# The science behind GPS

### The distance of the satellites

Velocity x Time = Distance and in a vacuum radio waves travel at the speed (velocity) of light: 299,792,458 meters per second. So it would seem quite simple to calculate the distance of the receiver from the satellite, except for an additional complication: timing.

If a satellite were right overhead, the journey of its signal to the receiver would take about 0.06 seconds. Imagine the satellite and receiver both starting to play a song at exactly the same time (and that standing by the receiver you could hear both songs). Your hearing of the version playing at the satellite is delayed by the time it takes it to travel to the receiver. So at the navigation device you hear two versions of the song, one running fractionally behind the other.

To work out exactly how delayed the satellite's version of the song is, you could delay the receiver's version until the two songs fall into perfect sync. The amount of time by which we need to delay the receiver's version to achieve that synch is the journey time of the satellite's version of the song.

Essentially, this is how GPS works. Only instead of a song, the satellites and receivers use a clever little trick called "Pseudo Random Noise Code".

Pseudo Random Noise Code
The Pseudo Random Noise Code is simply a digital code (or sequence of on/off pulses) that is so complicated that its signal could be mistaken for random electrical noise. There are three main reasons for the code being so complicated:

1. To ensure the navigation device doesn't accidentally sync up with another signal, as no stray signal is likely to have the same pattern.

2. Each satellite has its own unique pseudo-random code. So your receiver doesn’t accidentally pick up another satellite's signal and the satellites can all operate on the same frequency without jamming each other.

3. The complex pattern of the code allows your navigation device to calculate the duration of the signal delay very accurately.

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