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The exactness of the position ranges from 10 to 15 m. Since the 2,May 2000, it can be used by everybody worldwide.
In excellent conditions the precision increases to 3m.
The American military can totally block civil use or can limit it in crisis zones!
Disturbances of the receiver:
Signals of satellites are straightly broadcast. However, interferences and inaccuracies are caused by valleys, mountains, buildings and tunnels. Metal, e.g. car roofs, are disruptive elements.
Orientation of satellites:
Generally speaking, for optimal use you have to get the geometry of the satellites in correct order. On the one hand this means that the satellites must not be too close together, because the signal's intersection can't get fixed clearly. On the other hand, the distance between the satellites mustn't get too far, as the signals can be disturbed through shadows.
The measuring of the altitude is 1.7 times more worse than the horizontal line in fixing the position.
The reason for that is the earth's atmosphere, because radio signals are being refracted.
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