Chapter 02 of 5
Four Satellites and a Stopwatch
How a phone works out where it is by timing radio signals.
- The Core Idea
A Radio Stopwatch
Each GPS satellite broadcasts a signal that says where it is and exactly when the signal was sent.
- Travel Time
How Far Away?
Your receiver notes when the signal arrived. The delay, multiplied by the speed of light, gives the distance to the satellite.
- Speed
Light Is Fast
Radio waves travel at about 300,000 kilometers a second. A signal from a satellite overhead reaches you in about a fifteenth of a second.
17 more cards in this chapter
Could you answer this?
Why does a GPS receiver need signals from at least four satellites?
🛰️ Why Your Phone Needs Einstein
- 01Beeps From SpaceHow listening to Sputnik led to satellite navigation.
- 02Four Satellites and a StopwatchHow a phone works out where it is by timing radio signals.
- 03A Billionth of a SecondWhy GPS depends on atomic clocks and timing measured in nanoseconds.
- 04Einstein in OrbitThe relativity corrections that keep your blue dot on the right street.
- 05Lost in the CityUrban canyons, jamming and how your phone fills the gaps.