How does GPS know where you are?
Your phone never sends anything to the satellites. It listens to their time stamps, turns each delay into a distance, and lets a fourth satellite fix its own clock.
The mechanism, in three steps
Satellites broadcast. Each GPS satellite carries atomic clocks and repeats two facts by radio: where it is, and the exact time the signal left.
Delay equals distance. Your phone compares the time stamp with its own clock. Radio moves at light speed, so a late signal means a far satellite, and one nanosecond of delay is about 30 centimetres.
Three distances give a point, and the fourth fixes the clock. One distance puts you on a sphere, three shrink it to a point, and a fourth satellite corrects the phone's own clock so the sums come out right.
Check yourself
Does your phone send anything to the GPS satellites?
No. A GPS receiver only receives. The satellites broadcast and never hear the phone.
Why does a position fix need four satellites and not three?
Three distances pin down a point, but the phone's own clock is slightly wrong, so a fourth satellite is used to solve for that clock error as well as the position.
Open a map and the blue dot lands within a few metres of where you stand. It feels as though the phone asked someone. It did not. Nothing your phone does is heard by a satellite. It listens, and then it does some arithmetic.
What the satellites are saying
About thirty GPS satellites circle the Earth, roughly 20,000 kilometres up. Each one carries atomic clocks, among the most precise timekeepers ever built, and each one repeats the same two facts over and over by radio: here is where I am, and here is the exact time this signal left me.
That is the whole broadcast. The satellite does not know you exist.
Turning a delay into a distance
Your phone catches one of those broadcasts and reads the time stamp. It compares that stamp with its own clock. The gap between the two is how long the signal took to arrive.
Radio waves travel at the speed of light, about 300,000 kilometres a second, so a known delay is a known distance. A signal that arrives 67 milliseconds after it was sent has travelled about 20,000 kilometres. The numbers are tiny and the stakes are large: a clock error of one nanosecond, a billionth of a second, is about 30 centimetres of error on the ground. That is why the satellites carry atomic clocks and not the kind found in a watch.
Why four, not three
One distance from one satellite puts you somewhere on the surface of an enormous sphere centred on that satellite. Two spheres cross in a circle. Three cross at a point, and that point is you.
There is a catch. The phone’s own clock is nowhere near as good as the satellites’, so every delay it measures is off by the same unknown amount. A fourth satellite gives one more equation, which is enough to solve for that clock error as well as the position. The phone ends up knowing where it is and, as a bonus, what time it really is.
Under open sky the answer is typically within a few metres. Between tall buildings the signals bounce and the phone leans on other sensors to help, but that is another question.
Sources
- GPS: The Global Positioning System GPS.gov
- Space Segment GPS.gov
- GPS Accuracy GPS.gov
- How does GPS work? NASA Space Place
- Time and Frequency from A to Z: G (GPS clocks) NIST
- SI base units: the metre, defined by the speed of light BIPM
Questions this also answers
How accurate is GPS on a phone?
Under open sky, civilian GPS is typically accurate to a few metres. Phones often do better by combining GPS with other sensors, which is a separate question.
How many GPS satellites are there?
The constellation is kept at a minimum of 24 operational satellites, and 31 were in service in 2024, so about thirty is the honest round number.