A satellite navigation receiver determines its position by measuring the distance between it and at least four navigation satellites in orbit around the Earth. The distances are measured by timing how long radio waves take to travel from the satellites to the receiver.
If a navigation receiver measures its distance from one satellite, the receiver position must lie on a sphere with a radius equal to the measured distance and with its centre located at that satellite. Using two satellites places the receiver position on a circle where two spheres, one centred on each satellite, intersect. Adding a third measurement fixes the receiver at one of two points where the two-satellite circle and the third sphere meet.
Three measurements might seem sufficient, since one of the two points can usually be ignored because it is not on the surface of the Earth, or is moving too quickly. However, adding a fourth satellite measurement not only resolves the two-point ambiguity, it also enables the receiver to derive its time reference from the clocks on the satellites, in effect using four spheres to locate the receiver in four dimensions – the three dimensions of space, plus time.
Distance measurement is performed by timing how long radio signals broadcast by the satellites take to reach the receiver. The satellites transmit very accurate time signals, derived from on-board atomic clocks. The receiver compares the signal arrival time with the satellite time, giving the transit time. When the transit time is multiplied by the speed of the radio waves, this gives the distance to the satellite.
From the advent of celestial navigation using sextants and chronometers, accurate timekeeping has been the key to navigation. Modern navigation technology, with its increased precision derived from extremely accurate clocks, has transferred the timekeeping task from the navigator to the navigation satellite service provider.