How does GPS know where you are?
Your phone works out its position from signals sent by satellites 20,000 km away. How satellite navigation measures distance with time, why it needs four satellites, and why phones use several systems at once.
Measuring distance with time
Each navigation satellite carries extremely precise atomic clocks and constantly broadcasts the time and its own position. Your phone receives the signal a fraction of a second later. Radio travels at the speed of light, so that delay tells the distance: one millionth of a second is 300 meters.
Why four satellites
Knowing your distance to one satellite puts you somewhere on a huge sphere around it. Two spheres meet in a circle, three in two points, one of which is usually far out in space. A fourth satellite is needed because the clock in your phone is nowhere near as accurate as the satellites'; the extra measurement lets the receiver correct its own clock error too.
Four systems, one sky
GPS (USA), Galileo (Europe), BeiDou (China) and GLONASS (Russia) all cover the whole world. Modern phones listen to all of them, so dozens of satellites are usually in view, which helps a lot between tall buildings. Korea is developing its own regional system, KPS, aimed at improving accuracy around the Korean peninsula.
Relativity matters
Clocks on the satellites run slightly fast compared with clocks on the ground, because of both their speed and the weaker gravity up there, as Einstein's theories predict. The difference is a few tens of microseconds a day, enough to make positions drift by kilometers, so it is corrected for in the system.