NorthStar 958 Operator's Manual page 192

What is Differential
GPS (DGPS)
Page 184
allow them to effectively circumvent SA's built-in errors. SA
was officially ended in the year 2000, but the advantage of
using DGPS over non-SA GPS is still clear and desirable for the
safety of marine navigation. That is, mariners who need better
than 100 meters will probably also need better than 30 meters.
The majority of mariners use GPS in conjunction with a chart
plotter, but for any given position solution there's an error
budget: dilution of GPS precision, errors in cartography from
both government and private sources, datum shifts, latency of
displayed position fix and so on, all compound errors in the
displayed position. For example, imagine a chart plotter at
full-scale zoom showing a channel 200 feet wide (60 meters). A
vessel navigating through that channel may well appear to be
"on the rocks" if it's 30 meters out of place! Although this
magnitude of error won't always be the case, prudent
navigators should always choose the maximum margin of
safety. Furthermore, the precise location of bottom features for
fishing and diving is significantly degraded without some kind
of DGPS, and therefore would usually be of little use unless
there's better than the 100 feet of accuracy (30 meters) of
non-differential GPS.
The 958 offers two kinds of differential corrections:
•
WAAS satellite differential
•
Radiobeacon differential
What is WAAS satellite differential?
One type of differential corrections involves the tracking of
WAAS satellites using Northstar's 2201, a 12-channel GPS/
WAAS pod. The self-contained 2201 provides the 958 with
accuracy of better than 2 meters 2dRMS.
What is radiobeacon satellite differential?
Another type of differential corrections comes from fixed
"reference stations," and is broadcast by marine radiobeacons
over a relatively limited area—usually a maximum broadcast
radius of 250 miles—to DGPS receivers. The precise geographic
958 Operator's Manual, Rev. A