# How GPS works: the satellites don't know where you are

> GPS satellites track nothing: they broadcast where they are and the time, and your phone turns each signal's delay into a distance. Four satellites fix your position and your phone's clock, tuned for relativity. A 49-second explainer, every figure sourced.

- URL: https://oliverz.dev/videos/gps
- Author: Oliver Zhang (https://oliverz.dev)
- Language: en
- Published: 2026-11-01
- Tags: gps, how-it-works, explainer
- Video: https://www.youtube.com/shorts/cfCqJHzKWAQ

> **In short:** GPS satellites don't track anyone. About 30 of them, roughly 20,200 km up and carrying atomic clocks, broadcast where they are and what time it is. Your phone only listens: each signal's delay times the speed of light is a distance, three distances meet at one point, and a fourth satellite corrects the phone's far less precise clock. The satellites' clocks run fast because of relativity, so they are tuned before launch. A smartphone ends up within about 4.9 m under open sky.

## The satellites track nothing

From gps.gov's FAQ: "GPS satellites themselves do not track anything on the ground. They are simply beacons, like lighthouses, broadcasting one-way reference signals."

## About 30 satellites, 20,200 km up

gps.gov, on the space segment: "the U.S. Space Force has been flying 31 operational GPS satellites for well over a decade", and "GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km (12,550 miles). Each satellite circles the Earth twice a day."

Each carries "multiple atomic clocks that contribute very precise time data to the GPS signals", and the signals "give the current GPS satellite position and time".

## Delay × the speed of light = distance

ESA: "The signal emitted from each satellite is a microwave radio wave containing the time it was transmitted and the satellite's current orbital position." From 20,200 km away a signal takes about 0.067 s to arrive (our arithmetic: 20,200 km ÷ 299,792 km/s), and that delay times the speed of light is the distance.

## Three distances meet at you, a fourth fixes the clock

ESA again: "Three are used to 'trilateralise' (the three-dimensional equivalent of triangulation) the user's longitude, latitude and altitude and a fourth to determine the time offset between the (precise) satellite clock and the (less precise) clock embedded within the receiver."

The clock matters: a millionth of a second of error is about 300 m (our arithmetic: 299,792 km/s × 10⁻⁶ s).

## Einstein's correction

In orbit the satellites' clocks gain about 38 microseconds a day. R. Pogge (Ohio State): "by about 38 microseconds per day (45-7=38)!" N. Ashby (Living Reviews in Relativity, 2003) gives the net offset as −4.4647 × 10⁻¹⁰: "the standard clock in orbit is beating too fast". So the clocks are set slow before launch, to 10.22999999543 MHz instead of 10.23 MHz.

Ashby in Physics Today (2002): "If these relativistic effects were not corrected for, satellite clock errors building up in just one day would cause navigational errors of more than 11 km, quickly rendering the system useless."

## About five metres

gps.gov: "GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky… However, their accuracy worsens near buildings, bridges, and trees."

## Takeaway

1. GPS is one-way: the satellites broadcast, your phone listens.
2. Time is distance: delays from four satellites fix where you are, and when.
3. Einstein matters: the satellites' clocks are tuned for relativity before launch.

## Sources

- gps.gov: [FAQ](https://www.gps.gov/faq), [Space Segment](https://www.gps.gov/space-segment), [GPS and telling time](https://www.gps.gov/gps-and-telling-time), [GPS accuracy](https://www.gps.gov/gps-accuracy)
- ESA, [How satellite navigation works](https://www.esa.int/Applications/Satellite_navigation/How_satellite_navigation_works)
- N. Ashby, "Relativity and the Global Positioning System", Physics Today 55(5), May 2002; "Relativity in the Global Positioning System", Living Reviews in Relativity 6:1, 2003
- R. Pogge, "Real-World Relativity: The GPS Navigation System", Ohio State University

The video is animated with code and the script was written with AI assistance; every quote above is checked against its source, and the two figures marked "our arithmetic" are ours. Illustrative: the satellites, the signal pulses and the distance spheres (drawn as circles). The video is narrated in English. Made with [Shotloop](https://oliverz.dev/products/shotloop), my free Mac app for code-made explainers, fixed one shot at a time.