Four clocks and some arithmetic
How Does a Phone Know Where It Is?
Satellites broadcast the time, and your phone turns the delays into distances to find itself.
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Step 1 of 5
The satellites know nothing about you.
They never receive anything. Each one simply shouts its identity and the exact time, endlessly, to whoever happens to be listening below.
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Step 2 of 5
Each signal says when it left.
Your phone reads the timestamp inside the message and compares it with its own clock, which tells it how long that signal spent travelling.
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Step 3 of 5
A delay is really a distance.
Radio covers a known distance every second, so multiplying the travel time by the speed of light turns each delay straight into a range.
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Step 4 of 5
One distance is a sphere; several pin a point.
Knowing you are 20,000 km from one satellite puts you anywhere on a huge ball. Each extra satellite carves that down until only one place survives.
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Step 5 of 5
The fourth satellite fixes your clock.
Satellites carry atomic clocks; your phone carries a cheap one. A fourth measurement lets it solve for its own clock error and trust the other three.
Light travels about 30 centimetres in a nanosecond, so a clock error of one millionth of a second moves your position by 300 metres.
The short version
Your phone finds itself by timing signals from several satellites, turning each delay into a distance, and solving for the only point that fits.
Try it yourself
Open a maps app in a tunnel or a deep basement. The dot drifts or freezes, because no satellite signal can reach you there.
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