Digital mapping: what a GPS actually does
Geography · Key Stage 2 · The School
A one-way signal
GPS stands for Global Positioning System. It uses a set of satellites that circle the Earth. A GPS device picks up signals from several of these satellites at once. It never sends anything back up to them. It only listens. This is exactly why GPS works even with no phone signal at all.
Finding a spot from timing
Each satellite signal carries the exact time it was sent. The device checks that against the exact time it was picked up. That tiny gap in time tells the device how far away that satellite is. Doing this with a few satellites at once pins down an exact spot on the ground.
A digital map can update itself
A printed map is fixed the moment it comes off the press. A digital map can update at once, showing a new road or a closed street straight away. It can also show your own dot moving in real time, which no paper map can ever do. That single feature has changed how most people now find their way.
A phone receives a signal from Satellite A. The timing shows it could be anywhere on a certain circle around that satellite. A signal from Satellite B narrows this down to just two possible points, where two circles cross. A signal from Satellite C rules out one of those two points completely. Only one position now fits all three signals. That is the phone's exact location. Three satellites, three overlapping circles, one single answer. A fourth satellite is often used too, to check the height above the ground.