How does GPS know where you are?

Above your head, about 30 GPS satellites circle the Earth, each carrying an atomic clock accurate to a billionth of a second. Every satellite constantly broadcasts one thing: the exact time and its own position.

Your phone listens. A signal from a satellite takes a little time to arrive — roughly 70 milliseconds. By comparing the time sent with the time received, the phone works out how far away that satellite is.

One distance tells you that you are somewhere on a huge sphere around that satellite. Three distances narrow it to a point; a fourth corrects your phone’s cheap clock. The maths is called trilateration.

GPS is American; India has its own system, NavIC, with seven satellites focused on the subcontinent. Your phone may be using both right now to put the blue dot on the map.