**DEFINITION**:

Using the fixed stars, you can determine your position anywhere on the Earth’s surface to within a few hundred metres, provided the weather is fine. But navigation satellites can tell you where you are to the nearest few metres or better, whatever the weather.

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 center 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 kilometers above the Earth’s surface. In practice, a fourth satellite is needed to synchronize 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.

It is only possible to determine a location on Earth if you know the location of the navigational satellites very precisely. This is achieved by placing the satellites in highly stable Medium Earth Orbits (MEOs) at an altitude of about 22 000 kilometers. MEOs are the orbits of choice for a number of reasons: their stability enables exact orbit predictions; the satellites travel relatively slowly and so can be observed over several hours, and the satellites can be arranged in a constellation so that at least four are visible from any point on the Earth’s surface at any time.

Benefits from satellite navigation data are well known by everybody since every smart phone incorporates a navigation sensor and associated applications.

(Source: PAE own elaboration from ESA’s web page: http://www.esa.int/Our_Activities/Navigation/About_satellite_navigation2 )

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