How does GPS work? Your phone receives radio signals from several satellites, each one stamped with the exact time it was sent. Compare send time to receive time, multiply by the speed of light, and you know how far away each satellite is. Do that with enough satellites and there is only one point on Earth that fits all the distances.
The clever part is that your phone never transmits anything. It is a pure receiver, listening.
It Is Trilateration, Not Triangulation
Triangulation uses angles. GPS uses distances:
- One satellite: you are somewhere on a sphere of that radius around it
- Two satellites: you are on the circle where two spheres intersect
- Three satellites: two points where three spheres intersect — one is usually in space or moving absurdly, so the other is your location
- Four or more: needed because your phone’s clock is not accurate enough
That fourth satellite is the subtle bit. Signal timing requires nanosecond accuracy, and an atomic clock costs far more than a phone. So the receiver solves for four unknowns — latitude, longitude, altitude, and time — using four satellites. It figures out the correct time as part of finding your position.
Why Timing Is Everything
Radio signals travel at light speed — about 300,000 km per second, or roughly 30 cm per nanosecond. Being off by a millionth of a second puts you 300 meters away from where you think you are.
This is why each GPS satellite carries multiple atomic clocks accurate to a few nanoseconds per day, and why the whole system depends on timing infrastructure as much as on satellites.
Relativity Is Not Optional Here
GPS is one of the few everyday technologies where you have to correct for relativity or it simply stops working:
- Special relativity — the satellites move at about 14,000 km/h relative to the ground, so their clocks run about 7 microseconds per day slower
- General relativity — they sit about 20,000 km up where gravity is weaker, so their clocks run about 45 microseconds per day faster
Net: satellite clocks gain roughly 38 microseconds per day. Uncorrected, that would accumulate to about 10 km of positioning error per day. The system corrects for it in the satellite’s transmitted signal.
So every time you navigate somewhere, you are relying on both Einstein theories being right.
Why It Is Inaccurate Sometimes
All of that assumes a clear view of the sky — the practical limit on how does GPS work day to day.
| Cause | What happens | Typical error |
|---|---|---|
| Ionospheric delay | Charged particles slow the signal | Up to several meters |
| Multipath | Signal bounces off buildings before arriving | Large in cities |
| Blocked sky view | Too few satellites in direct line of sight | Fails outright indoors |
| Satellite geometry | Satellites clustered close together in the sky | Amplifies all other errors |
| Receiver quality | Cheap antenna and clock | 1-5 meters typically |
The “urban canyon” effect you notice between tall buildings is mostly multipath — your phone hears the same signal arriving from several directions after bouncing, and cannot tell which is the direct path.
Modern phones do better than the raw GPS figure because they combine multiple satellite systems (GPS, Galileo, GLONASS, BeiDou), plus Wi-Fi positioning, cell tower triangulation, and inertial sensors. That fusion is why your blue dot is usually accurate to a few meters even in a city.
Accuracy Can Be Much Better
Consumer GPS is typically good to about 3-5 meters. Survey-grade equipment using RTK (real-time kinematic) correction — comparing against a fixed ground station at a known position — reaches centimeter accuracy. That is what machine-guided agriculture and precision construction use.
The raw GPS signal deliberately had a higher-accuracy military channel; the “Selective Availability” that degraded civilian signals was turned off in 2000, which is why consumer accuracy improved overnight for everyone.
Keep Exploring
- What is gravity? — why satellites stay up there at all
- What is a light-year? — light speed, at a larger scale
- What is an algorithm? — what the receiver solves
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