Furuno GD-280, Руководство оператора

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10-2
Position-fixing accuracy (HDOP)
In radar position-fixing, most accurate position fixes are obtained when the targets used are
spaced nearly 90° from each other. Similarly, 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 the figure below. 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
The index for position-fixing accuracy is known as HDOP (Horizontal Dilution of Precision). 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 proportional to the HDOP value
as shown in the figure below.
Time (90% of the time)
50 m
DOP
1 2 3 4
Note: Built-in GPS receiver's position fixing error under ideal receiving conditions.