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Chapter 5 - Understanding Position Coordinates
Using DGPS to determine position
Using DGPS to
increase repeatable
accuracy
DGPS-in-use
indicator
DGPS status screen
Page 5-4
ference, all of which affect accuracy. HDOP can range from an
ideal value of slightly less than one up to a poor value of 10 or
more; any value less than two indicates excellent performance.
For a higher level of repeatable accuracy than basic GPS, you'll
want to use differential GPS, or DGPS.
The 961XD includes an internal differential receiver, and uses
DGPS whenever signals are available. To use DGPS with a
961X, you must convert your Northstar 961X to a Northstar
961XD by adding an internal differential receiver.
DGPS corrections remove the errors caused by Selective Avail-
ability (SA) and atmospheric variations, and provide an accu-
racy that can exceed that which is obtained by military users
of the GPS system. With good DGPS corrections, your 961
should provide better than four-meter accuracy (repeatable
and absolute) 95 percent of the time, SOG readings accurate to
0.1 knot, and COG measurements accurate to about 0.5
degree. For more information about GPS, see "Using GPS to
determine position" starting on page 5-2.
When using DGPS as your navigation source, your 961's position
coordinates may actually exceed the accuracy of some charts.
The DGPS indicator appears as the word DGPS in the upper
left-hand corner of the CHART screen. When the 961 displays
this indicator, it means that you're using DGPS's higher level
of accuracy for received data. When you require this addi-
tional accuracy, check for the presence of this indicator, since
its absence means DGPS isn't in use.
To display more information about received DGPS corrections,
press the STAR key to display the SERVICE MENU screen,
then press the GPS STATUS key. The 961 displays the GPS
SATELLITE STATUS screen, showing the DGPS differential
NOTE:
961 Operations and Reference Manual Rev. D
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