How does GPS know where you are?

GPS does not track you. Satellites broadcast signals, and your device listens. Each signal carries the time it was sent and the satellite's position. Because the signal travels at the speed of light, the delay before it arrives reveals how far away that satellite is. With distances to several satellites, the device can work out its own longitude, latitude and altitude. [1] [2]

The European Space Agency compares the idea to counting the seconds between a lightning flash and thunder to judge how far away a storm is. A time value is converted into a distance. GPS is maintained by the U.S. Space Force and has 31 satellites orbiting about 20,200 kilometers above Earth. Twenty-seven are in operation and the others serve as backups. Their orbits are arranged so that at least four can be seen from nearly anywhere on Earth. [2] [1]

Each satellite carries atomic clocks, which NIST says keep time to within three billionths of a second. They are regularly corrected by even more accurate clocks on the ground. Precise timing matters because the travel times involved are tiny fractions of a second. [1] [3]

Each distance places you on a sphere around a satellite, and combining several spheres narrows your position to a point. The ESA describes this as trilateration, the three-dimensional counterpart of triangulation. Three satellites fix longitude, latitude and altitude. A fourth is used to find the offset between the precise satellite clocks and the less precise clock inside your receiver. [2]

The FAA makes the same point: a fourth measurement means the receiver does not need its own atomic clock, so it can compute latitude, longitude, altitude and time from four satellites. Some ESA material describes the minimum differently, saying that two satellites can narrow a position on the Earth's surface and a third is needed for clock synchronisation. Most sources, however, give four as the working minimum for a full fix, and more satellites improve accuracy. [4] [3]

The signals are very faint. The ESA likens them to car headlights shone from one end of Europe to the other. Receivers hold records of each satellite's code and an almanac of expected satellite positions, which cuts the time needed to lock on from minutes to seconds. [2]

Accuracy figures vary by source and conditions. NIST cites about 4.9 meters (16 feet), while the FAA describes basic service as approximately 7.0 meters, 95 percent of the time. Errors come from the ionosphere and troposphere, which delay signals, and from multipath, where signals bounce off nearby surfaces. Dual-frequency receivers can remove most ionospheric error, while phone-sized single-frequency receivers use software models to remove up to half of it. Specialised receivers can do far better, and NOAA's National Geodetic Survey says some reach a fraction of an inch. [1] [4] [2] [5]