How GPS Works

The Global Positioning System (GPS) is a satellite-based navigation system that lets a receiver determine its position anywhere on or near the Earth. It relies on a constellation of satellites orbiting at high altitude, each continuously broadcasting radio signals that carry the satellite's identity and the precise time the signal was sent, derived from atomic clocks aboard the spacecraft.

A receiver works out its location by measuring how long signals take to arrive from several satellites at once. Because radio waves travel at the known speed of light, each travel time converts into a distance to that satellite. The receiver lies somewhere on a sphere of that radius around each satellite, and the intersection of these spheres pinpoints its position. Signals from at least four satellites are normally needed: three to fix latitude, longitude and altitude, and a fourth to correct the receiver's own inexpensive clock.

The system was developed by the United States and is one of several global satellite navigation systems now in service alongside others operated by different nations. Accuracy is affected by atmospheric delays, signal reflections off buildings and the geometry of the visible satellites, and various correction techniques can improve it to within a few metres or better. GPS underpins everyday navigation, surveying, timing for communication networks and countless mobile applications.

Frequently asked questions

How does a GPS receiver find its position?

It measures the time signals take to arrive from several satellites, converts each into a distance, and finds the single point where those distances intersect.

Why are at least four satellites needed?

Three give latitude, longitude and altitude, while a fourth lets the receiver correct its own clock error, which is essential for accurate timing.

What reduces GPS accuracy?

Atmospheric delays, signals bouncing off buildings and unfavourable satellite geometry can all introduce errors, though correction methods can reduce them.