The Global Positioning System has 31 satellites orbiting Earth at 20,200 kilometers altitude, each carrying an atomic clock accurate to within a billionth of a second. The satellites broadcast a continuous signal: 'I am satellite N. The current time, by my atomic clock, is T.'

That's it. Satellites do not track you. They have no idea where you are. They simply broadcast their identity and the time, constantly, in every direction.

Your phone does the rest. Its receiver picks up signals from at least four GPS satellites simultaneously. It compares the time stamps to the time on its own clock. The difference, multiplied by the speed of light, gives the distance from your phone to each satellite. With four distances and the known orbital positions of the four satellites, your phone solves four simultaneous equations to pinpoint where it must be: a single x, y, z, t coordinate in space and time.

The math is called trilateration. With three satellites you'd locate yourself in 3D space. You need a fourth because your phone's clock is much less accurate than the satellites'. The fourth satellite resolves the time-error component too — letting your phone correct its own clock to nanosecond accuracy as a side effect.

This means civilian GPS is fundamentally a one-way system. The satellites have no idea who is using them. Hundreds of millions of devices receive the signals at the same moment, each calculating its own location independently. There is no central server tracking phones.

The free public service runs on US Air Force satellites that cost billions per year to maintain. Almost every smartphone, plane, ship, and missile in the world depends on it.