NorthStar 951X Operator's Manual page 37

Loran
951/952 Operator's Manual Rev. B
tor instructs the DGPS receiver to switch automatically to
other available satellites.
The 951XD/952XD contains an internal, fully automatic differ-
ential receiver. The 951X/952X doesn't have an internal differ-
ential receiver, but can accept corrections from an external
differential receiver, such as the Northstar 8401 or 8800.
Loran can outperform non-differential GPS in repeatable accu-
racy. Although susceptible to weather and limited to coastal
use, it's a dependable system familiar to thousands of boaters.
Since the 1970's, loran users have relied on this system's abil-
ity to repeatedly return them to a previously saved location
with extreme accuracy. Fishermen have especially appreciated
this capability because 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.
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 opti-
mum coastline navigation, it's easy to see why loran's repeat-
able accuracy has served harbormasters and fishermen so well
for the last 25 years.
Loran, however, isn't as proficient as GPS in absolute accuracy,
or when visiting a spot for the first time. Because loran trans-
mits at a relatively low frequency (100 kHz—similar to that of
your AM radio), your ability to get an accurate position "fix" is
directly related to:
•
your distance and line geometry from the loran transmit-
ters
•
the weather between you and the transmitters
In these two areas GPS consistently excels when it comes to
absolute accuracy.
Chapter 2 - Introducing the Northstar 951/952
Page 27