Engineering · Three spheres
How does your phone know where you are?
What you’ll see
Your phone has no idea where it is, until it listens. Each GPS satellite broadcasts the time. A signal that took seven hundredths of a second travelled about twenty thousand kilometres. So one satellite tells you only this: you're somewhere on a sphere that size. A second satellite adds a second sphere, and the two meet in a circle. A third sphere cuts that circle in just two points, and one of them is far out in space. You're at the other. Three distances, one place. A fourth satellite corrects your phone's own clock.
