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Nowadays almost everyone who have owned a smartphone or some type of GPS device somewhere. These devices also seemly update in real time. How is the GPS satellite capable of responding to potentially millions of requests from millions of different devices, and update all the millions of devices in real time without lag.

As I understand it, websites which get traffic even in the thousands gets slowed if it's not properly prepared for it, how does the GPS cope with amounts of traffic that is seemly impossible to deal with, even difficult for a super computer.

user10037
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    -1. Please read: a text on GPS such as "Global Positioning System" by P.Enge, or do a search on Altavista "how GPS works" – Nick Alexeev Feb 10 '12 at 03:17
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    @Nick - Altavista? That's so 1990s! – stevenvh Feb 10 '12 at 12:16
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    @NickAlexeev You could argue that this is off-topic, but I don't think it is a bad question. The idea of stackexchange is to be the source of information, so when you "search on altavista" for "how a GPS works" you will be brought to this question, where ultimately SE will make money off of ad space. – Kellenjb Feb 10 '12 at 14:10
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    @Kellenjb I argue that one should do homework (i.e. preliminary research) before posting. The answer to original question is like on the 2nd page of any popular article or chapter on principles of operation of GPS. I didn't argue that this is off-topic. – Nick Alexeev Feb 10 '12 at 17:15
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    @NickAlexeev which paragraph of which article should I read (is there a link?). I would have not posted here if I could find the answer myself. This is a specific question on the GPS, why would I know to search for "how GPS work"? AFAIK I searched for this question and there's nothing that came up in the results. – user10037 Feb 10 '12 at 17:45

4 Answers4

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If someone stands on a hilltop over a large town and screams "the Mongols are coming!" then everybody knows what's up and they get out of town. The lookout doesn't have to say "Hey Timmy: The Mongols are coming! Hey John: The Mongols are coming! Hey Sarah..."

GPS is just a bunch of satellites in orbit screaming "I'm over here!" in radio frequency. A GPS receiver just tries to make out the different satellites screaming their positions and does the number crunching for "If satellite 1 is over there, and satellite 2 is over THERE, and satellite 3 is just about in THAT spot... then I must be around HERE someplace".

Technically, the receiver is listening for each GPS satellite's timestamp and orbital position. It calculates the time the different satellites' signals took to reach the receiver, which gives the receiver the distance from each satellite. Given the distance to each satellite, you know your own position.

How? Imagine three satellites in orbit and you on the earth, with long sticks in between. Those sticks are only going to meet in one spot. With one satellite and one fixed length stick, you could be anywhere on a sphere around the satellite. With two satellites, you could be anywhere on a circle centered between the two satellites. With three satellites, your position generally can only be in one spot. Usually, four satellites are required for any precision, though. (The calculation of distance from the satellites is usually not that precise, so knowing the distance to more satellites is better)

darron
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    +1 for the Mongols coming. No-one really remembers, but GPS (Global Positioning System) is the system. The little box with the knobs and buttons to frob is known as a GPSR (Global Positioning System Receiver), because it's a Receiver for Global Positioning System data, and the hint is in ‘Receiver’. Of course, the only people who remember this are sad, uncool nerds who already know GPS is one-way. Oh, wait. – Alexios Feb 10 '12 at 16:59
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    +1 for mentioning that you need information from at least THREE GPS satellites before getting a fix and explaining why. – Jon L Feb 13 '12 at 20:43
  • It actually takes four satellites to get a good fix. You actually have four unknowns to solve for when determining your location: 3 spatial dimensions and time. The satellites all know exactly what time it is (because they have atomic clocks onboard), but your receiver does not know what time it is. You can do some clever assumptions to get a reasonable fix with 3, but the accuracy improves dramatically with 4. – Cort Ammon Aug 25 '16 at 23:21
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GPS satellite is only a transmitter (as far as GPS signal is concerned) and the other end is only a receiver. There is no bi-directional communication and therefore there is no need for satellite to care about the number of devices. Basically satellite transmits its own location at certain time and the receiver calculates its position by using that information, so all work is done by the receiver.

In short, there are no "requests", just like there are no "requests" for traditional analogue radio and television.

AndrejaKo
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Satellites don't respond. They transmit signal and GPS clients receive it. I mean your GPS device doesn't need to send anything to satellite, it just receives from satellite and that's enough. Your GPS device needs to receive signals from more satellites and then it does some math to compute its own position.

So the short answer is: It's massively parallelized. :-)

Al Kepp
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The distance from each satellite to your position is calculated from the time it takes for the signal to travel the 20,200 km (12,600 miles) when overhead, which goes up to 26,600 km (16,500 miles) when on the horizon, to your receiver. With the signal travelling at 300,000 km/s, the time taken is between 89 and 67 milliseconds, and so this has to be measured with nanosecond accuracy. One of the most remarkable things about GPS is how the cheap and simple clock in the receiver is made to have the same accuracy as the very complex and expensive clocks in the satellites.

Darron explained how the fix is obtained using four satellites. One defines a sphere, the second intersects this as a circle, the third cuts the circle in two points and the fourth distinguishes these two points. If the assumption is made that the receiver is near the surface of the Earth, then this can be used instead of the fourth satellite measurement. Ideally these should all intersect at a single point, but in practice, without correction, they would be spread out slightly due to the receiver clock running fast or slow. By adjusting the clock rate to get as close a match as possible, the timing accuracy needed is obtained. The remaining spread is a measure of the accuracy of the fix.

In the old days of navigation by chronometer for longitude, all that was really necessary was an extremely stable clock. Although the clock would run slightly slow or fast, this did not matter as long as the rate was known; it was easy then to calculate the exact time from the rate and how long since the clock was checked against an accurate time, such as the firing of a noon gun in port. Similarly what is really needed in the GPS receiver is a simple but stable clock, with the rate worked out as above, to give you the equivalent of an "atomic clock in your hand".

Harry Weston
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