NorthStar 958 Operator's Manual page 182

Chapter 12 - Setting Up Your 958
Choosing beacon
frequency and baud
Page 174
Differential corrections are invaluable for navigating in narrow
or congested areas, such as ports and waterways, or whenever
you require extreme accuracy in order to locate a precise area
rate
(retrieving lobster pots, returning to a diving spot, and so on).
You can interface with 958 with an optional external Northstar
beacon receiver. This receiver takes corrections broadcast by
shore-based marine radiobeacons situated along most of the
U.S. coastline, then the 958 uses these corrections to improve
its position accuracy to four meters or better—with one to
three meters accuracy common. This two-channel receiver
automatically tunes to the appropriate differential frequency
as you move from one differential coverage area to another, or
as weather conditions change. The receiver constantly
evaluates the quality of the radiobeacon signals, selecting only
the best station. As you travel, the receiver will switch to
another frequency automatically as conditions require.
Automatic mode
Automatic mode can usually be used for all applications. The
958 enters automatic differential mode as soon as it receives
corrections, and requires no further supervision of differential
operation. It maintains automatic operation by using two
independent receiver channels. Channel 1 tracks the best
available differential signal (the one with the lowest data error
rate) and sends the demodulated DGPS corrections to the
unit's GPS receiver. Channel 2 continuously scans the entire
differential frequency band (283.5 to 325kHz), locating and
measuring received DGPS differential signals. The 958 then
stores this information as a DGPS differential "directory" in its
battery-powered memory.
If the signal that's received and monitored by Channel 1
degrades for any reason (such as bad weather between your
vessel and the transmitter), the 958 tries to select a better
frequency from the directory it has compiled. This function
allows it to switch to the best differential signal before your
GPS accuracy is affected negatively.
958 Operator's Manual, Rev. A