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Chapter 5 - Understanding Position Coordinates
differential receiver. Other brands of differential receivers may
supply DGPS corrections, but they omit the additional signal
information shown on the 961's screen.
Understanding Loran-C
Loran is an acronym for Long Range Navigation, a system of
What is loran and
land-based radio transmitters that give off, or emit, precisely
loran TDs?
synchronized pulses. These pulses are then picked up, or
received, by a loran receiver. The relatives times of arrival of
the pulses are called time differences, or TDs. Charts in loran
areas usually show TD lines of position.
Although loran is susceptible to weather conditions, and lim-
About loran's
ited to coastal and inland use, it's a dependable system familiar
repeatable accuracy
to thousands of experienced mariners. Loran easily outper-
forms uncorrected GPS in repeatable accuracy. Since the 1970s,
loran users have relied on its ability to repeatedly return them
to previously saved locations—with extreme accuracy. This
capability has been especially appreciated by captains of com-
mercial fishing vessels, since each subsequent visit to a lobster
trap or buoy would be virtually on the mark, saving valuable
time and money. During that time, loran's repeatable accuracy
was superior to anything else commercially available.
In areas with very good coverage (dependent on a host of fac-
tors), loran typically positions your vessel to within 14 meters,
or approximately 50 feet, of a previously visited target. On the
other hand, uncorrected GPS with selective availability turned
on can be counted on to position your vessel to within only
about 100 meters (approximately 300 feet) of your intended
destination. Obviously, uncorrected GPS isn't much help in
finding a mark in dense fog.
Because most loran chains (a group of loran transmitters) were
arranged for optimal coastline navigation, it's easy to see why
loran's repeatable accuracy has served harbormasters and fish-
ing vessels so well for such a long time.
961 Operations and Reference Manual Rev. D
Page 5-7
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