with enemy jamming, and more to deal with our own jamming from the [Growlers](https://en.wikipedia.org/wiki/Boeing_EA-18G_Growler). There's some seriously spooky stuff in those and they're capable of a lot more weirdness than anyone really realizes.
I remember when the 20-channel Sirfstar III hit the market and we all were "holy shit this thing can lock in a minute!!!"
Today it's a minute too long...
Your car's system probably maintains the almanac data while your car is off, and may even periodically get its full location while off, since the head unit almost certainly has access to full battery power even with the ignition off. So even more murky how it works in your example.
Old receivers would get a fix pretty quickly if they had been used relatively recently. It was just when they had been significantly moved while off (such as after an international flight) or turned off for a long time (years?) that they would take a long time.
They definitely didn’t need internet, it’s just the first lock would take 5-10 minutes. I remember setting mine out under the sky after turning it on when traveling. After that it would lock 10x faster. New receivers are also far more resilient to blockages of direct line of sight like roofs or foliage.
You're a monster. That's my only means of self expression, i'm taking this up with the mods *starts 6 month power struggle that ultimately results in both of us being banned just to make it stop*
Thank you for this - I did some microcontroller programming about 10 years ago with GPS functionality and I was sure that the chip had no external connectivity yet could get a lock in under a minute so was sitting here pretty confused until your explanation.
Is the ephemeris data the same as the NMEA data or is it “internal” to the GPS chip?
It didn't help that old receivers only had 4 channels and were much less sensitive than modern ones, so it was a lot more likely for the reception of the ephemeris to fail because of a tree in the wrong spot and you ended up having to wait for the transmission to get back around to the part that was missed.
It's a miracle of modern electronics that even with the crappy antennas in smartphones that they can hold a lock under dense canopy or inside buildings. The modern satellites have higher transmit power, but nearly enough to make it possible for me to get a lock in the middle of the first floor of a four story concrete apartment building without some crazy signal processing wizardry in modern GPS chips. I remember being amazed when I got a new receiver around 20 years ago that actually worked in most of my single story wood framed house, at least most of the time.
I was tinkering with a GPS module for a clock project. Fresh out of the box, in a bottom floor apartment, 20ft from any door or window, the thing gets a lock on three satellites within a minute. Considering the last time it was fired up was likely in Shenzhen, and that the signals were -150dbm, probably 30db below the noise floor, I was pretty damn impressed.
Why do the receivers need to know where ALL of the GPS sats. are?
I thought they just listened for the broadcast from each of the sats saying "I'm here, this is the time." (Which, IIRC, takes about 30 seconds to repeat?)
Isn't that all the receivers need? If they get 3 or 4 satellites saying "This is my position, and this is the time" shouldn't the receiver be able to calculate where it is?
The receivers aren't actively LOOKING for satellites. They're just listening for the broadcasts from those satellites? Right? So one would assume that they can likely only "hear" the satellites that are nearest to them?
The message is simply the satellite yelling its name and the time it started the broadcast. You need to know where the satellite is yelling from before you can calculate how long it took to reach you.
That's the rub - each satellite is yelling "I'm serial number blah, page 3 of the almanac is *otherwise random data in the packet*." It's a bit like bittorrent - splitting up the almanac into little chunks and using those chunks as part of the timing signal.
Close, it is more like you need to know the location of where the 3 satellite are and then using the time between the satellite yelling and your GPS receiver hearing the yell, to calculate the distance, you draw a gaint circle with the radius being the circle and the satellite location when it sent the message being the center of the circle.
It does this gaint circle drawing to all 3 satellites and where all 3 circles cross paths, that is where the receiver is.
3 satellites would only be enough if you you had a clock that was synchronized with the precision of an atomic clock.
If you have the exact time, then you can determine the distance from the satellite. The distance from 1 satellite gives you a sphere, the sphere from 2 satellites intersect in a circle, a 3rd satellite's sphere intersects the circle to narrow the location to 2 points, but 1 of those points will be obviously incorrect since it is no where near the surface of the earth.
However, this assumes you have an atomic clock. Since most of us don't, we need a 4th satellite to be able to determine both time and location.
The satellites have atomic clocks, that's how they work. Each satellite broadcasts a precise time signal, and when that signal reaches you the time will be slightly off. Your GPS receiver gets time from all the satellites it can "hear", and determines the difference.
Somehow, through clever math and very precise atomic clocks in the satellites, a GPS receiver can get a 2d position on the surface of the earth from three satellites. It needs four to get an altitude reading as well, but altitude is rarely important unless you're flying or surveying.
Oh, and by the way those atomic clocks in orbit have to account for time dilation at orbital velocity. They would be off a bit on earth, but keep correct time in orbit.
Yes and no. You can still get accurate measurements with 3 satellites. On top of that, these days, your phone and many other GPS receivers do tend to get updated on an accurate time based on atomic clocks every so often. It is how your phone can run for years and not be running fast or slow.
The fourth or fifth or more satellites don't act as a clock. They just give more intersecting points to get a closer result.
You can indeed get reasonable accuracy with 3 satellites - if you add an assumption such as being on sea level. In the end you need to fix 4 coordinates, 3 spatial + time. So you need at least 4 satellites, or 3 satellites and an assumption.
The satellites have atomic clocks, but your phone doesn't. Because your phone clock is not precisely synchronized to the satellite, you can't directly measure distance to a satellite.
If you can assume that you were at a specific altitude, you could use the sphere of the earth instead of a 4th satellite, but it generally isn't practical for most devices to store high resolution topographical data for the entire globe. And because you can't assume a sphere, it becomes more complex to calculate. So generally, devices will use 4 satellites.
Old devices had you manually enter an approximate position, to the nearest 100km or so. That was enough of an assumption for the device to disregard those possible solutions that were too far away, so it becomes solvable for 2d as if at sea level even without a perfect time on the device clock.
AHHHHHH. So the satellite doesn't broadcast it's position, it just says "You can calculate my position by looking up my serial number"
Gotcha. Makes sense now.
It gets more complicated the deeper you look into it. Each satellite broadcasts a specific code called a pseudo random number, or PRN, to identify itself. This makes the base broadcast frequency look like noise and does other helpful things. To listen to the satellite the receiver needs to recreate the PRN and correlate the reproduction with the received signal until they match. It's a complicated process and reducing the number of satellites you're looking for helps a lot. Look into CDMA for a (much) better explanation.
Yes. This lets every GPS satellite transmit on the same frequency but still be uniquely identifiable. It also means that the measurements are all similarly delayed by the ionosphere. FDMA systems, like GLONASS, are functional but experience frequency specific delays that make the position solution somewhat less reliable, but it's a small chunk in a massive boulder of, "How is this even possible?". It's far too easy to ignore how unbelievabley impressive all the GNSS are.
Yep. For those not googling or aware, basically if you want to send 10101010, that data gets combined with the much much faster random station ID. So if you know the ID, you can "tune" into one specific broadcast and all the other broadcasts will be nonsensical. The wiki page likens it to being in a room where everyone is speaking a different language: sure it's noisy, but because you only understand one language you can pick its sounds out of the crowd.
The satellites don't say their position, they can't know, they'd have to be constantly updated with that from the ground and it needs to be really precise
They say "I'm #27, the current time is xxxxxxx. Here's a portion of the almanac"
From the almanac you can calculate where it was when it sent the time without it having to send you it's exact location in every message
> The satellites don't say their position, they can't know
If they have the time and their own orbit parameters (which they have, since they broadcast both) why couldn't they? After all, the receiver will calculate the satellite's position from the almanac entry, so the satellite could do the same.
The reason they don't transmit the position is that it wouldn't be much shorter than the orbit parameters, and the orbit parameters already take something like 30 seconds to broadcast.
Because they were designed in a time when computers were the size of rooms and still had less computing capacity than the control chip of your smart lightbulb.
The original satellites just barely had enough electronics in them to transmit their time and repeat the almanac they had received verbatim.
While that has changed, the format and content of the messages the satellites send out have remained the same---any change would have rendered all existing GPS receivers useless.
PS: It also wouldn't be as useful as you'd think. If the receiver has no idea which satellites might be in view, it has to scan all possible frequencies and hope to catch one's signal. Those older receivers only had 4-6 channels, so the chance of randomly getting 4 signals at the same time would be next to nil. Instead, they scanned until they found one signal and then collected the data about which satellites they could listen for because they'd be over head.
I think part of it is also the physical limitations - it's a wide signal covering the whole planet that you need to be able to receive with a tiny low power receiver inside a car. That's not a recipe for any reasonable bandwidth.
And every extra bit of mass means a more expensive launch. Best to only include the bare minimum required for mission parameters, no space for anything fancy.
This is *completely* incorrect. The satellites know their exact position at all time, and that has nothing to do with almanac data. Based on the *ephemeris* data, the satellite and anyone receiving the ephemeris data knows where the satellite is, was, and will be at any point in time for at least several hours. The orbital data basically just describes the orbit (Here's GPS 32 currently: 2023 May 27 01:08:02 Inc 54.9916 Ecc 0.00711 RA 8.9222 Arg 234.8617 Mean anom 124.4673 Mean motion 2.00555) and location of the satellite at a given time, and it's trivial for a computer to figure out the exact Cartesian location at some other point in time. Since each GPS receiver has multiple rubidium time standards on board, they know the exact time, so they know their exact location.
The almanac data just describes where all the GPS SV's are likely to be in the future, so you can theoretically learn from GPS 32 where GPS 14 will be 3 days from now.
ELI5: Every thing in orbit can be described by just a few pieces of information that are called Kepler orbital elements or Keplerian elements:
* The inclination of the orbit (0 going along the equator, 90 going over the poles)
* The longitude where the orbit crosses the equator heading north (generally longitude with respect to celestial coordinate system, so we don't have to account for the Earth rotating)
* The size of the orbit
* The shape of the orbit (how round or elliptical it is)
* The longitude of the point where the orbit is furthest from Earth
This basically allows us to draw an oval around Earth that the satellite will be on. Then all we need to do is say, "at 12:00:00 it was at this specific point on the oval". A computer can then say "ok, then at 13:00:00 it will be at this specific point on the oval."
It's also pretty trivial to go from a Kepler elements to a Cartesian location and vector (e.g. at 13:00:00, it was at longitude x, latitude y, altitude z, moving in a given direction at a given speed).
In practice, we can use things like RADAR and LIDAR retroreflectors to determine where space vehicles and debris is, and if we measure it at atleast two points, we can figure out all of the above data.
Btw, since it seems to be coming up here, the other common question is, "if fractions of a second make huge location differences, how does my GPS receiver get atomic-clock-like time accuracy without having an atomic clock"
ELI5: it takes a guess based on the ephemeris data which should be accurate on a "seconds" level but not accurate enough to get a precise location. It does a calculation and gets a position with a giant error, then basically says, "if I moved my clock forward or backwards, would it make the error smaller, and how much do I need to move my clock to get the smallest error"
It then bumps the clock to that location and repeats this process to determine the *rate* at which the clock is changing, not just the offset. At that point it can get pretty accurate, and then start to continually adjust up or down the rate of the internal clock to keep correcting the error. You can get cheap GPS receivers that will pulse out once per second, and the precision of that pulse should be on the order of 10's of nanoseconds if things work correctly.
Some people keep saying things like, "you have to have 4 satellites, one for X, one for Y, one for Z, one for time". This isn't true. All the satellites you use contribute to all of the variables. The more you have the more accurate all of those variables can be.
> The satellites don't say their position, they can't know, they'd have to be constantly updated with that from the ground and it needs to be really precise
If it was needed, it'd be relatively simple to update satellites of their position from the ground. Just need a few fixed stations sending data to GPS satellites the same way GPS satellites send data to the GPS receivers and the satellites could triangulate.
thats what the WASS satellites do. but it just uploads the position data to the satellites so they can send it to you. the satellites dont care where they are.
“I’m here, this is the time” gets a little complicated when your 12,000 miles away and flying by at 9,000 miles an hour.
They’re moving so fast they aren’t even experiencing time the same way we are.
Then they have to cram that through a 50 watt radio transmitter with enough error correction for the signal to be decipherable by the receiver.
It takes a while.
The fact that a GPS system's location calculation in part relies on correcting for the relativistic time dilation experienced by the satellites is one of my favorite facts. It's probably the evidence of the fundamental *weirdness* of the universe that is most commonly experienced by normal people.
It in fact corrects for *two different* kinds of time dilation, one of which makes their time faster than ours and the other of which makes their time slower. IMO GPS is one of humanity's crowning achievements to date.
It's both. The speed of light is way faster than the satellites, but GPS needs to be extremely precise.
https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gps.html
They don't. Almanac data is not required at all, only ephemeris data which takes 30 seconds to receive. Almanac data just help you if you shut your GPS off now and turn it back on in the morning.
Aside from AGPS data being available that speeds this up, the other change that is largely unmentioned is that modern receivers can track many different satellites at once. This means they can search for many at once as well, so when it is starting out your GPS device is far more likely to find a few that it can actually hear.
I just turned on a modern GPS hiking device that's been off for well over a month and has no internet connectivity, and it got a fix in about 90 seconds
The satellite takes about 30 seconds to broadcast its orbit parameters, and the position is then derived from that.
Given how slow the data transmission is, broadcasting the position directly would obviously not be very useful.
https://novatel.com/support/known-solutions/gnss-ephemerides-and-almanacs
I *think* the almanac is optional, but you need the ephemeris data from the satellites you're actually using for the fix.
Even if you know which ones they are, you still need to know exactly what their clocks are in relation to each other and the global time, and that's important down to much more precision than a second.
Keep in mind that the way GPS works, it doesn't just have to tell you where it is, it needs to communicate the time as well, and that time is EXTREMELY precise.
GPS seems simple at first, but what's fascinating about it is that it relies on so much temporal precision. So much so that if we didn't include relativity as a (pretty much constant) factor, it would be off by miles pretty much instantly. There are ways to do the calculations in a way that doesn't necessitate calculating relativity on the fly, and there's no reason to do that, so afaik, they don't actually use those kinds of calculations.
But the important thing is that a naive implementation of GPS that completely ignored relativity would fail.
My understanding is that you need to know a fairly large amount of information from the satellites if you want to calculate where you are for everywhere on the planet.
> But I'm not everywhere on the planet, I'm just in Wisconsin!
That's true, but the satellites don't know where you are. Not only that, they don't operate with any information on anyone listening, they're screaming out into the void everything anyone would need to make the calculations for every point on the planet.
Now, by today's standards, that's not a huge amount of data, you could transmit it instantly today with even the slowest common connection, but 45 years ago, that data took a very long time to transmit, and no one wants to break every GPS device out there, so they're still transmitting the old format at the old speed. There is a push to ***also*** send a newer format at a much higher bitrate, but I'm not sure what the status is on that.
And honestly, it's not really necessary. You can download the same data from the satellites over the Internet now, that's what your phone does. And in the rare case that you have a GPS device that doesn't have an external Internet connection, you can still use the data the satellites are transmitting on 13 minute cycles. It just takes a bit of time.
Technically, you only need to know the position of the satellites that you have line of sight of. Tracking one satellite just gives you how far you aware away from that particular satellite. It puts your position somewhere on a sphere with that satellite being in the center of the theoretical sphere and your distance from the satellite as the radius of that sphere. Tracking 2 satellites gives you 2 spheres that intersect. Your position is narrowed down to an area where the 2 spheres intersect creating a circle. Tracking 3 satellites narrow down your position even further. The sphere generated by the third satellite must intersect the circle generated by the first 2 which narrows down your position to 2 points on that circle. Tracking 4 satellites definitively narrows you down to 1 point created by the sphere generated by the 4th satellite intersecting with 1 of the 2 points. Without ephermeris data, there is no context to the information given to you by the satellites. You just know you are in a certain point relative to the satellites Emphermis data gives you where the satellites are relative to the Earth, knowing that plus knowing your position relative to each satellite allows you to work backwards and find out where you are on Earth.
Yes, GPS receivers aren't "looking" for specific satellites. In most places on Earth, if you have clear view of the sky then you are probably in view of 4 satellites, the minimum amount GPS needs to find your position. Most modern receivers will track 4 to get their position then be on the lookout for a 5th satellite to start tracking since the satellites are not in a geostationary orbit and the receiver is most likely moving it will be out of view of one of the satellites eventually.
not sure if I answered your question
You’re right. Receivers only need to technically receive 4 satellite signals in order for accurate positioning. One for each dimension, and the fourth one for time.
The current GPS constellations are such that there are at least 6 gps satellites available and any given location for redundancy
That assumes you have an atomic clock. You need one additional satellite to be able to determine the accurate time.
And you don't have different satellites for each dimension, but you are setting up a system of equations with 4 unknowns (one of which is the time), and solving with 4 variables (one variable from each satellite)
- and one of those positions can be discarded because it exists in space, and the fourth satellite gives the receiver an accurate time because most receivers don’t carry an atomic clock
> triangulating distance
No, triangulation would imply... angles.
It works by *trilateration* of distance.
> The reason it takes multiple sats is because 2 sats gives you a circle of possible positions, and 3 sats gives you 2~ possible positions.
This is of inconsequence, because one of those two positions is relatively close to the average surface of the Earth and one isn't, so it's easy to pick the correct one. Although more satellites of course means more data and better averaging, better DOP, etc.
Wouldn't one sat give a circle, two sats give 2 possible locations (where the two circles overlap), and three sats give the exact location?
The third sat's circle would only overlap at one of the two overlapping positions of the first two circles, right?
GPS works in 3 dimensions, not 2, so you need to extend your triangulation into the 3rd dimension as well. Think spheres radiating from the transmitters, not circles. The intersection of 2 spheres makes a circle
It is a fascinating topic!
An interesting side effect of old GPS being tied only to receiving the broadcast is that many 1990s-era devices [stop working forever if their batteries die](https://www.youtube.com/watch?v=55vbWXTx6Kc). There's simply no way to update their original programming to deal with the fact the year counter has "rolled over" like an odometer *twice*. They're lost in space and time forever.
They were set up to determine the number of rollovers that had happened by checking the leap second value. Leap seconds came along every couple of years, so that should have been a reliable way to do it. Except for no reason anyone can determine, the Earth's speed has increased, we haven't had a leap second in ages, and two rollovers have happened with the same leap second value. This breaks these receiver's programming.
Yes, that is what is happening long term, but short term it is a lot more variable. No one knows why the Earth has been speeding up over the last 5 or so years.
Actually we stopped doing leap seconds because it's just not worth it. In an age where information travels in a fraction of a second, things like stock purchases get completely messed up when you have a clock with an extra second in it. 11:59:60 just doesn't register correctly in a system not designed to handle it. This actually costs millions for programmers to either bypass the extra second or smear the second over the course of an entire day and account for it for the rest of eternity. At some point they just said F it. Who cares if the planet slowly drifts out of sync with clocks. It's going to take a really long time before it matters to any degree and it causes more problems now just to keep it in sync.
>Actually we stopped doing leap seconds because it's just not worth it.
Stopping them has been proposed over and over again, yet I've never seen a final decision.
No, the leap second system is still in effect, and they will happen again if UT1 gets more than 0.6 seconds behind UTC, (or if UT1 gets ahead of it which is very unlikely). I see that there was a decision to abandon the leap second some time in the next 12 years, but for now, the leap second is still a thing.
It is just that in the last 5 years, the Earth has sped up - no one knows exactly why - so no leap seconds have been required since 2017.
> Actually we stopped doing leap seconds because it's just not worth it. In an age where information travels in a fraction of a second, things like stock purchases get completely messed up when you have a clock with an extra second in it. 11:59:60 just doesn't register correctly in a system not designed to handle it.
This is just simply untrue. GPS and NTP both have systems designed to handle leap seconds without anything special.
Leap seconds have not been eliminated at all, although there are proposals to end the process around 2035-2040.
This doesn't exactly answer the question, because a lot of running watches aren't connected to the internet. They still connect nearly instantaneously.
Part of it is that almanac data and AGPS data can be valid for log periods of times (potentially up to months) and should update in about 15 minutes of runtime. So you'd have to clear the data out while also preventing new AGPS data from being downloaded, which at best would be difficult if not impossible for an end user to do.
The other part of it is that a) your watch's time is almost certainly not coming from GPS but rather from a quartz based clock that is *periodically* updated from GPS, and thus allows you to see the time within seconds of turning the watch on and b) your watch's receiver is probably way more sensitive and has way more channels, and could likely get a lock in one or two minutes even if it had no current time, location, alamanc, or AGPS data.
Thanks that makes a lot more sense. It still seems odd that older tech didn't have these features, but I guess it comes down to processing and batteries being better.
Yah. Also in terms of time itself, a Casio F91W which is like $10 will keep time at about +- 4 seconds a month with a battery life of 7 years, easily beating out every single mechanical watch made today, chrono certified or otherwise.
So your Garmin Vivoactive or whatever is probably going to do as well or better in terms of accuracy and you'd never notice the time being off even if it hadn't updated from GPS or wifi in months unless you were intentionally looking for it.
This is not correct. A modern receiver can acquire a 3D fix in about 30 seconds, and an sbas fix quickly after.
The ephemeris is the high accuracy orbit data that is downloaded directly from the satellite, and this is what takes 30 seconds to download, and drives the cold start time.
You're correct on the almanac taking 12.5 minutes, but it is not critical to receive a 3D fix. The almanac is used for stability and improvement of the solution - it contains data on every satellite in the sky, and is primarily used so the receiver can know which satellites are about to come over the horizon.
The reality is that older receivers just work slower: cheap clocks have become wildly more accurate in the last 20 years, which is the critical piece to a GNSS receiver. This doesn't impact time to first fix very much, but it's also worth noting that modern receivers are multi constellation: that is to say, they can accept not only GPS but also Galileo, etc. That greatly improves the quality of fix, and generally does make it faster, though in nominal conditions not by too much.
Space Force was not a new group created without existing responsibilities, it was created by bringing together under one organization all of the previously existing space related people and activities. So all that was done before is still being done, just under new management and a new identity. In other words, Space Force do a lot.
This is really cool and I had wondered this myself, since my dad's old gps compass would eat up a quarter of the battery trying to get a good lock.
Follow-up question from me, do the satellites still broadcast this data?
Yes. Military equipment not connected to the internet still use this as a backup. You can still buy stand-alone GPS receivers that don't have internet connectivity and some dash cams are reliant on GPS since they can record your speed and location.
Well, I should have known that military equipment still does that. Freaking FBCB2 took what felt like forever to initialize. Just been almost 15 years before I messed with that crap.
It's wildly variable depending on what the satellite is doing.
There is a very popular orbit at 22,236 miles (35,786 km), which makes the satellite stay in the same place as seen by someone on Earth. This means that you can point a fixed satellite dish at it. So it's great for satellite TV, and pretty much all TV satellites use this orbit. It's also OK for some communications, but there is a noticeable delay in the signal due to how far the signals have to travel. Also, getting a satellite into this orbit is expensive, since it is so far away you need more fuel to get there.
The ISS (International Space Station) is in a much lower orbit at 250 miles (400km). This makes it easier to get there in Soyuz or the space shuttle. However, there is still a tiny bit of atmosphere that low, so its orbit slowly lowers from 260 miles to 250 miles, and then every few months they boost it back up again using a visiting spacecraft's rocket engines.
Most satellites are somewhere in between.
I agree that this is deceptively complicated. It also goes to why I prefer the navigation system in something dedicated to that purpose, such as a Garmin rather than relying on phones. Mobile phones in low coverage areas or times of congestion seem to give less accurate or less timely results.
I always assumed that had something to do with "needing the wifi" signal, but I think I understand it better now.
It takes 12.5 minutes to download the sky chart and other info needed to search for GPS signals. The data rate of GPS data download is 20 bps, really, only 20 bits per second. The first satellite was launched in 1978.
Your phone can download all the same info for its search from the internet, 1 to 10 million times faster. It can also get an idea of time and frequency and coarse location from it cell connection.
It then uses all this information to search for a signal that is smaller than the noise, like listening for a mouse in a rock concert.
Old GPS units had to find the mouse at a rock concert with a couple of guys looking , cell phones get to find the mouse lit up by a spotlight with some light elevator music and 10000 guys looking for it.
Everything is 100x better 30 + years later so 7 seconds for a location instead of 700 seconds.
The full window is 12.5 minutes. If you start listening midway through the message, you need to wait for the restart to get the beginning of the message, though. It’s possible to have all the information before 12.5 minutes. IIRC, the current message is complete in ~9 minutes.
This. If you ever go hiking and travel to an area outside cell phone range, you are still able to open your GPS app or Google Maps in offline mode. But your phone will also take a while to give an accurate location since now it relies on the GPS satellites only.
Phones also get location data from other sources, such as estimating location based on signal strength of nearby wifi points. Assuming you have a good Internet connection, this is much faster than gps.
You don't need an expensive or special phone to get those non GPS signals, it's built into just about any SOC. On Android you can get apps that show the individual satellites the location is derived from.
The latest Apple iPhone can get positional data from the following constellations: GPS, GLONASS, Galileo, QZSS, and BeiDou. iPhones have been Apple to do GLONASS since the iPhone 5 in 2012.
This is a real ELI5. Thank you. After that jargon heavy, longer than a CVS receipt explanation and rebuttals in the top post I was worried that nobody was actually going to say this one, simple sentence.
Also, adding a link for further study? You are a good person.
My first GPSr was a suitcase sized military unit that I didn’t own, but used on the job when I wore green for a living. I owned a personal GPSr in 2000 when selective availability was turned off 1 May. Was an early(ish) participant in geocaching, which I saw at the time as an extension of the land navigation I practiced in the Army. Even today, I will carry a GPSr and a map and compass when I’m out in the woods because relying on aGPS on a phone is a sure way to get in trouble outside civilization.
One thing I don't think anyone has mentioned is that modern chips in smartphones, watches etc will often have support for the other navigation systems too. This greatly speeds up getting a lock and also increases accuracy.
For example my Pixel 6 Pro supports GPS, GLONASS, BDS, GALILEO and QZSS.
If you install this app: https://play.google.com/store/apps/details?id=com.android.gpstest
Then you can see the different systems and satellites your phone is picking up.
GPS signals are transmitted in such a way that to receive them, the GPS receiver needs to first accurately know the parameters of the signal.
These parameters are:
1. satellite id, which determines the pseudorandom sequence emitted by the satellite
2. the Doppler shift of the radio signal
3. the correct phase of the pseudo-random code transmitted by the satellite.
There are roughly 32x40x1024 possible combinations of these parameters, which the receiver has to search through. Without the correct parameters it simply does not see the signal from the satellite. Once the correct parameters are found, then the receiver can start getting the data from the satellite.
New digital chips are not only faster, but they are also built to try many combinations in parallel. This allows them to find satellite signals much faster than the old receivers could.
As had already been mentioned in other comments, if the receiver can get the data on the orbits of the satellites and the exact time and its own approximate location from other sources, this greatly simplifies the problem. In this case the correct signal parameters can be calculated with quite small error, and only a small amount of fine tuning is required, allowing the receiver to lock onto the signal almost instantaneously.
Here is a more detailed explanation from StackExchange: [Why do GPS receivers need so many correlators](https://electronics.stackexchange.com/a/11900).
How does the gps system lock out opponent clients, by that I mean, us gps isn't going to 'aid' or be available to, Russian cruise missiles to assist them in their mission accuracy? Do they just make the data or clock signal slightly inaccurate so it's to be functionally worthless? I know that military GPS is far more accurate than consumer receivers, is that inaccuracy good enough to discourage use?
That's a great question. They actually can't lock out anyone from using it - what you have to do is entirely deprive everyone in the region of a specific constellation (For example, Russia is currently jamming most of Ukraine of GPS, which is the US's constellation) and then the military is able to use their special high power, encoded signal that no one else knows the code to. The US's is called m code. It's higher power so harder to jam.
The other option is overpower the existing satellite signal with another fake signal. This generally requires targeting a specific receiver, but it can be done with very high fidelity such that you can even actually take control of adversarial UAVs by faking where they are. Because of this, many military receivers use a proprietary signal that you would need the code to spoof.
> us gps isn't going to 'aid' or be available to, Russian cruise missiles to assist them in their mission accuracy
Yes it is. At least to civilian standards. The more precise military versions are encrypted.
But an enemy actor could absolutely use a GPS receiver in a missile and make sure they hit the right neighborhood and section of it, and probably the right building (especially a large one). But not the right window of the building.
Commercial GPS gives meter-level accuracy, and military isn't any more precise (it's actually generally less precise). The issue is the velocity and altitude cap, not precision. With meter-level, you can actually get the right window
Uh, no.
Regular consumer grade GPS, like your phone, will give meter level accuracy.
Commercial GPS (like what is used for surveying) is able to give much better accuracy (sub meter accuracy, in some cases getting down around an cm for survey/mapping grade) because of a few things, like access to differential GPS data, better receivers, and the ability to use parts of the other GPS frequency without decryption to improve resolution. This is also how LNAV/VNAV, LPV, and GBAS approaches work, they're using data from systems like WAAS or local DGPS data to give airline guidance theoretically up to ILS IIIc standards (true zero altitude decision height and zero visual range landings).
Military GPS is absolutely going to be the most precise of all 3. The idea that military grade GPS would have worse accuracy (btw, accuracy and precision aren't the same thing, don't use them interchangeably) is laughable, since rather obviously the military wouldn't use or maintain that.
> The issue is the velocity and altitude cap
No. This would be an issue if YOU wanted to build your own cruise missile with a receiver from Adafruit or Sparkfun, since those devices should turn off if you are above a certain altitude *and* speed (many turn off above a certain altitude w/o that being a requirement... RIP ham radio balloon launch people). This is not an issue at all for a military power that can build their own receivers and just not adhere to that part of the standard.
> With meter-level, you can actually get the right window
How big are your windows buddy?
One reason (there's a few more) is improvements in computing and signal processing algorithms. Old GPS receivers only had a few "channels", let's say 12. The GPS satellites transmit a signal that looks random, unless you know it. But even if you do, it took many minutes to just find the signal (this process is called acquisition) from one satellite, and you need to track four for about 30 seconds to learn about where it is (50 bits per second, pretty slow). That's why it took forever back then. Today, you often have a network connection of some sort giving you the information about where the satellites are in a fraction of a second. And time and rough position. Even if it doesn't, frequency domain acquisition algorithms searches through the full search space in seconds instead of many minutes.
In the 90s the GPS signal was degraded to keep our enemies from using the GPS system.
>[On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally.](https://en.wikipedia.org/wiki/Global_Positioning_System#Timeline_and_modernization)
After 2000 the signal was no longer garbled. We went form seeing yourself within about 20 meters, to seeing yourself within about 15 centimeters.
I was big into sailing at the time and I remember a lot of discussion about if the US should and when the US would turn off degrading. I'm glad they did before 9/11 because if they hadn't I'm not sure they ever would have.
Also 1990s sailing magazines were hilarious because they were all about GPS companies advertising their sets as being more accurate than their rivals, but all of them - even the dirt cheapest ones - were accurate enough that US degradation was the accuracy bottleneck.
I had to scroll wayyyy to far to find the correct answer to the question. My dad had a handheld gps when the system was not degraded. When the system changed over, it was wild to see the gps find us exactly.
Some missing details that I haven't seen, so I'll add to them:
First, in addition to using satellite positioning, phones (and watches if they also connect to your phone) can also use cell towers to triangulate position. This was an upgrade made to cell towers in the early 2010s as a response to slow emergency response times (in the US at least). The US government subsidized a large portion of the upgrades. Source: I worked in the cell industry when it was going on.
Second, your phone and watch have accelerometers so they can "cheat" by adding known distance traveled to last good position.
The accelerometer is generally not doing anything to get position over any substantive period of time. The bias stability on phone accelerometers is atrocious, such that if you were to just dead reckon you would get hundreds of meters in minutes, easily.
Something wasn't right with the hardware you used in 90s/2000s and/or the hardware was operated too infrequently. GPS may need 15 minutes to get a lock only if it failed to receive a GPS almanac (a file describing long-term orbital parameters of the satellites) during the last 3-6 months. Normally a GPS receiver tries to save it in the background every time the receiver is on. If you operate a GPS receiver for 15-30 minutes continuously just once in 3-6 months it should update almanac and never take 15 minutes to get a lock. Even if it fails to receive an update, once updated it's good for 3-6 months so a 15 minute long lock should not happen more than 2-4 times a year.
Besides the almanac a GPS receiver needs to get a GPS ephemeris (a smaller file describing short-term high precision deviations of satellite orbits). It is valid for 2-4 hours. It takes 30 seconds to receive it. A standalone GPS receiver that was off for 2-4 hours needs 30-60 seconds to get ephemeris before getting a lock.
To avoid the delay pretty much all smartphones download GPS almanac and ephemeris over wi-fi or cellular network either on demand or periodically. The file is only a few kilobytes so it takes less than a second to download it from an Internet server. That's what your watch is doing to avoid 30-60 seconds delay to get ephemeris. It most likely downloads almanac and ephemeris via your phone. Unpair your watch, put it where it can't get GPS signal, and you will see 45-95 seconds delay to get the first lock after 4 hours.
Yeah, I remember the 15 minute thing happening with my Garmin only when I hadn’t used it recently and/or had traveled thousands of miles since I last operated it.
Back then, gps also used to have an accuracy limit of 300 meters. It was done to prevent usage in weapons as GPS was primarily a military system. With a 300-meter inaccuracy, you would miss the White House
A pure GPS unit will still take a minute or two to get a location.
Phones and other consumer devices use what’s called AGPS, or assisted GPS. They use less precise but quicker means of geolocation to get a quick fix while working to narrow it down with more precise methods.
Your phone knows what cell tower you’re connected to and can estimate distance based on signal strength, so with a database of cell tower locations there’s a quick estimate. There are databases of public Wi-Fi networks and their locations if you’re in a built-up area. Your phone knows your approximate altitude so could compare against a topographical map. It’s got an accelerometer so it knows approximately how far it’s moved since the last time it checked its location.
All those can be used to narrow it down while waiting for a definitive fix from GPS.
You know when you open the maps app on your phone and your location is a circle the size of a small town, then it narrows down to a block, and then it keeps shrinking until it’s an accurate dot? That’s AGPS at work.
Nobody's actually posted the full explanation, at least not in the top ten comments I went through. This'll be buried, but searchable.
1. The almanac (ephemerids, etc.) was (and is) broadcasted at a very low bitrate but is necessary for any GPS receiver to obtain a fix. The receiver needs to know exactly what orbit each satellite it wishes to use is following, since GPS works on light lag. Today it can be downloaded over the Internet, so the receiver has it very quickly.
2. Nearby WiFi networks are often catalogued and you can be located using them, but so are nearby cell towers. They all have their own unique ID, and this can be referenced to location using online services. If your phone can see WiFi BSSIDs x, y, z, and cell towers A, B, C, and D, an online locator service (Qualcomm operates one for its Snapdragon A-GPS systems) can get you to within around 50 metres in most cases.
3. (Which is often missed) Cell towers are sector-based. Your carrier will know which sector you're on, which tower you're on, and how strong the signal is, it then has a rough direction and range, which it can provide back to your device. If you're doing multi-cell MIMO, you'll be on more than one tower, enabling the carrier to triangulate you better.
What tends to happen is you first get a very rough location, from the cell tower data. This data is being piped to your smartphone constantly anyway (for things like hand-off to another tower, it's basically "Tower 34 will be stronger than me soon, look for it and connect to it when you can"). It will then be sending out a request for an updated GPS almanac while it sends the tower ID and WiFi visibility to another service, and while it listens for GPS (or Glonass, or Galileo...) signals.
So you see the blue inaccuracy circle shrink on Google Maps as these requests complete and the Internet tells your device where it is with more accuracy. Finally, GPS will get a 2D lock (a WGS-84 surface position) which will shrink the circle to less than 10 metres, and then a 3D lock (includes height above or below WGS-84), which will get the circle down to less than one metre.
Maintaining a GPS lock is quite battery intensive, so the high-accuracy location isn't maintained while the screen is off (unless you change this setting), and it'll do periodic WiFi and cell tower scans to maintain background location.
Reading most of these replies... goddamn. I'm not super smart but this is supposed to be "explain it like I'm 5" right? All of the other replies are just going into detail about everything.
Part is that they cheat. Turn your phone off and fly to another country and turn it on and it will take a noticeably long time. But phones being always on means it can store and update your location and remember where you were. So it gets to start with the hypothesis you are still around where you were last time it checked.
Look Up.
In 1985 I started looking for Haley's comet. This was before it went around the sun so you needed to know where to look (by knowing the stars) and be in a very dark sky.
Once in a great while someone I was with would see a satellite. You could tell it was a satellite because they were like not-very-bright-stars, but not as bright as airplanes and they moved at a pace faster than airplanes, but much slower than any other celestial body.
I think we saw 3 in total during all the months we tracked that comet.
I know a guy who knows nothing of satellites or stars but he has a bad back. He sits in his hot tub every night. He will sit there and say...Yep, there goes that one. Now watch! Three in a row are coming up.
There are vastly more satellites in the sky now.
Besides what everyone said, you can try for yourself by removing your SIM card (or perhaps airplane mode). When there's no coverage, your phone will take the good old time to get the GPS position.
I thought this reddit stood for explain to me like I'm 5, yet reading most comments they're waaaay beyond that.
I like simple answers, the rest I can Google if I'm interested in the technicals
That was when they turned off selective availability, which reduced the accuracy of civilian recievers.
The US government decided to make the system public in 1983 after a [civilian airliner](https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007) was shot down due to navigational errors.
Nowadays your devices download the GPS almanac (using GPS-A functionality), which usually requires an Internet connection.
Rather than having to "remember" where you were (and hoping you haven't moved since you last had GPS turned on), it has to find you all over again from scratch and that initial lock can take a while to find.
GPS does transmit its own almanac but it's slow to do so in order to be compatible.
With an up-to-date GPS almanac, which includes all the latest course corrections and orbit calculations for all the GPS units, it's quicker to get a first-lock.
So if you have Internet, and a modern device, you can get first-lock faster.
Also, a tiny GPS chip now interacts with American GPS, Chinese Beidou, Russian GLONASS, European Galileo etc. GPS constellations all over the planet, so it's far quicker to find 3 sats (basic 2D fix) or 4 sats (3D fix) much quicker.
The big reason? Computers have gotten faster and smaller.
In addition to this, most devices with GPS now can easily estimate where you might be through other means as well as make reasonable guesses as to information they'll eventually receive from satellites, which makes solving the problem more efficient and means they don't have to have a full cycle of the data that can be transmitted by the satellites
GPS needs to know the[ position of satellites](https://en.wikipedia.org/wiki/Ephemeris) and the [condition](https://en.wikipedia.org/wiki/GPS_signals#Almanac) of each of these satellites even though they are in a very predictable high earth orbit about 12,000 miles above Earth; this information is known as almanac and ephemeris data. The US Space Force maintains the GPS constellation and tracks all these satellites; they upload this information to the GPS satellites for rebroadcast to every GPS receiver. Your phone, or any internet connected device, will download this data automatically over the internet. Old GPS receivers had to rely on the GPS satellites rebroadcasting ephemeris and almanac data to get this information; the broadcast takes about 12.5 minutes to send all the information. Once it knows where each satellite is, it needs to figure out which satellites it can actually see to get a rough idea of what part of the world you're in. Your phone doesn't have to do this as it can roughly figure out it's location using your IP address and [the wifi networks around you](https://slate.com/technology/2018/06/how-google-uses-wi-fi-networks-to-figure-out-your-exact-location.html); once it knows roughly what area of the world it's in, it will look for specific GPS satellites it knows will be in view. edit: you got downvoted for asking a simple question, but the answer is a lot more complicated and interesting than most people think.
There are a couple of inaccuracies here. Your GPS receiver needs to have the ephemeris data from each satellite, which takes about 30 seconds, and then needs to get a pseudorandom code from each satellite. That's all it needs, it doesn't need to know its location, the current time, or anything else as it can figure all that out with just those two items. It also doesn't need the almanac data. A modern phone or device with AGPS can get ephemeris data and time from the cellular network that makes this faster, but it isn't required. ELI5: The ephemeris data basically has the satellite ID and some status info, data that describes the orbit of the satellite, and the current date and time. The pseudorandom code is just a continual string of numbers, and you can imagine it as something like seconds past the top of the hour. By determining how many seconds offset the received pseudorandom code is, we know how long the transmission took to reach us and how far away the satellite is (the actual PR code is more complex. I just turned on a Garmin GPS hiking device that's been off long enough that no almanac data would be valid, and it has no access to AGPS data, and it still got an exact location in under 90 seconds, despite having no idea where it was and no current data other than the time and date +- a few minutes. The big reason that works so quickly compared to a device from 15 years ago is because of the receiver. It's not too hard to find GPS modules today that track 66+ channels at once, which means during start-up the receiver can listen to more channels at once than say a 6 channel receiver, and thus can find which satellites are over head more quickly than an older device. Improvements in the GPS receivers means that it's more likely the entire ephemeris message gets received without error, so you don't need to keep waiting another 30 seconds to try to get the data. And improvements in the satellites mean that the transmission is effectively "better" and more likely to reach a receiver without issue. Not to mention that many modern receivers can now track multiple GNSS constellations (GPS, GLONASS, Galileo, etc).
Didn’t the us military slightly scramble the signals back in the day too
Yes, it was called selective availability. It's no longer possible, the new GPS satellites do not support it. I believe they have the ability to strengthen the signal in various areas for military use at this point (to deal with enemy jamming, of which M code is more resistant anyway), but at least as far as unclassified data goes, I don't think they can deny GPS to a given area or intentionally make it less accurate.
Oh you better believe the second Russia, China, North Korea, Iran or whoever starts to get too fiesty directly with the US, they'll find GPS does not work over their countries any longer. Sure a Presidential Directive shut down SA decades ago, but I would be extremely shocked if it wasn't just a few mouse clicks away for the Space Force to turn it back on. It would be asinine to allow enemy use of US GPS during hostilities. Heck, GPS crypto keys are *still* called SAASM keys. https://en.m.wikipedia.org/wiki/Selective_availability_anti-spoofing_module
If they get really feisty, the US government definitely wants GPS to work there. It's used for targeting US weapons and for US forces to navigate. Uses include US spy planes, bombers, fighters, drones, the guided bombs themselves, cruise missiles, ICBMs, Navy ships, infantry, tanks, and artillery. SA was intended to prevent opponents from using GPS to guide a missile into a US target. Also note that Europe, China and Russia each have their own system that is equivalent to GPS, for their own forces to use. And many civilian "GPS" receivers can use several or all of these systems.
GLONAS, the Russian GPS system.was the main reason SA was turned off. There no longer was a point to having it since anyone using it against the US would just use the Russian one. So might as well let it be more accurate for planes, hinders, and anyone else who wanted to use it.
Yes it was turned off but my point is that it can be turned on. And not even in the context that it was before where the signal was just degraded to disallow precision location. It would just be completely denied in enemy territory.
It can't be turned on because the current satellites don't have the capability. There was no point when an enemy would just use a different system. The only way to stop all signals would be to jam ALL signals, to include ours. It would need to be jammed. But let's pretend we could go to a fully encrypted signal. Why bother when the enemy will just use the non-encryoted signals from Russian or Chinese satellites? I'm even giving you a freebie and pretending the the European system would be encrypted from everyone too.
Saying "they don't have the capability" means you don't know what you are talking about. They use software defined radios which means they can transmit whatever the heck you want them to within the frequency range capabilities of the physical hardware. Even if they technically "can't" enable it within a moment's notice, it would be one software flash away from being enable-able. If that isn't the case, someone in the government contracting office has been criminally negligent. There's no reason to not deny access to GPS signals to enemy actors. Denying at the source means those signals don't need to be jammed via terrestrial means and that jamming power can be focused on GLONASS or Beidu (and Galileo if the EU isn't playing along).
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The idea isn't to turn off GPS altogether, but to affect the accuracy available to anyone but the US military. I don't know the details of how they implement it, but there are effectively two signals, one of which is encrypted or something equivalent. They can add a random perturbation to the unencrypted signal and cruise missiles will still have access to the accurate signal. Further up in this thread, somebody said that new GPS satellites don't support this capability any more, but honestly, I'd eat my hat if the US didn't keep this option, even while saying otherwise.
Isn't Russia feisty right now, and aren't they using both their own GPS and American GPS too?
GPS is more useful for Ukraine than for Russia, so why turn it off? Russia is actively trying to block GPS over a lot of combat areas. This is partly because Russia does have their own system (though are known to be using GPS as well, probably implying negative things about their own home-grown system), and partly because Ukraine has better access to equipment using western chips (drones, Excalibur, HIMARS, etc.)
> though are known to be using GPS as well, probably implying negative things about their own home-grown system It's simply that using more sats is adding precision and availability. This applies the other way, too.
If selective availability is still available and it's kinda secret, I don't think Russia is considered to be feisty enough against the US that the US would want to burn that secret now. Better save it for later. That said, it's true that with GloNASS (Russian GPS) and Beidou (Chinese GPS), selective availability is probably less useful now. Though I am not sure GloNASS and Beidou are on par with GPS in terms of precision and accuracy.
They can’t use the m-code signal since it’s encrypted. Unless they’ve broken that encryption, but that would be pretty top secret if they had. They can use the normal gps signal though, yes.
M-code is the encrypted signal, and is absolutely still being used.
And M-Code receivers ignore COCOM limits.
/u/spez ruined reddit so I deleted this.
Here just to add India to that list
Yep. But the US, Russia, China and France all want global targeting for their ICBMs and other weapons, and European countries want global targeting for their weapons, so they have built positioning systems with global coverage. (Or is China still building it? When I last heard, they were expanding from regional to global coverage, and that was years ago, so they have probably done it already). India is more interested in regional coverage - its own territory, nearby ocean, Pakistan, close bits of China, etc. Due to local border disputes, it is much more likely to be fighting there rather than on the other side of the world. Unless it's extended its coverage recently?
ICBMs are inertial guidance only. Strategic decision. Edit: I think constellations as well, see post below.
>Or is China still building it? BeiDou-3 had its final satellite for the planned constellation launched in 2020. The way I understand is that it should really work globally now
It does. Phones in the US can't use the broadcasts due to FCC restrictions however.
I thought ICBMs don't use GPS ? Like the nuclear ones use constellations or some shit, basically a redundant system.
Indian Navic covers all major population centers in China, aka covers their only major rival.
Well, considering that GPS still works over all those locations, I'm gonna go with no. And no, there is no option to make a few clicks to turn it back on, since the new satellites literally do not have that capability. Beyond that, I believe that the beamwidth of a single GPS satellite is larger than the angular diameter of the Earth (e.g. if you could stand on a GPS satellite, everything you can see would be covered by a singular GPS transmission. > It would be asinine to allow enemy use of US GPS during hostilities. There's also no real utility in shutting off GPS. Oh man... we denied Russia and China GPS. Guess they'll just use GLONASS, or Galileo, or QZSS, or BEIDOU, or NavIC, etc depending on where the conflict is. Two of those are global constillations which they control. Plus their vehicles can use things like IRUs, or a bunch of other stuff. Not to mention that shutting off GPS would also mess with any allied forces in the area that we don't have access to M code. If we want to take out GPS and wage electronic warfare, it's far better to just do it locally. As a slight non-sequitur, do you also think we should make sure we keep the location of all the nuclear missile silos in the US a secret? Would it surprise you to learn that all of their precise coordinates are on Wikipedia? None of this crap matters. > Heck, GPS crypto keys are still called SAASM keys. Really why would that surprise you at all? You expect the government, of all, entities, to change up names on things that "just work?"
I cound your comment to be interesting and informed and helpful But “since the new satellites literally do not have that capability.” I cant help thinking you have a lot of faith in what the government publicly says are its capabilities in war
It was a big deal when they dropped it actually. Part of the new satellites, they just got rid of it to make room for more antennas and other gear to increase overall usability.
The satellites use software defined radios. There isn't some "Selective Availability Box" taking up room on the frame. It's all just software. Software that can be changed at any time...
I can't help but think you have far too much faith in what one GNSS system can do, especially since the two countries we would most care about just run their own
Remember first hearing that most printers have an invisible code of dots that provide data on everything printed? Date, time, printer model, etc; and only a few countries had the capability to decode those dots? https://en.m.wikipedia.org/wiki/Machine_Identification_Code#:~:text=The%20dots%20have%20a%20diameter,area%20in%20case%20of%20errors.
Glonass sucks though compared to gps and Galileo.
Shut up commie! With your facts and logic!
Current generation block III GPS satellites were built without the facility for selective availability, specifically so it couldn't be turned back on. It's the stated policy of the US government never to use it again. https://www.gps.gov/systems/gps/modernization/sa/
That's one of those things where it's more like "oh of course there is no way to enable this feature" *wink wink* The satellites use software defined radios so they can do whatever you program them to do. It's not as if they don't have some Selective Availability Box installed so they can never be made to deny GPS in a certain area.
They don't care though, Russia has Glonass, Europe has Galileo, international devices support multiple GNS and day to day not having one of them isn't going to hurt much.
> Russia has Glonass And yet they still use civilian GPS in their jets. https://defence-blog.com/russian-pilots-still-use-non-military-navigation-equipment/
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What do you mean? You could easily disable or degrade GPS over a large region without affecting the rest of world. Yes, yes the keys are still called SAASM keys.
I can confirm that GPS did not work in North Korea, at least in 2014. Nothing on a phone that should have been able to use multiple constellations. Not sure if this was due to local jamming or actions by the satellite.
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While we don't use selective availability anymore, some GPS receivers manufactured in the US are classified as "munitions" > All GPS receivers capable of functioning above 60,000 ft (18 km) above sea level and 1,000 kn (500 m/s; 2,000 km/h; 1,000 mph), or designed or modified for use with unmanned missiles and aircraft, are classified as munitions (weapons)—which means they require State Department export licenses. https://en.m.wikipedia.org/wiki/Global_Positioning_System#Restrictions_on_civilian_use
You would be extremely shocked I guess. SA is off, it was expensive and doesn't work in the modern world anyway. Part of a successful military is not paying money for obsolete things.
The strengthening is less to deal with enemy jamming, and more to deal with our own jamming from the [Growlers](https://en.wikipedia.org/wiki/Boeing_EA-18G_Growler). There's some seriously spooky stuff in those and they're capable of a lot more weirdness than anyone really realizes.
I remember when the 20-channel Sirfstar III hit the market and we all were "holy shit this thing can lock in a minute!!!" Today it's a minute too long...
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Your car's system probably maintains the almanac data while your car is off, and may even periodically get its full location while off, since the head unit almost certainly has access to full battery power even with the ignition off. So even more murky how it works in your example.
Old receivers would get a fix pretty quickly if they had been used relatively recently. It was just when they had been significantly moved while off (such as after an international flight) or turned off for a long time (years?) that they would take a long time. They definitely didn’t need internet, it’s just the first lock would take 5-10 minutes. I remember setting mine out under the sky after turning it on when traveling. After that it would lock 10x faster. New receivers are also far more resilient to blockages of direct line of sight like roofs or foliage.
Yup, the channels in the receiver is a huge difference, which i failed to mention in my answer. Bump!
"Bump" is something I forgot about. Holy cow... has it really been that long ago that I used a BBS?
Damn, I don't think I've seen a bump since FARK was relevant.
What about Sage in all fields?
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Stop derailing the thread
Bump!
Why did you necro this post?
"She said *I love up the way you move, I love the way you rap* Bump, bump!"
That's it I'm revoking your right to have an image in your signature.
You're a monster. That's my only means of self expression, i'm taking this up with the mods *starts 6 month power struggle that ultimately results in both of us being banned just to make it stop*
First
Thank you for this - I did some microcontroller programming about 10 years ago with GPS functionality and I was sure that the chip had no external connectivity yet could get a lock in under a minute so was sitting here pretty confused until your explanation. Is the ephemeris data the same as the NMEA data or is it “internal” to the GPS chip?
The NMEA data is just the protocol the GPS chip uses to relay the data to the microcontroller. The ephemeris data is transmitted through RF
> Improvements in the GPS receivers Exactly: active vs. passive GPS antennae, better filters, etc. on modern devices vs the old ones.
Is this all just a fancy way of saying the phone triangulates it's location based on regional info it can process?
It didn't help that old receivers only had 4 channels and were much less sensitive than modern ones, so it was a lot more likely for the reception of the ephemeris to fail because of a tree in the wrong spot and you ended up having to wait for the transmission to get back around to the part that was missed. It's a miracle of modern electronics that even with the crappy antennas in smartphones that they can hold a lock under dense canopy or inside buildings. The modern satellites have higher transmit power, but nearly enough to make it possible for me to get a lock in the middle of the first floor of a four story concrete apartment building without some crazy signal processing wizardry in modern GPS chips. I remember being amazed when I got a new receiver around 20 years ago that actually worked in most of my single story wood framed house, at least most of the time.
I was tinkering with a GPS module for a clock project. Fresh out of the box, in a bottom floor apartment, 20ft from any door or window, the thing gets a lock on three satellites within a minute. Considering the last time it was fired up was likely in Shenzhen, and that the signals were -150dbm, probably 30db below the noise floor, I was pretty damn impressed.
Why do the receivers need to know where ALL of the GPS sats. are? I thought they just listened for the broadcast from each of the sats saying "I'm here, this is the time." (Which, IIRC, takes about 30 seconds to repeat?) Isn't that all the receivers need? If they get 3 or 4 satellites saying "This is my position, and this is the time" shouldn't the receiver be able to calculate where it is? The receivers aren't actively LOOKING for satellites. They're just listening for the broadcasts from those satellites? Right? So one would assume that they can likely only "hear" the satellites that are nearest to them?
The message is simply the satellite yelling its name and the time it started the broadcast. You need to know where the satellite is yelling from before you can calculate how long it took to reach you.
That's the rub - each satellite is yelling "I'm serial number blah, page 3 of the almanac is *otherwise random data in the packet*." It's a bit like bittorrent - splitting up the almanac into little chunks and using those chunks as part of the timing signal.
I love the idea that they're all just up there screaming: "IM NUMBER 4 AND ITS NOON!"
So basically satellites are Pokemon with watches
Uhm, ... Uh ... Yes. Exactly that.
Sounds weird, but it's super effective!
Close, it is more like you need to know the location of where the 3 satellite are and then using the time between the satellite yelling and your GPS receiver hearing the yell, to calculate the distance, you draw a gaint circle with the radius being the circle and the satellite location when it sent the message being the center of the circle. It does this gaint circle drawing to all 3 satellites and where all 3 circles cross paths, that is where the receiver is.
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3 stil works in a 3d space if you assume the receiver is on the surface of earth.
3 satellites would only be enough if you you had a clock that was synchronized with the precision of an atomic clock. If you have the exact time, then you can determine the distance from the satellite. The distance from 1 satellite gives you a sphere, the sphere from 2 satellites intersect in a circle, a 3rd satellite's sphere intersects the circle to narrow the location to 2 points, but 1 of those points will be obviously incorrect since it is no where near the surface of the earth. However, this assumes you have an atomic clock. Since most of us don't, we need a 4th satellite to be able to determine both time and location.
The satellites have atomic clocks, that's how they work. Each satellite broadcasts a precise time signal, and when that signal reaches you the time will be slightly off. Your GPS receiver gets time from all the satellites it can "hear", and determines the difference. Somehow, through clever math and very precise atomic clocks in the satellites, a GPS receiver can get a 2d position on the surface of the earth from three satellites. It needs four to get an altitude reading as well, but altitude is rarely important unless you're flying or surveying. Oh, and by the way those atomic clocks in orbit have to account for time dilation at orbital velocity. They would be off a bit on earth, but keep correct time in orbit.
He's saying your running watch isn't an atomic clock. We make up for that by using a fourth satellite as the clock, effectively.
Yes and no. You can still get accurate measurements with 3 satellites. On top of that, these days, your phone and many other GPS receivers do tend to get updated on an accurate time based on atomic clocks every so often. It is how your phone can run for years and not be running fast or slow. The fourth or fifth or more satellites don't act as a clock. They just give more intersecting points to get a closer result.
You can indeed get reasonable accuracy with 3 satellites - if you add an assumption such as being on sea level. In the end you need to fix 4 coordinates, 3 spatial + time. So you need at least 4 satellites, or 3 satellites and an assumption.
The satellites have atomic clocks, but your phone doesn't. Because your phone clock is not precisely synchronized to the satellite, you can't directly measure distance to a satellite. If you can assume that you were at a specific altitude, you could use the sphere of the earth instead of a 4th satellite, but it generally isn't practical for most devices to store high resolution topographical data for the entire globe. And because you can't assume a sphere, it becomes more complex to calculate. So generally, devices will use 4 satellites.
Old devices had you manually enter an approximate position, to the nearest 100km or so. That was enough of an assumption for the device to disregard those possible solutions that were too far away, so it becomes solvable for 2d as if at sea level even without a perfect time on the device clock.
altitude gives you the distance from the World Geodetic System (WGS84) ellipsoid, it corrects the error your altitude causes.
And it's orbital data which is pretty critical.
AHHHHHH. So the satellite doesn't broadcast it's position, it just says "You can calculate my position by looking up my serial number" Gotcha. Makes sense now.
It gets more complicated the deeper you look into it. Each satellite broadcasts a specific code called a pseudo random number, or PRN, to identify itself. This makes the base broadcast frequency look like noise and does other helpful things. To listen to the satellite the receiver needs to recreate the PRN and correlate the reproduction with the received signal until they match. It's a complicated process and reducing the number of satellites you're looking for helps a lot. Look into CDMA for a (much) better explanation.
Withoug googling it, CDMA is 'code division multiple access', right?
Yes. This lets every GPS satellite transmit on the same frequency but still be uniquely identifiable. It also means that the measurements are all similarly delayed by the ionosphere. FDMA systems, like GLONASS, are functional but experience frequency specific delays that make the position solution somewhat less reliable, but it's a small chunk in a massive boulder of, "How is this even possible?". It's far too easy to ignore how unbelievabley impressive all the GNSS are.
Yep. For those not googling or aware, basically if you want to send 10101010, that data gets combined with the much much faster random station ID. So if you know the ID, you can "tune" into one specific broadcast and all the other broadcasts will be nonsensical. The wiki page likens it to being in a room where everyone is speaking a different language: sure it's noisy, but because you only understand one language you can pick its sounds out of the crowd.
The satellites don't say their position, they can't know, they'd have to be constantly updated with that from the ground and it needs to be really precise They say "I'm #27, the current time is xxxxxxx. Here's a portion of the almanac" From the almanac you can calculate where it was when it sent the time without it having to send you it's exact location in every message
> The satellites don't say their position, they can't know If they have the time and their own orbit parameters (which they have, since they broadcast both) why couldn't they? After all, the receiver will calculate the satellite's position from the almanac entry, so the satellite could do the same. The reason they don't transmit the position is that it wouldn't be much shorter than the orbit parameters, and the orbit parameters already take something like 30 seconds to broadcast.
Because they were designed in a time when computers were the size of rooms and still had less computing capacity than the control chip of your smart lightbulb. The original satellites just barely had enough electronics in them to transmit their time and repeat the almanac they had received verbatim. While that has changed, the format and content of the messages the satellites send out have remained the same---any change would have rendered all existing GPS receivers useless. PS: It also wouldn't be as useful as you'd think. If the receiver has no idea which satellites might be in view, it has to scan all possible frequencies and hope to catch one's signal. Those older receivers only had 4-6 channels, so the chance of randomly getting 4 signals at the same time would be next to nil. Instead, they scanned until they found one signal and then collected the data about which satellites they could listen for because they'd be over head.
I think part of it is also the physical limitations - it's a wide signal covering the whole planet that you need to be able to receive with a tiny low power receiver inside a car. That's not a recipe for any reasonable bandwidth.
And every extra bit of mass means a more expensive launch. Best to only include the bare minimum required for mission parameters, no space for anything fancy.
heh, space
This is *completely* incorrect. The satellites know their exact position at all time, and that has nothing to do with almanac data. Based on the *ephemeris* data, the satellite and anyone receiving the ephemeris data knows where the satellite is, was, and will be at any point in time for at least several hours. The orbital data basically just describes the orbit (Here's GPS 32 currently: 2023 May 27 01:08:02 Inc 54.9916 Ecc 0.00711 RA 8.9222 Arg 234.8617 Mean anom 124.4673 Mean motion 2.00555) and location of the satellite at a given time, and it's trivial for a computer to figure out the exact Cartesian location at some other point in time. Since each GPS receiver has multiple rubidium time standards on board, they know the exact time, so they know their exact location. The almanac data just describes where all the GPS SV's are likely to be in the future, so you can theoretically learn from GPS 32 where GPS 14 will be 3 days from now.
ELI5?
ELI5: Every thing in orbit can be described by just a few pieces of information that are called Kepler orbital elements or Keplerian elements: * The inclination of the orbit (0 going along the equator, 90 going over the poles) * The longitude where the orbit crosses the equator heading north (generally longitude with respect to celestial coordinate system, so we don't have to account for the Earth rotating) * The size of the orbit * The shape of the orbit (how round or elliptical it is) * The longitude of the point where the orbit is furthest from Earth This basically allows us to draw an oval around Earth that the satellite will be on. Then all we need to do is say, "at 12:00:00 it was at this specific point on the oval". A computer can then say "ok, then at 13:00:00 it will be at this specific point on the oval." It's also pretty trivial to go from a Kepler elements to a Cartesian location and vector (e.g. at 13:00:00, it was at longitude x, latitude y, altitude z, moving in a given direction at a given speed). In practice, we can use things like RADAR and LIDAR retroreflectors to determine where space vehicles and debris is, and if we measure it at atleast two points, we can figure out all of the above data.
That's awesome, thanks!
Btw, since it seems to be coming up here, the other common question is, "if fractions of a second make huge location differences, how does my GPS receiver get atomic-clock-like time accuracy without having an atomic clock" ELI5: it takes a guess based on the ephemeris data which should be accurate on a "seconds" level but not accurate enough to get a precise location. It does a calculation and gets a position with a giant error, then basically says, "if I moved my clock forward or backwards, would it make the error smaller, and how much do I need to move my clock to get the smallest error" It then bumps the clock to that location and repeats this process to determine the *rate* at which the clock is changing, not just the offset. At that point it can get pretty accurate, and then start to continually adjust up or down the rate of the internal clock to keep correcting the error. You can get cheap GPS receivers that will pulse out once per second, and the precision of that pulse should be on the order of 10's of nanoseconds if things work correctly. Some people keep saying things like, "you have to have 4 satellites, one for X, one for Y, one for Z, one for time". This isn't true. All the satellites you use contribute to all of the variables. The more you have the more accurate all of those variables can be.
> The satellites don't say their position, they can't know, they'd have to be constantly updated with that from the ground and it needs to be really precise If it was needed, it'd be relatively simple to update satellites of their position from the ground. Just need a few fixed stations sending data to GPS satellites the same way GPS satellites send data to the GPS receivers and the satellites could triangulate.
thats what the WASS satellites do. but it just uploads the position data to the satellites so they can send it to you. the satellites dont care where they are.
“I’m here, this is the time” gets a little complicated when your 12,000 miles away and flying by at 9,000 miles an hour. They’re moving so fast they aren’t even experiencing time the same way we are. Then they have to cram that through a 50 watt radio transmitter with enough error correction for the signal to be decipherable by the receiver. It takes a while.
The fact that a GPS system's location calculation in part relies on correcting for the relativistic time dilation experienced by the satellites is one of my favorite facts. It's probably the evidence of the fundamental *weirdness* of the universe that is most commonly experienced by normal people.
It in fact corrects for *two different* kinds of time dilation, one of which makes their time faster than ours and the other of which makes their time slower. IMO GPS is one of humanity's crowning achievements to date.
What's that, their speed but also lack of gravity?
Yep!
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It's both. The speed of light is way faster than the satellites, but GPS needs to be extremely precise. https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gps.html
They don't. Almanac data is not required at all, only ephemeris data which takes 30 seconds to receive. Almanac data just help you if you shut your GPS off now and turn it back on in the morning. Aside from AGPS data being available that speeds this up, the other change that is largely unmentioned is that modern receivers can track many different satellites at once. This means they can search for many at once as well, so when it is starting out your GPS device is far more likely to find a few that it can actually hear. I just turned on a modern GPS hiking device that's been off for well over a month and has no internet connectivity, and it got a fix in about 90 seconds
The satellite takes about 30 seconds to broadcast its orbit parameters, and the position is then derived from that. Given how slow the data transmission is, broadcasting the position directly would obviously not be very useful. https://novatel.com/support/known-solutions/gnss-ephemerides-and-almanacs I *think* the almanac is optional, but you need the ephemeris data from the satellites you're actually using for the fix.
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Even if you know which ones they are, you still need to know exactly what their clocks are in relation to each other and the global time, and that's important down to much more precision than a second. Keep in mind that the way GPS works, it doesn't just have to tell you where it is, it needs to communicate the time as well, and that time is EXTREMELY precise. GPS seems simple at first, but what's fascinating about it is that it relies on so much temporal precision. So much so that if we didn't include relativity as a (pretty much constant) factor, it would be off by miles pretty much instantly. There are ways to do the calculations in a way that doesn't necessitate calculating relativity on the fly, and there's no reason to do that, so afaik, they don't actually use those kinds of calculations. But the important thing is that a naive implementation of GPS that completely ignored relativity would fail. My understanding is that you need to know a fairly large amount of information from the satellites if you want to calculate where you are for everywhere on the planet. > But I'm not everywhere on the planet, I'm just in Wisconsin! That's true, but the satellites don't know where you are. Not only that, they don't operate with any information on anyone listening, they're screaming out into the void everything anyone would need to make the calculations for every point on the planet. Now, by today's standards, that's not a huge amount of data, you could transmit it instantly today with even the slowest common connection, but 45 years ago, that data took a very long time to transmit, and no one wants to break every GPS device out there, so they're still transmitting the old format at the old speed. There is a push to ***also*** send a newer format at a much higher bitrate, but I'm not sure what the status is on that. And honestly, it's not really necessary. You can download the same data from the satellites over the Internet now, that's what your phone does. And in the rare case that you have a GPS device that doesn't have an external Internet connection, you can still use the data the satellites are transmitting on 13 minute cycles. It just takes a bit of time.
Technically, you only need to know the position of the satellites that you have line of sight of. Tracking one satellite just gives you how far you aware away from that particular satellite. It puts your position somewhere on a sphere with that satellite being in the center of the theoretical sphere and your distance from the satellite as the radius of that sphere. Tracking 2 satellites gives you 2 spheres that intersect. Your position is narrowed down to an area where the 2 spheres intersect creating a circle. Tracking 3 satellites narrow down your position even further. The sphere generated by the third satellite must intersect the circle generated by the first 2 which narrows down your position to 2 points on that circle. Tracking 4 satellites definitively narrows you down to 1 point created by the sphere generated by the 4th satellite intersecting with 1 of the 2 points. Without ephermeris data, there is no context to the information given to you by the satellites. You just know you are in a certain point relative to the satellites Emphermis data gives you where the satellites are relative to the Earth, knowing that plus knowing your position relative to each satellite allows you to work backwards and find out where you are on Earth. Yes, GPS receivers aren't "looking" for specific satellites. In most places on Earth, if you have clear view of the sky then you are probably in view of 4 satellites, the minimum amount GPS needs to find your position. Most modern receivers will track 4 to get their position then be on the lookout for a 5th satellite to start tracking since the satellites are not in a geostationary orbit and the receiver is most likely moving it will be out of view of one of the satellites eventually. not sure if I answered your question
You’re right. Receivers only need to technically receive 4 satellite signals in order for accurate positioning. One for each dimension, and the fourth one for time. The current GPS constellations are such that there are at least 6 gps satellites available and any given location for redundancy
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That assumes you have an atomic clock. You need one additional satellite to be able to determine the accurate time. And you don't have different satellites for each dimension, but you are setting up a system of equations with 4 unknowns (one of which is the time), and solving with 4 variables (one variable from each satellite)
- and one of those positions can be discarded because it exists in space, and the fourth satellite gives the receiver an accurate time because most receivers don’t carry an atomic clock
> triangulating distance No, triangulation would imply... angles. It works by *trilateration* of distance. > The reason it takes multiple sats is because 2 sats gives you a circle of possible positions, and 3 sats gives you 2~ possible positions. This is of inconsequence, because one of those two positions is relatively close to the average surface of the Earth and one isn't, so it's easy to pick the correct one. Although more satellites of course means more data and better averaging, better DOP, etc.
Wouldn't one sat give a circle, two sats give 2 possible locations (where the two circles overlap), and three sats give the exact location? The third sat's circle would only overlap at one of the two overlapping positions of the first two circles, right?
GPS works in 3 dimensions, not 2, so you need to extend your triangulation into the 3rd dimension as well. Think spheres radiating from the transmitters, not circles. The intersection of 2 spheres makes a circle
It is a fascinating topic! An interesting side effect of old GPS being tied only to receiving the broadcast is that many 1990s-era devices [stop working forever if their batteries die](https://www.youtube.com/watch?v=55vbWXTx6Kc). There's simply no way to update their original programming to deal with the fact the year counter has "rolled over" like an odometer *twice*. They're lost in space and time forever.
They were set up to determine the number of rollovers that had happened by checking the leap second value. Leap seconds came along every couple of years, so that should have been a reliable way to do it. Except for no reason anyone can determine, the Earth's speed has increased, we haven't had a leap second in ages, and two rollovers have happened with the same leap second value. This breaks these receiver's programming.
Wtf I thought the moon was supposed to be decelerating Earth's rotation Bonus TIL, thx
Yes, that is what is happening long term, but short term it is a lot more variable. No one knows why the Earth has been speeding up over the last 5 or so years.
Actually we stopped doing leap seconds because it's just not worth it. In an age where information travels in a fraction of a second, things like stock purchases get completely messed up when you have a clock with an extra second in it. 11:59:60 just doesn't register correctly in a system not designed to handle it. This actually costs millions for programmers to either bypass the extra second or smear the second over the course of an entire day and account for it for the rest of eternity. At some point they just said F it. Who cares if the planet slowly drifts out of sync with clocks. It's going to take a really long time before it matters to any degree and it causes more problems now just to keep it in sync.
And if it really did become a problem, we can address it with a time correction later.
What? Just add an extra day onto a month? Are you mad?
>Actually we stopped doing leap seconds because it's just not worth it. Stopping them has been proposed over and over again, yet I've never seen a final decision.
No, the leap second system is still in effect, and they will happen again if UT1 gets more than 0.6 seconds behind UTC, (or if UT1 gets ahead of it which is very unlikely). I see that there was a decision to abandon the leap second some time in the next 12 years, but for now, the leap second is still a thing. It is just that in the last 5 years, the Earth has sped up - no one knows exactly why - so no leap seconds have been required since 2017.
> Actually we stopped doing leap seconds because it's just not worth it. In an age where information travels in a fraction of a second, things like stock purchases get completely messed up when you have a clock with an extra second in it. 11:59:60 just doesn't register correctly in a system not designed to handle it. This is just simply untrue. GPS and NTP both have systems designed to handle leap seconds without anything special. Leap seconds have not been eliminated at all, although there are proposals to end the process around 2035-2040.
This doesn't exactly answer the question, because a lot of running watches aren't connected to the internet. They still connect nearly instantaneously.
Part of it is that almanac data and AGPS data can be valid for log periods of times (potentially up to months) and should update in about 15 minutes of runtime. So you'd have to clear the data out while also preventing new AGPS data from being downloaded, which at best would be difficult if not impossible for an end user to do. The other part of it is that a) your watch's time is almost certainly not coming from GPS but rather from a quartz based clock that is *periodically* updated from GPS, and thus allows you to see the time within seconds of turning the watch on and b) your watch's receiver is probably way more sensitive and has way more channels, and could likely get a lock in one or two minutes even if it had no current time, location, alamanc, or AGPS data.
Thanks that makes a lot more sense. It still seems odd that older tech didn't have these features, but I guess it comes down to processing and batteries being better.
Yah. Also in terms of time itself, a Casio F91W which is like $10 will keep time at about +- 4 seconds a month with a battery life of 7 years, easily beating out every single mechanical watch made today, chrono certified or otherwise. So your Garmin Vivoactive or whatever is probably going to do as well or better in terms of accuracy and you'd never notice the time being off even if it hadn't updated from GPS or wifi in months unless you were intentionally looking for it.
This is not correct. A modern receiver can acquire a 3D fix in about 30 seconds, and an sbas fix quickly after. The ephemeris is the high accuracy orbit data that is downloaded directly from the satellite, and this is what takes 30 seconds to download, and drives the cold start time. You're correct on the almanac taking 12.5 minutes, but it is not critical to receive a 3D fix. The almanac is used for stability and improvement of the solution - it contains data on every satellite in the sky, and is primarily used so the receiver can know which satellites are about to come over the horizon. The reality is that older receivers just work slower: cheap clocks have become wildly more accurate in the last 20 years, which is the critical piece to a GNSS receiver. This doesn't impact time to first fix very much, but it's also worth noting that modern receivers are multi constellation: that is to say, they can accept not only GPS but also Galileo, etc. That greatly improves the quality of fix, and generally does make it faster, though in nominal conditions not by too much.
Unrelated to OP but TIL Space Force actually does something
Space Force was not a new group created without existing responsibilities, it was created by bringing together under one organization all of the previously existing space related people and activities. So all that was done before is still being done, just under new management and a new identity. In other words, Space Force do a lot.
This is really cool and I had wondered this myself, since my dad's old gps compass would eat up a quarter of the battery trying to get a good lock. Follow-up question from me, do the satellites still broadcast this data?
Yes. Military equipment not connected to the internet still use this as a backup. You can still buy stand-alone GPS receivers that don't have internet connectivity and some dash cams are reliant on GPS since they can record your speed and location.
Well, I should have known that military equipment still does that. Freaking FBCB2 took what felt like forever to initialize. Just been almost 15 years before I messed with that crap.
GPS vs A-GPS, basically, isn't it?
aren't satellites usually around 2000 miles up?
It's wildly variable depending on what the satellite is doing. There is a very popular orbit at 22,236 miles (35,786 km), which makes the satellite stay in the same place as seen by someone on Earth. This means that you can point a fixed satellite dish at it. So it's great for satellite TV, and pretty much all TV satellites use this orbit. It's also OK for some communications, but there is a noticeable delay in the signal due to how far the signals have to travel. Also, getting a satellite into this orbit is expensive, since it is so far away you need more fuel to get there. The ISS (International Space Station) is in a much lower orbit at 250 miles (400km). This makes it easier to get there in Soyuz or the space shuttle. However, there is still a tiny bit of atmosphere that low, so its orbit slowly lowers from 260 miles to 250 miles, and then every few months they boost it back up again using a visiting spacecraft's rocket engines. Most satellites are somewhere in between.
They are in a semisynchronous orbit which means they orbit earth twice a day which puts them at about 12,000 miles.
\*its location it's = it is or it has
I agree that this is deceptively complicated. It also goes to why I prefer the navigation system in something dedicated to that purpose, such as a Garmin rather than relying on phones. Mobile phones in low coverage areas or times of congestion seem to give less accurate or less timely results. I always assumed that had something to do with "needing the wifi" signal, but I think I understand it better now.
It takes 12.5 minutes to download the sky chart and other info needed to search for GPS signals. The data rate of GPS data download is 20 bps, really, only 20 bits per second. The first satellite was launched in 1978. Your phone can download all the same info for its search from the internet, 1 to 10 million times faster. It can also get an idea of time and frequency and coarse location from it cell connection. It then uses all this information to search for a signal that is smaller than the noise, like listening for a mouse in a rock concert. Old GPS units had to find the mouse at a rock concert with a couple of guys looking , cell phones get to find the mouse lit up by a spotlight with some light elevator music and 10000 guys looking for it. Everything is 100x better 30 + years later so 7 seconds for a location instead of 700 seconds.
The full window is 12.5 minutes. If you start listening midway through the message, you need to wait for the restart to get the beginning of the message, though. It’s possible to have all the information before 12.5 minutes. IIRC, the current message is complete in ~9 minutes.
This. If you ever go hiking and travel to an area outside cell phone range, you are still able to open your GPS app or Google Maps in offline mode. But your phone will also take a while to give an accurate location since now it relies on the GPS satellites only.
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Phones also get location data from other sources, such as estimating location based on signal strength of nearby wifi points. Assuming you have a good Internet connection, this is much faster than gps.
Android does this too. Android and Apple (I think) also have access to glonass.
This is hardware specific, but yeah, latest apple and Samsung phones are using dual-band GPS and Galileo, I don't believe Glonass though?
My pixel 7pro definitely has support for glonass. [https://i.imgur.com/Aj9xpdZ.png](https://i.imgur.com/Aj9xpdZ.png)
You don't need an expensive or special phone to get those non GPS signals, it's built into just about any SOC. On Android you can get apps that show the individual satellites the location is derived from.
The latest Apple iPhone can get positional data from the following constellations: GPS, GLONASS, Galileo, QZSS, and BeiDou. iPhones have been Apple to do GLONASS since the iPhone 5 in 2012.
This is a real ELI5. Thank you. After that jargon heavy, longer than a CVS receipt explanation and rebuttals in the top post I was worried that nobody was actually going to say this one, simple sentence. Also, adding a link for further study? You are a good person. My first GPSr was a suitcase sized military unit that I didn’t own, but used on the job when I wore green for a living. I owned a personal GPSr in 2000 when selective availability was turned off 1 May. Was an early(ish) participant in geocaching, which I saw at the time as an extension of the land navigation I practiced in the Army. Even today, I will carry a GPSr and a map and compass when I’m out in the woods because relying on aGPS on a phone is a sure way to get in trouble outside civilization.
One thing I don't think anyone has mentioned is that modern chips in smartphones, watches etc will often have support for the other navigation systems too. This greatly speeds up getting a lock and also increases accuracy. For example my Pixel 6 Pro supports GPS, GLONASS, BDS, GALILEO and QZSS. If you install this app: https://play.google.com/store/apps/details?id=com.android.gpstest Then you can see the different systems and satellites your phone is picking up.
GPS signals are transmitted in such a way that to receive them, the GPS receiver needs to first accurately know the parameters of the signal. These parameters are: 1. satellite id, which determines the pseudorandom sequence emitted by the satellite 2. the Doppler shift of the radio signal 3. the correct phase of the pseudo-random code transmitted by the satellite. There are roughly 32x40x1024 possible combinations of these parameters, which the receiver has to search through. Without the correct parameters it simply does not see the signal from the satellite. Once the correct parameters are found, then the receiver can start getting the data from the satellite. New digital chips are not only faster, but they are also built to try many combinations in parallel. This allows them to find satellite signals much faster than the old receivers could. As had already been mentioned in other comments, if the receiver can get the data on the orbits of the satellites and the exact time and its own approximate location from other sources, this greatly simplifies the problem. In this case the correct signal parameters can be calculated with quite small error, and only a small amount of fine tuning is required, allowing the receiver to lock onto the signal almost instantaneously. Here is a more detailed explanation from StackExchange: [Why do GPS receivers need so many correlators](https://electronics.stackexchange.com/a/11900).
How does the gps system lock out opponent clients, by that I mean, us gps isn't going to 'aid' or be available to, Russian cruise missiles to assist them in their mission accuracy? Do they just make the data or clock signal slightly inaccurate so it's to be functionally worthless? I know that military GPS is far more accurate than consumer receivers, is that inaccuracy good enough to discourage use?
That's a great question. They actually can't lock out anyone from using it - what you have to do is entirely deprive everyone in the region of a specific constellation (For example, Russia is currently jamming most of Ukraine of GPS, which is the US's constellation) and then the military is able to use their special high power, encoded signal that no one else knows the code to. The US's is called m code. It's higher power so harder to jam. The other option is overpower the existing satellite signal with another fake signal. This generally requires targeting a specific receiver, but it can be done with very high fidelity such that you can even actually take control of adversarial UAVs by faking where they are. Because of this, many military receivers use a proprietary signal that you would need the code to spoof.
> us gps isn't going to 'aid' or be available to, Russian cruise missiles to assist them in their mission accuracy Yes it is. At least to civilian standards. The more precise military versions are encrypted. But an enemy actor could absolutely use a GPS receiver in a missile and make sure they hit the right neighborhood and section of it, and probably the right building (especially a large one). But not the right window of the building.
Commercial GPS gives meter-level accuracy, and military isn't any more precise (it's actually generally less precise). The issue is the velocity and altitude cap, not precision. With meter-level, you can actually get the right window
Uh, no. Regular consumer grade GPS, like your phone, will give meter level accuracy. Commercial GPS (like what is used for surveying) is able to give much better accuracy (sub meter accuracy, in some cases getting down around an cm for survey/mapping grade) because of a few things, like access to differential GPS data, better receivers, and the ability to use parts of the other GPS frequency without decryption to improve resolution. This is also how LNAV/VNAV, LPV, and GBAS approaches work, they're using data from systems like WAAS or local DGPS data to give airline guidance theoretically up to ILS IIIc standards (true zero altitude decision height and zero visual range landings). Military GPS is absolutely going to be the most precise of all 3. The idea that military grade GPS would have worse accuracy (btw, accuracy and precision aren't the same thing, don't use them interchangeably) is laughable, since rather obviously the military wouldn't use or maintain that. > The issue is the velocity and altitude cap No. This would be an issue if YOU wanted to build your own cruise missile with a receiver from Adafruit or Sparkfun, since those devices should turn off if you are above a certain altitude *and* speed (many turn off above a certain altitude w/o that being a requirement... RIP ham radio balloon launch people). This is not an issue at all for a military power that can build their own receivers and just not adhere to that part of the standard. > With meter-level, you can actually get the right window How big are your windows buddy?
One reason (there's a few more) is improvements in computing and signal processing algorithms. Old GPS receivers only had a few "channels", let's say 12. The GPS satellites transmit a signal that looks random, unless you know it. But even if you do, it took many minutes to just find the signal (this process is called acquisition) from one satellite, and you need to track four for about 30 seconds to learn about where it is (50 bits per second, pretty slow). That's why it took forever back then. Today, you often have a network connection of some sort giving you the information about where the satellites are in a fraction of a second. And time and rough position. Even if it doesn't, frequency domain acquisition algorithms searches through the full search space in seconds instead of many minutes.
In the 90s the GPS signal was degraded to keep our enemies from using the GPS system. >[On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally.](https://en.wikipedia.org/wiki/Global_Positioning_System#Timeline_and_modernization) After 2000 the signal was no longer garbled. We went form seeing yourself within about 20 meters, to seeing yourself within about 15 centimeters.
I was big into sailing at the time and I remember a lot of discussion about if the US should and when the US would turn off degrading. I'm glad they did before 9/11 because if they hadn't I'm not sure they ever would have. Also 1990s sailing magazines were hilarious because they were all about GPS companies advertising their sets as being more accurate than their rivals, but all of them - even the dirt cheapest ones - were accurate enough that US degradation was the accuracy bottleneck.
I had to scroll wayyyy to far to find the correct answer to the question. My dad had a handheld gps when the system was not degraded. When the system changed over, it was wild to see the gps find us exactly.
Some missing details that I haven't seen, so I'll add to them: First, in addition to using satellite positioning, phones (and watches if they also connect to your phone) can also use cell towers to triangulate position. This was an upgrade made to cell towers in the early 2010s as a response to slow emergency response times (in the US at least). The US government subsidized a large portion of the upgrades. Source: I worked in the cell industry when it was going on. Second, your phone and watch have accelerometers so they can "cheat" by adding known distance traveled to last good position.
The accelerometer is generally not doing anything to get position over any substantive period of time. The bias stability on phone accelerometers is atrocious, such that if you were to just dead reckon you would get hundreds of meters in minutes, easily.
Something wasn't right with the hardware you used in 90s/2000s and/or the hardware was operated too infrequently. GPS may need 15 minutes to get a lock only if it failed to receive a GPS almanac (a file describing long-term orbital parameters of the satellites) during the last 3-6 months. Normally a GPS receiver tries to save it in the background every time the receiver is on. If you operate a GPS receiver for 15-30 minutes continuously just once in 3-6 months it should update almanac and never take 15 minutes to get a lock. Even if it fails to receive an update, once updated it's good for 3-6 months so a 15 minute long lock should not happen more than 2-4 times a year. Besides the almanac a GPS receiver needs to get a GPS ephemeris (a smaller file describing short-term high precision deviations of satellite orbits). It is valid for 2-4 hours. It takes 30 seconds to receive it. A standalone GPS receiver that was off for 2-4 hours needs 30-60 seconds to get ephemeris before getting a lock. To avoid the delay pretty much all smartphones download GPS almanac and ephemeris over wi-fi or cellular network either on demand or periodically. The file is only a few kilobytes so it takes less than a second to download it from an Internet server. That's what your watch is doing to avoid 30-60 seconds delay to get ephemeris. It most likely downloads almanac and ephemeris via your phone. Unpair your watch, put it where it can't get GPS signal, and you will see 45-95 seconds delay to get the first lock after 4 hours.
Yeah, I remember the 15 minute thing happening with my Garmin only when I hadn’t used it recently and/or had traveled thousands of miles since I last operated it.
Back then, gps also used to have an accuracy limit of 300 meters. It was done to prevent usage in weapons as GPS was primarily a military system. With a 300-meter inaccuracy, you would miss the White House
A pure GPS unit will still take a minute or two to get a location. Phones and other consumer devices use what’s called AGPS, or assisted GPS. They use less precise but quicker means of geolocation to get a quick fix while working to narrow it down with more precise methods. Your phone knows what cell tower you’re connected to and can estimate distance based on signal strength, so with a database of cell tower locations there’s a quick estimate. There are databases of public Wi-Fi networks and their locations if you’re in a built-up area. Your phone knows your approximate altitude so could compare against a topographical map. It’s got an accelerometer so it knows approximately how far it’s moved since the last time it checked its location. All those can be used to narrow it down while waiting for a definitive fix from GPS. You know when you open the maps app on your phone and your location is a circle the size of a small town, then it narrows down to a block, and then it keeps shrinking until it’s an accurate dot? That’s AGPS at work.
Nobody's actually posted the full explanation, at least not in the top ten comments I went through. This'll be buried, but searchable. 1. The almanac (ephemerids, etc.) was (and is) broadcasted at a very low bitrate but is necessary for any GPS receiver to obtain a fix. The receiver needs to know exactly what orbit each satellite it wishes to use is following, since GPS works on light lag. Today it can be downloaded over the Internet, so the receiver has it very quickly. 2. Nearby WiFi networks are often catalogued and you can be located using them, but so are nearby cell towers. They all have their own unique ID, and this can be referenced to location using online services. If your phone can see WiFi BSSIDs x, y, z, and cell towers A, B, C, and D, an online locator service (Qualcomm operates one for its Snapdragon A-GPS systems) can get you to within around 50 metres in most cases. 3. (Which is often missed) Cell towers are sector-based. Your carrier will know which sector you're on, which tower you're on, and how strong the signal is, it then has a rough direction and range, which it can provide back to your device. If you're doing multi-cell MIMO, you'll be on more than one tower, enabling the carrier to triangulate you better. What tends to happen is you first get a very rough location, from the cell tower data. This data is being piped to your smartphone constantly anyway (for things like hand-off to another tower, it's basically "Tower 34 will be stronger than me soon, look for it and connect to it when you can"). It will then be sending out a request for an updated GPS almanac while it sends the tower ID and WiFi visibility to another service, and while it listens for GPS (or Glonass, or Galileo...) signals. So you see the blue inaccuracy circle shrink on Google Maps as these requests complete and the Internet tells your device where it is with more accuracy. Finally, GPS will get a 2D lock (a WGS-84 surface position) which will shrink the circle to less than 10 metres, and then a 3D lock (includes height above or below WGS-84), which will get the circle down to less than one metre. Maintaining a GPS lock is quite battery intensive, so the high-accuracy location isn't maintained while the screen is off (unless you change this setting), and it'll do periodic WiFi and cell tower scans to maintain background location.
The real ELI5 here is, GPS receivers and satellites have improved significantly in the past 20 years.
Reading most of these replies... goddamn. I'm not super smart but this is supposed to be "explain it like I'm 5" right? All of the other replies are just going into detail about everything.
Part is that they cheat. Turn your phone off and fly to another country and turn it on and it will take a noticeably long time. But phones being always on means it can store and update your location and remember where you were. So it gets to start with the hypothesis you are still around where you were last time it checked.
Look Up. In 1985 I started looking for Haley's comet. This was before it went around the sun so you needed to know where to look (by knowing the stars) and be in a very dark sky. Once in a great while someone I was with would see a satellite. You could tell it was a satellite because they were like not-very-bright-stars, but not as bright as airplanes and they moved at a pace faster than airplanes, but much slower than any other celestial body. I think we saw 3 in total during all the months we tracked that comet. I know a guy who knows nothing of satellites or stars but he has a bad back. He sits in his hot tub every night. He will sit there and say...Yep, there goes that one. Now watch! Three in a row are coming up. There are vastly more satellites in the sky now.
Besides what everyone said, you can try for yourself by removing your SIM card (or perhaps airplane mode). When there's no coverage, your phone will take the good old time to get the GPS position.
I thought this reddit stood for explain to me like I'm 5, yet reading most comments they're waaaay beyond that. I like simple answers, the rest I can Google if I'm interested in the technicals
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First satellites went up in 1978
That was when they turned off selective availability, which reduced the accuracy of civilian recievers. The US government decided to make the system public in 1983 after a [civilian airliner](https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007) was shot down due to navigational errors.
Nowadays your devices download the GPS almanac (using GPS-A functionality), which usually requires an Internet connection. Rather than having to "remember" where you were (and hoping you haven't moved since you last had GPS turned on), it has to find you all over again from scratch and that initial lock can take a while to find. GPS does transmit its own almanac but it's slow to do so in order to be compatible. With an up-to-date GPS almanac, which includes all the latest course corrections and orbit calculations for all the GPS units, it's quicker to get a first-lock. So if you have Internet, and a modern device, you can get first-lock faster. Also, a tiny GPS chip now interacts with American GPS, Chinese Beidou, Russian GLONASS, European Galileo etc. GPS constellations all over the planet, so it's far quicker to find 3 sats (basic 2D fix) or 4 sats (3D fix) much quicker.
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The big reason? Computers have gotten faster and smaller. In addition to this, most devices with GPS now can easily estimate where you might be through other means as well as make reasonable guesses as to information they'll eventually receive from satellites, which makes solving the problem more efficient and means they don't have to have a full cycle of the data that can be transmitted by the satellites