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Chapter 5 - Understanding Position Coordinates
About loran's
absolute accuracy
Page 5-8
Loran isn't as proficient as GPS in absolute accuracy; that is,
when you're visiting a spot for the first time. Because loran
transmits at a relatively low frequency (100 kHz—similar to
that of an AM radio), your ability to obtain an accurate posi-
tion fix is related to several factors:
•
your distance from the loran transmitters, and their power
•
line geometry
•
weather conditions between you and the transmitters
In these areas, GPS consistently excels when it comes to abso-
lute accuracy. Even with selective availability turned on, GPS
provides an absolute accuracy of about 300 feet, 95 percent of
the time, almost everywhere in the world, under virtually all
conditions—far surpassing the 600-foot absolute accuracy of a
good loran receiver under ideal conditions.
If you're used to relying on a loran receiver's repeatable accu-
racy to return you to within one vessel's length of a lobster
trap or buoy time after time, you won't necessarily see this
same performance with uncorrected GPS: You could be as far
as 600 feet away from your expected position. This is espe-
cially apparent when using the 961's CHART screen, where
you may see your vessel's plotted position appear to wander
within an area up to 600 feet in diameter. See Figure 4, "Posi-
tion variances with uncorrected GPS," below. When using the
-nm chart scale, this 600-foot area covers almost the full
screen.
961 Operations and Reference Manual Rev. D
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