Position Fixing Accuracy: Hdop - Interphase Chart Master Chart Master 11 CVS+ User Manual

2. In this receiving process, the GPS receiver measures very accurate
distances to the satellites, using the "Spread Spectrum Modulation"
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:
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 accu-
racy 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:
The index for position-fixing accuracy is called HDOP ("Horizontal Dilution Of Pre-
cision", see also Chapter 7 for the HDOP definition).
The HDOP value indicates higher position accuracy. Ranges of 1.0 to 1.2 provide
the highest degree of accuracy. Higher values indicate less position accuracy. This
is due to the satellites being gathered in a narrow space. Satellites which are
widely scattered in the sky produce the highest degree of accuracy.
User Manual
d2
d1
Fig. 1.3a - The GPS position calculation
Fig. 1.3b - HDOP
d3
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