09/25/02 GPS_field_guide 8:19 A9/P9
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3. Repeats the operation mentioned under point 2 with a third satellite. In our example
the receiver finds out that it is 20,000 km from the third satellite which narrows its
position down even further, to the two points (Figure 5 point b) where the 20,000 km
sphere cuts through the circle reported on Figure 4 (corresponding to the intersection
of the first two spheres).
Figure 5 - Potential locations of the GPS receiver with 3 satellites signals (b)
Most of the time one of these two points is absurd and is rejected by the receiver that
is then able to give the exact location. In some cases, a fourth satellite is necessary to
know which of the two points is the correct one. In any case, a configuration with 4
satellites is better in order to increase the accuracy and/or reduce the sources of error.
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