How does it work?

To pinpoint your location accurately, your receiver needs to receive signals from at least three navigational satellites. The receiver determines your distance from each of the satellites by measuring the time taken by the signal to travel from the satellite to your receiver antenna.

Distance measurements from two satellites tell you that you are situated somewhere on the circle where two spheres intersect. The spheres each have one of the two satellites at their centre and their radii are the satellite-receiver distances. Knowing your distance from a third satellite fixes your position at one of the two points where the circle intersects the third sphere. One of the intersection points can usually be discounted – for instance, it may be thousands of kilometres above the Earth’s surface.

In practice, a fourth satellite is needed to synchronise your receiver’s clock with a common time standard which is strictly adhered to by the clocks on board all the satellites. The use of a fourth satellite also resolves the position ambiguity that occurs with only three satellites. In general, the more satellites used, the greater the positioning accuracy. Many receivers have channels for receiving signals from up to 15 satellites.

Last update: 3 August 2011

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