# WhyBench

> Big questions. Clear answers. One idea at a time.
> Questions people actually type into a search box, each answered with one
> mechanism in three numbered steps and one line to say back. Every number
> has a source; the sources are listed on every question.

Sources here are agency and standards-body pages, textbooks, peer-reviewed
papers and manufacturers' technical documents. When citing, cite the source
the question names and link the question page as the explanation.
Corrections are appended to the question they affect, dated.

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## Technology

### [How does GPS know where you are?](https://whybench.com/questions/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.
Video: https://youtu.be/qVALC3Lhhpg

1. Satellites broadcast. Each GPS satellite carries atomic clocks and repeats two facts by radio: where it is, and the exact time the signal left.
2. 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.
3. 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.

- **Say it back:** Four satellites, four clocks, and your phone just listens.
- **Topics:** GPS, how GPS works, satellites, atomic clocks, trilateration, positioning, navigation