Explainer #11
How does GPS know where you are?
About 30 GPS satellites circle the Earth some 20,000 km up, and each one keeps sending its position and the exact time. Your phone notes when each signal arrives. The longer a signal took, the farther away that satellite is. Each distance puts you somewhere on a circle around one satellite, and only one spot fits them all. A fourth satellite lets the phone fix its own, much less exact clock. The satellites' clocks have to be extremely exact: a millionth of a second off means about 300 metres off.
The principle
Your phone listens to satellites. About 30 GPS satellites circle the Earth, roughly 20,000 km up, each going round twice a day. They don’t know where you are, and your phone doesn’t send them anything. It only listens.
Step 1: Satellites send the time. Each satellite carries a very exact clock. It keeps sending a signal that says, in effect: “I am here, and right now the time is …”.
Step 2: Travel time gives distance. The signals travel at the speed of light, so from a satellite to you they take only about a fifteenth of a second. Your phone notes when each signal arrives and compares it with the time written in it. The farther away a satellite is, the longer its signal took. Travel time multiplied by the speed of light gives the distance.
Step 3: Three distances meet at you. Knowing your distance to one satellite only tells you that you are somewhere on a circle around it (really a ball, because space has three directions). With three satellites, there is only one spot near the Earth that sits at the right distance from all of them: that is you.
Step 4: A 4th one fixes your clock. The clock in your phone is far less exact than the satellites’ clocks. If it is off by just a tiny bit, every distance comes out wrong by the same amount, and the circles no longer meet in one spot. A fourth satellite gives the phone one more distance, and there is only one clock setting that makes all four circles meet. The phone picks that one, and so it finds both where you are and the exact time.
Step 5: The clocks must be perfect. Because the signals are so fast, tiny time errors matter a lot: a millionth of a second off means about 300 metres off. Up there, time even runs a tiny bit differently. Einstein showed that clocks run faster where gravity is weaker and slower when they move fast. Together, a satellite clock would gain about 38 millionths of a second a day, so the clocks are set to tick slightly slower before launch.
Common myth
GPS satellites track your phone
They don’t. GPS only works one way: the satellites send, and your phone listens and does all the work itself. That is why GPS still finds you in flight mode or far out at sea. Apps that share your location send it over the phone network or the internet, not through GPS.How this video was made: Rendered 3D animation, simplified and drawn flat. The satellites are drawn far closer, the signals are slowed down enormously so you can see them move, and the clock error in step 4 is hugely exaggerated.
