How does GPS know where you are?

GPS (Global Positioning System) determines your location through a process called trilateration, which relies on precise timing and geometry. Here is how it works:

1. The Constellation: There are at least 24 GPS satellites orbiting the Earth. Each satellite carries an incredibly accurate atomic clock and constantly broadcasts a signal containing its exact position and the current time.
2. Timing the Signal: Your device (like a phone or GPS receiver) receives these signals. Because radio waves travel at the speed of light, your device can calculate how long it took for the signal to travel from the satellite to you.
3. Measuring Distance: By multiplying that time by the speed of light, your device determines how far away it is from each specific satellite. If you know how far you are from one satellite, you know you are somewhere on the surface of a giant sphere surrounding that satellite.
4. Pinpointing the Spot:
   • If you measure your distance from two satellites, you are at the intersection of two spheres, which creates a circle.
   • With three satellites, you narrow your location down to two points where those spheres intersect.
   • A fourth satellite is usually used to confirm the location and correct for timing errors between the satellite's atomic clock and the less precise clock in your device.

Once your device has calculated the intersection of these four spheres, it can pinpoint your location on Earth with high accuracy.