Si-tex NAVSTAR 10, Operation Manual

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NAVSTAR 10/12 series Introduction
[The GPS position calculation]
As illustrated in the previous figure, the position is calculated as the meeting point of three
spheres, which are drawn around the three satellites with diameters d1, d2 and d3.
Position Fixing Accuracy: HDOP
The GPS fix accuracy is due to the locations of 3 satellites in the sky. High accuracy is obtainable
when the satellites are widely scattered in the sky; on the
contrary, accuracy is reduced when the satellites have gathered in a narrow space. In the following
figure, in both cases it is possible to obtain the GPS fix, but in the left case the accuracy will be higher
than the right:
[HDOP]
The index for position-fixing accuracy is called HDOP ("Horizontal Dilution Of Precision"). The smaller
the HDOP value, the more accurately the position can be fixed.