Space around Earth · 4 min · 3 steps
How your phone knows where it is
Goal. Explain how satellite navigation turns signal travel times into a position.
Steps
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Predict
How many navigation satellites does your phone need to work out its position?
- One
- At least four
- At least forty
Show the answer
At least four. At least four: three for the position and one more to correct the phone's own clock.
These are the navigation satellites of GPS (USA), Galileo (Europe), BeiDou (China) and GLONASS (Russia). Most orbit 19,000 to 23,000 km up, circling the Earth about twice a day.
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Each satellite carries atomic clocks and keeps broadcasting the time and its own position. Your phone measures how long each signal took to arrive. The signals are radio waves, which travel at the speed of light: 300 km in a thousandth of a second. So the delay gives the distance.
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With the distances to several satellites, only one point on Earth fits them all. The clocks must agree to within billionths of a second, and the system even corrects for Einstein's relativity.
The common mistake
Myth
GPS satellites track your phone.
Fact. Navigation satellites only broadcast. Your phone listens and works out its own position; nothing is sent back to the satellites.
Check yourself
Question 1
What does a navigation satellite broadcast?
- Photos of the ground
- The time and its own position
- Your location
Show the answer
The time and its own position. It sends only its time and orbit. Everything else is calculated in your phone.
Question 2
Why does a phone need a fourth satellite?
- To correct its own clock
- For the internet
- To take photos
Show the answer
To correct its own clock. The phone's clock is not precise enough on its own; a fourth distance lets it solve for the clock error too.
Question 3
About how far does light travel in one thousandth of a second?
- 3 km
- 300 km
- 300,000 km
Show the answer
300 km. Light travels about 300,000 km a second, so 300 km in a thousandth of a second.