Measuring range to satellite

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Ranges between the receiver and the satellites are measured.

Each satellite contains a very accurate clock (actually four very accurate atomic clocks) and the clock is used to generate a unique coded signal for each satellite.

The receiver on the ground generates the same coded signal at the same time and compares the received code with the one being generated.

The time difference between the two codes gives the time of signal travel between the satellite and receiver and:

range to satellite (D) = signal travel time (dt) x speed of light

A range measured in this way is often called a Pseudorange. Pseudo because not all the errors in the measurement are taken into account and therefore the range measured is not the true one.