How does the Global Positioning System work?

GPS was developed by the US Department of Defense as a worldwide all weather navigation and positioning and timing resource, mainly for military use. It is based on a constellation of 24 satellites orbiting the earth which act as reference points.

By measuring the travel time of signals transmitted from four satellites, a receiver can measure its distance from each satellite and combine these measurements to calculate its latitude, longitude, altitude, course and speed.

GPS positions are accurate 95% of the time to ± 20 metres and updates can be received every second. A higher accuracy level is required for some specialised navigation and positioning purposes and the integrity of the system needs to be monitored for safety-related applications.