Figure 1-Max. Position Variances With Uncorrected Gps - NorthStar 941X Reference Manual

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Loran, however, isn't as proficient as GPS in absolute accuracy , or visit-
ing a spot for the first time. Since it transmits at a relatively low fre-
quency (100 kiloHertz—similar to that on your AM radio), your ability to
get an accurate position "fix" is directly related to: 1) your distance from
the loran transmitters, and 2) the weather between you and the trans-
mitters. It's in these two areas that GPS consistently excels when it
comes to absolute accuracy.
Even with SA, GPS provides an absolute accuracy of around 300 feet
nearly everywhere in the world, under virtually any conditions. With a
"constellation" of satellites orbiting the earth and transmitting their po-
sition data on an extremely high frequency (1.575 GigaHertz, or 1.5 bil-
lion cycles per second), GPS is impervious to weather and your location
on the surface of the earth. You can receive position fixes at any time,
anywhere, in any weather. This far surpasses the absolute accuracy that
is achievable from a good loran receiver in a strong signal coverage area,
in favorable weather conditions: usually 600 feet.
If you are accustomed to using your Northstar loran's repeatable accu-
racy to return to within a boat length of a lobster trap or a buoy, time af-
ter time, you will not necessarily see this same performance with
uncorrected GPS—you could be as much as 600 feet from your expected
position. This will be especially apparent when using the 941X's plotter
screen (without DGPS), where you may see your boat's plotted position
appear to wander around within an area up to 600 feet in diameter (see
Figure 1 below).
Position circle of 300-foot radius,
showing possible 600-foot error
ACTUAL
40°00.000 N
72°00.000 W
Figure 1—Max. Position Variances with Uncorrected GPS
1 — Introduction 1 — Introduction
1 — Introduction 1 — Introduction
between fixes.
300'
You could be
anywhere within
the circle, 95% of
the time.
300'
Rev. D