GPS WORKS

The actual principle of GPS is very easy to appreciate, since it is exactly the same as traditional “triangulation” (although this is not quite correct, as GPS does not use angles). If one imagines an orienteer needing to locate them on a map, they first need to be able to find at least three points that they recognize in the real world, which allows them to pinpoint their location on the map. They can then measure, using a compass, the azimuth that would be needed to take them from the point on the map to their current position. A line is then drawn from each of the three points, and where the three lines meet is where they are on the map.

Translating this into the GPS world, we can replace the known points with satellites, and the azimuth with time taken for a signal to travel from each of the known points to the GPS receiver. This enables the system to work out roughly where it is located - it is where the circles representing the distance from the satellite, calculated on the basis of the travel time of the signal, intersect. Of course, this requires that the GPS locator has the same coordinated time as the satellites, which have atomic clocks on board. To do this, it cross-checks the intersection of the three circles with a fourth circle, which it acquires from another satellite.

If the four circles no longer intersect at the same point, then the GPS system knows that there is an error in it’s clock, and can adjust it by finding one common value (one second, half a second and so on) that can be applied to the three initial signals which would cause the circles to intersect in the same place. Behind the scenes, there are also many complex calculations taking place which enable the system to compensate for atmospheric distortion of the signals, and so forth, but the principle remains the same.