How GPS works

The Global Positioning System is run by the United States Space Force. It relies on a constellation of about 31 operational satellites in orbits roughly 20,200 kilometres above Earth, arranged so that at least four are visible from almost anywhere on the planet at any time.

GPS is one of several global navigation satellite systems. Russia operates GLONASS, the European Union operates Galileo, and China operates BeiDou. Most modern phones listen to several systems at once, which improves accuracy and reliability.

Each satellite carries extremely precise atomic clocks and continuously broadcasts a signal saying who it is, where it is and exactly what time the signal left. The signal travels at the speed of light, about 300,000 kilometres per second.

A receiver compares the time a signal was sent with the time it arrived. Multiply that delay by the speed of light and you get the distance to the satellite. A timing error of just one millionth of a second would mean a distance error of about 300 metres, which is why timing precision is everything.

Knowing your distance from one satellite places you somewhere on a huge imaginary sphere around it. Distance from a second satellite narrows your position to a circle where two spheres meet, and a third reduces it to two points, one of which is usually far out in space and can be ruled out.

In practice receivers need a fourth satellite. A phone's cheap quartz clock is far less accurate than an atomic clock, so the receiver treats its own clock error as an extra unknown. With four distances it can solve for its three position coordinates and the clock error at the same time.

GPS has to account for relativity. Because the satellites move fast, special relativity says their clocks run slightly slow compared with clocks on the ground, by about 7 microseconds a day. Because they are farther from Earth's gravity, general relativity says their clocks run faster, by about 45 microseconds a day.

The net effect is that satellite clocks gain about 38 microseconds per day. Uncorrected, that would cause position errors of around 10 kilometres per day. The satellite clocks are therefore deliberately set to tick slightly slower before launch.

• The ionosphere and lower atmosphere slow signals slightly; receivers use models, and dual-frequency receivers can measure the delay.
• Signals bounce off buildings before reaching the receiver, called multipath, which is a major problem in cities.
• Satellite orbit and clock data contain small errors that ground stations continually correct.
• Phones combine GPS with Wi-Fi and cell tower positions, compasses and motion sensors.
• Differential and real-time kinematic systems use fixed reference stations to reach centimetre accuracy for surveying and farming.

GPS began as a military project in the 1970s, and the first satellite was launched in 1978. After a Korean Air Lines passenger jet strayed into Soviet airspace and was shot down in 1983, the United States announced that GPS would be made available for civilian use. An intentional degradation of civilian accuracy, called Selective Availability, was switched off in 2000.

Beyond navigation, the precise time signals from GPS are used to synchronise mobile phone networks, power grids and financial transactions, making it quiet but critical infrastructure.