Furuno GP-1610CF Operator's Manual page 56

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 4-2. In both
situations a fix is obtainable in the Northern
Pacific region because three satellites are in
line-of-sight. However, accuracy will be
higher in the bottom figure since the satellites
are spread farther apart than the satellites in
the top figure.
Low accuracy
High accuracy
Figure 4-2 Satellite positions and
accuracy of position fix
4-2
The index for position-fixing accuracy is
known as HDOP (Horizontal Dilution of Pre-
cision). In simpler terms it is the geometrical
relationship among 3 (or 4) satellites. The
higher the HDOP value the less accurate the
position fix. The error in distance is propor-
tional to the HDOP value as shown in Figure
4-3.
ERROR (RMS)
60 m
45 m
30 m
15 m
3
Figure 4-3 HDOP rate and position error
In this manual HDOP is referred to as DOP.
DOP
6 9 12