GPS systems are navigation systems that can locate a person to within a few meters. The system was created and previously restricted to the military, but for an unknown reason, they decided to open it up for everyone to use. The Global Positioning System works off of 24 satellites that are strategically positioned around the entire globe. There are really 27 satellites, but three extras are kept up there incase one of the satellites decides to become dysfunctional. These satellites have about a 12, 000 mile radius and revolve around the earth about twice a day.
The GPS finds the receiver's location through a process called trilateration. This is a process where the receiver sends radio signals (at the speed of light) to at least three satellites (but it is usually more to gain accuracy). These satellites then return the signal. The receiver knows how far it took for the signal to travel back and forth and can then determine how far away the satellite is from the receiver.
There is actually a delay between the signals. The receiver knows to take the delay between the signals and multiply it by the speed of light. When this is multiplied by the speed of light, it will give the receiver the exact distance the satellite is from it. To make a time measurement this specific, the receiver and the satellite must contain a state-of-the-art clock. All the satellites contain atomic clocks, which are extremely expensive. They are far too expensive to put into the receivers, so someone came up with the idea of putting quartz clocks into them.
If the receiver is off, it will measure each of the satellites incorrectly. However, with four being measured (what is usually measured, not three), the measurement will be proportionally incorrect, and it will be able to make the necessary adjustments and correspond it to the atomic clock of the satellites. What this means is that the quartz clock is essentially as accurate as the atomic on the satellite, except on a much cheaper scale. These clocks measure in nanoseconds (this makes them extremely accurate). There are more technicalities to how GPS systems work, but essentially this is what goes on. The clocks and the trilateration are what make it such an effective system.
Information used from the website: web.