10–2
10.3 Position-fixing Accuracy
(HDOP)
In radar position-fixing, most accurate posi-
tion fixes are obtained when the targets used
are spaced nearly 90° from each other. Simi-
larly, GPS position fixing accuracy is subject
to satellite location. Generally, the further
apart the satellites are from one another, the
greater the position-fixing accuracy.
For example, take a look at Figure 10-2. In
both situations a fix is obtainable in the North-
ern Pacific region because three satellites are
in line-of-sight. However, accuracy will be
higher in the bottom figure since the satel-
lites are spread farther apart than the satel-
lites in the top figure.
Figure 10-2 Satellite positions and accuracy
of position fix
The index for position-fixing accuracy is
known as HDOP (Horizontal Dilution of Pre-
cision) for 2D mode or PDOP for 3D mode.
In simpler terms it is the geometrical relation-
ship among 3 (or 4) satellites. The higher the
HDOP value the less accurate the position fix.
The error in distance is proportional to the
HDOP value as shown in Figure 10-3.
Figure 10-3 HDOP rate and position error
Note: In this manual HDOP is referred to as DOP.
3 6 9 12
60 m
45 m
30 m
15 m
DOP
ERROR (RMS)
LOW ACCURACY
HIGH ACCURACY
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