NorthStar 957 Operator's Manual page 167

These corrections are then broadcast to DGPS receivers, which
then convert this data into extremely precise position,
heading, and speed information. Referred to as "beacons," the
transmitters are situated at various locations along the U.S.
coastline and Great Lakes. Most of them are former marine
radiobeacon sites.
When using beacon differential through an optional external
Northstar radiobeacon receiver, the 957 provides an accuracy
of one to three meters 50 percent of the time, and an accuracy
of four meters 95 percent of the time. Lat/lon positions are
displayed to thousandths of minutes (approximately six feet).
In addition to accurate positioning information, beacon
differential provides critical integrity monitoring of these
received GPS signals, enabling a satellite problem to be
corrected in seconds, as opposed to hours using uncorrected
GPS. The integrity monitor tells the DGPS receiver to switch
automatically to other available satellites.
What is Loran?
Although susceptible to weather, and limited to coastal use,
loran is a dependable system familiar to thousands of
mariners. Since the 1970's, loran users have relied on its ability
to repeatedly return them to a previously saved location with
extreme accuracy. Fishermen have especially appreciated this
capability when each subsequent visit to a lobster trap or buoy
was virtually "on the mark," saving them valuable time and
money. Loran's repeatable accuracy was superior to anything
else commercially available at the time, and still outperforms
non-differential GPS in repeatable accuracy.
In very good coverage areas, loran can typically get you within
50 feet (or approximately 14 meters) of your previously visited
target. On the other hand, uncorrected GPS can only be
counted on to get you to within about 100 feet (20-30 meters),
maximum, of your intended destination. Obviously then,
uncorrected GPS wouldn't be as much help in finding a mark
in dense fog. Since most loran chains were configured for
optimum coastline navigation, it's easy to see why loran's
repeatable accuracy has served harbormasters and fishermen
so well over the last 25 years.
957 Operator's Manual, Rev. B Page 159