NorthStar 961 Reference Manual page 57

Chapter 2 - Introducing the 961/962
a source of accurate targeting data against U.S. interests.)
When it was enabled, SA reduced GPS's absolute and
repeatable accuracy from a maximum of 30 meters (better
than 100 feet) 95 percent of the time, to 100 meters (better
than 330 feet) 95 percent of the time. However, this accuracy
didn't meet the Coast Guard's requirement of 8-20 meters for
navigating in harbors, so the Coast Guard developed a system
of differential corrections, or differential GPS (DGPS) to allow
them to effectively circumvent SA's built-in errors. SA was
officially ended early 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: Mariners who need better than
100 meters probably will also need better than 30 meters.
The majority of mariners use GPS in conjunction with a chart
Differential GPS
plotter, and for any given position solution there is an error
corrections
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 the error in the
resulting displayed position fix. 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 is 30 meters out of place!
Although this magnitude of error won't always be the case, the
prudent navigator should choose the maximum margin of
safety. Furthermore, the precise location of bottom features for
fishing and diving is significantly degraded without 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.
These differential corrections are generated by fixed reference
stations, and then broadcast by marine radiobeacons over a
relatively limited area—usually a maximum broadcast radius of
250 miles in any direction—to differential-capable GPS (DGPS)
receivers. (The exact geographic positions of these fixed
reference stations are used to calculate the position
corrections, which are based on GPS data received from the
earth-orbiting GPS satellites.) These corrections are broadcast
to the DGPS receivers, which convert this data into extremely
precise position, heading, and speed data. Referred to as
961/962 Operations and Reference Manual, Rev. A Page 2-13