How GPS and GNSS Systems Work

The Global Positioning System (GPS), formally NAVSTAR GPS, determines a receiver's position through a technique called trilateration, which relies on measuring the time it takes radio signals to travel from multiple satellites to a receiver on or near the Earth's surface. Each satellite continuously broadcasts a signal encoding its own precise orbital position, known as ephemeris data, along with a time stamp generated by an onboard atomic clock, typically cesium or rubidium, accurate to within a few billionths of a second. A receiver calculates its distance, or pseudorange, to each satellite by multiplying the signal's travel time by the speed of light, and because the receiver's own clock is a comparatively cheap quartz oscillator rather than an atomic standard, a fourth satellite measurement is needed to solve simultaneously for three spatial coordinates and the receiver's clock offset, a method formalized in the original GPS design led by Bradford Parkinson at the U.S. Air Force in the 1970s.