How GPS Works: Satellites, Clocks and Your Position
GPS finds your position by timing radio signals from satellites about 20,200 km up. Here is how the satellites, atomic clocks and your receiver fit together.
In this article
A GPS receiver works out where you are by measuring how long radio signals take to reach it from several satellites. Each satellite broadcasts its own position and the exact time. Because the signals travel at a known speed, the delay tells the receiver how far away each satellite is, and a few such distances pin down one spot on Earth. Your phone only listens; it sends nothing to the satellites.
The three parts of the system
The US-run Global Positioning System has three parts, as described by the National Oceanic and Atmospheric Administration (NOAA): satellites, ground stations and receivers. The satellites do the broadcasting. The ground stations track the satellites and work out where each one is now and where it will be soon. Receivers are the phones, car units and watches that pick up the signals.
NASA says the system has at least 24 satellites arranged in six orbital planes, each tilted 55 degrees to the equator. They orbit at about 20,200 kilometres (12,550 miles) and take roughly 12 hours to go round once, which is two orbits a day. More than 30 satellites are working today, according to both NASA and NOAA. The layout is planned so that at least four satellites are above the horizon from almost anywhere on Earth.
What a satellite sends
Every satellite carries atomic clocks, and NASA notes that the redundancy is deliberate: if one clock fails, another takes over. The satellite repeatedly broadcasts two things: where it is, and the time its clock showed when the signal left. The ground stations keep this information up to date.
A receiver compares the time stamp on an incoming signal with its own clock. The difference is the travel time. Radio waves move at about 300,000 kilometres per second, so even a tiny timing error becomes a large distance error. A mistake of one millionth of a second would put a distance estimate off by around 300 metres. That is why the satellites need such precise clocks. NASA gives the time accuracy of the system as about 10 nanoseconds, which is ten billionths of a second.
From distances to a position
One distance tells you that you are somewhere on a sphere around that satellite. A second distance narrows it to a circle where two spheres meet. A third narrows it to two points, and one of those is usually far out in space and can be ignored. This method of fixing a position from distances is called trilateration. It differs from triangulation, which uses angles.
NOAA says a receiver needs distances from four or more satellites to know where it is. The reason for the fourth is practical. The clock in a phone is far less precise than an atomic clock, so the receiver does not know the exact time. Treating its own clock error as one more unknown, the receiver needs a fourth measurement to solve for it. With more satellites in view, the calculation has extra checks and the result is steadier.
- Satellite 1 gives a sphere of possible positions
- Satellite 2 reduces it to a circle
- Satellite 3 reduces it to two points
- Satellite 4 corrects the receiver's own clock error
How accurate it is
NASA states that the standard civilian service is built to meet about 10 metres horizontally and 20 metres vertically at 95 percent confidence, and that actual global-average error in recent years has been about 1 metre or better. NOAA describes typical results as within a few yards, with specialised receivers reaching far finer detail. Real-world results vary. Tall buildings, dense trees and tunnels block or bounce signals, and that can make a phone's position drift.
Who uses it, and what it is not
Because every satellite also carries a time signal, GPS is used for more than maps. Anything that needs precise time can take it from the satellites. Navigation apps, ride booking, surveying and mapping all use the same signals.
GPS does not know the roads, the traffic or the name of the street you are on. It supplies only coordinates and time. A map app adds the map, the route and the traffic information on top, usually through a mobile data connection. If the data connection is lost, the receiver can still calculate its position, though the map may not load.
The short version
Satellites broadcast their position and the time. A receiver measures the delay of four or more signals, turns the delays into distances and solves for its own position and clock error. The accuracy depends on atomic clocks in space and on the ground stations that keep the satellites' positions up to date. This is general information drawn from NASA and NOAA, as of 1 November 2026.
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