You open a map app and a blue dot snaps to your exact spot. Behind that everyday magic is a fleet of satellites and a trick of timing that’s genuinely clever.
A sky full of clocks
Around 30 GPS satellites orbit Earth, each carrying an ultra-precise atomic clock. Every satellite constantly broadcasts one simple message: who it is, where it is, and the exact time.
It’s all about how long the signal takes
Your phone listens for these signals. Because the messages travel at the speed of light, the tiny delay between when a satellite sent its signal and when your phone received it reveals how far away that satellite is.
Crossing the distances
One distance puts you somewhere on a huge sphere. Two narrow it down, three pin it to a point, and a fourth corrects your phone’s cheaper clock. With signals from four or more satellites, your device solves the puzzle and lands on your position — often within a few metres.
The system is so sensitive that engineers even have to account for Einstein’s relativity: time runs slightly differently for fast-moving satellites. Ignore it, and your blue dot would drift kilometres off within a day.
Photo: public domain.