Seiwa Techmarine, User Manual

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method. Excellence in GPS's position-fixing accuracy is mainly due to
this technology.
3. When the satellite locations and their distances are known, the GPS
receiver fixes its own position by triangulation:
Fig. 6.1a - 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.
6.1.1 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:
Fig. 6.1.1 - 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.