Chapter 5 - Determining Your Position
957 Operator’s Manual, Rev. B Page 55
• ACCURACY: The estimated accuracy--in feet--of the GPS
system to be expected at the current time. Your position
readings should be more accurate than this estimate 95
percent of the time.
• HDOP: This is a technical measure of the quality of your
fix. HDOP (Horizontal Dilution of Precision) is calculated
from the satellites’ current positions. You’ll usually see an
HDOP value displayed immediately after the 957 is
turned on, and before it’s ready to navigate. HDOP can
range from an ideal value of slightly less than one, up to
poor values of 10 or more. Any value less than two indi-
cates excellent satellite configuration.
Using DGPS
When using the 957 with optional radiobeacon DGPS , you’ll
usually see position measurements accurate to about one to
three meters, speed measurements accurate to about 0.1 knot,
and COG measurements accurate to about 0.5 degree. DGPS
corrections remove most of the errors caused by atmospheric
variations.
NOTE:
When you’re using DGPS as your navigation source, your position coordi-
nates, as calculated by the 957, may actually exceed the accuracy of
some charts.
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