How-to · Step-by-step
How does GPS work?
GPS works out where you are by measuring how long radio signals take to reach your device from several satellites, each broadcasting its position and a precise time.
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Step by step
How to do it
1.Satellites broadcast their position and time
Each GPS satellite continuously transmits a radio signal containing its exact orbital position and a timestamp from its onboard atomic clock, marking the precise moment the signal was sent.
2.Your device receives the signals
Your GPS receiver picks up the signals from every satellite it can “see” in the sky. It needs a reasonably clear view, which is why GPS struggles indoors or in deep urban canyons.
3.It measures the travel time
The receiver compares the timestamp in each signal with the time it arrived. Because radio waves travel at the speed of light, that tiny delay reveals the distance to each satellite.
4.Distance becomes a sphere
Knowing the distance to one satellite places you somewhere on a sphere around it. The signal from one satellite alone is not enough to fix your location.
5.Signals from several satellites are combined
By combining distances from at least four satellites, the receiver finds the single point where all the spheres intersect — a method called trilateration. The fourth satellite corrects the receiver’s own clock error.
6.Your position is calculated
The device computes your latitude, longitude and altitude, then displays it on a map. Continuous updates track your movement and speed as the distances change.
Satellites, signals and atomic clocks
The Global Positioning System relies on a constellation of around 30 satellites orbiting roughly 20,000 kilometres above the Earth, arranged so that several are visible from anywhere at any time. Each satellite carries an extremely accurate atomic clock and continuously broadcasts a radio signal stating exactly where it is and the precise instant the signal left. Your GPS receiver — in a phone, car or watch — listens passively to these signals; it never transmits anything back, which is why unlimited devices can use GPS at once and it works even with no phone signal.
Common questions
FAQ
How many satellites does GPS need to find my location?+
At least four. Three satellites can in principle fix a position in three dimensions, but a fourth is needed to correct the small timing error in your device’s clock. With four or more satellites in view, the receiver can calculate accurate latitude, longitude and altitude.
Does GPS track me or send my location somewhere?+
Basic GPS is one-way: your device only listens to satellite signals and works out its own position. The satellites do not know you exist, and standard GPS does not transmit your location anywhere. Tracking happens only if an app on your device chooses to send your calculated position over the internet or mobile network.
Why is GPS sometimes inaccurate?+
Accuracy drops when signals are blocked or bounced — by tall buildings, dense trees, tunnels or indoors — because the receiver needs a clear line of sight to several satellites. The atmosphere can also slightly delay signals. Phones improve accuracy by combining GPS with Wi-Fi, mobile masts and motion sensors.
Going deeper
