Right now, more than 100 satellites are orbiting 20,000 km above your head, each shouting the same message into space: “I am satellite X, and it is exactly this time.” From nothing but those whispers and the speed of light, your device pinpoints itself to a few metres. Scroll down to see exactly how the trick works — interactively.
Orbit height of GPS satellites
The speed of light — radio waves
From space to your pocket
To fix position and time
“GPS” is just one of several GNSS constellations. The USA runs GPS, the EU runs Galileo, Russia runs GLONASS and China runs BeiDou. Together they blanket Earth so that from almost anywhere, several satellites are always in view. Each carries an atomic clock and constantly broadcasts its identity, its position, and the time.
The blue marble is Earth. The glowing rings are orbital planes; each dot is a satellite. The green pulse marks you. Lines light up to satellites currently above your horizon — only those can be used.
Forget satellites for a second. Suppose you only know your distance from a few known points. One distance puts you on a circle. Two circles cross at two spots. A third circle nails the single point. That’s trilateration — and it’s the whole secret of GPS, just done in 3D with spheres.
Each beacon measured its distance to the hidden receiver (★). Each distance becomes a ring of “possible positions.” Watch how adding rings collapses the possibilities down to one point.
A satellite can’t hand you a tape measure. Instead it stamps every signal with the exact time it was sent. Your receiver notes when it arrived, subtracts, and multiplies by the speed of light. The catch: light is so fast that a clock error of just one microsecond = 300 metres of position error. Precision is everything.
Press the button to broadcast a timestamped pulse from the satellite. Watch it crawl across space at light speed and see the distance computed on arrival.
Satellites carry atomic clocks worth a fortune. Your phone has a cheap quartz clock that drifts. If your clock is even slightly off, every distance is wrong by the same amount — so all your rings miss the true point and leave a gap. The receiver uses that mismatch as a clue: it adjusts its clock until everything snaps together. Three satellites give 3D position; the fourth solves for time.
Drag the slider to push your receiver’s clock off. See the rings bloat or shrink together — they stop meeting at one point. Hit auto-correct to watch the receiver hunt for the offset that makes them agree again.
In a vacuum the maths is clean. Reality adds wobble: the atmosphere bends signals, buildings bounce them, and geometry matters. Toggle the gremlins below to see your accuracy degrade — and read how engineers fight each one.
Augmentation systems (WAAS/EGNOS) and RTK / differential GPS use ground stations at known locations to measure the current errors and broadcast corrections. Dual-frequency receivers also cancel most of the ionosphere by comparing two signals. Result: survey gear gets centimetre accuracy.
Because every fix is also a time fix from atomic clocks, GPS is the silent heartbeat of power grids, stock exchanges, and mobile networks — they synchronise to GPS time. If GPS blinked off, far more than maps would break.
Here’s the whole pipeline in one place. Press play and watch a receiver acquire satellites, time their signals, intersect the spheres, correct its clock, and lock onto a position.
Distant clocks, the speed of light, and a bit of geometry — repeated a thousand times a second so a dot on a map can follow you down the street. Now you know what’s happening every time it says “you are here.”