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Rev. D 1 — Introduction
1 — Introduction1 — Introduction
1 — Introduction Page 7
“beacons,” the transmitters are being situated at various locations along
the U.S. coastline and Great Lakes, most of which are former marine ra-
diobeacon sites.)
With DGPS, navigators are guaranteed position accuracy on the order of
2-5 meters
50% of the time, and to 10 meters 95% of the time (usually,
much better). Lat/lon positions displayed to thousandths of minutes
(roughly 6 feet) are common with DGPS. When using a radar or elec-
tronic plotter, DGPS corrections improve the steadiness of your dis-
played plot line dramatically.
In addition to accurate positioning information, DGPS provides critical
“integrity monitoring” of the received GPS signals, enabling the user to
be notified in seconds (as opposed to hours, with uncorrected GPS) that
an anomaly has been detected in a transmitted satellite signal. The integ-
rity monitor instructs the DGPS receiver not to use data from the satel-
lite in question.
Loran, on the other hand, can still outperform non-differential GPS—
when SA is in effect—in
repeatable accuracy
. Though susceptible to
weather and characterized by a relatively limited range (when compared
to GPS), it’s a dependable system familiar to thousands of boat owners.
Since the 1970’s, LORAN users have relied upon that system’s ability to
repeatedly get them back to a previously-saved location with extreme
accuracy. Fishermen 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 to within 50 feet
(or roughly 14 meters) of your previously-visited target. On the other
hand, at its very best, uncorrected GPS with SA enabled will get you to
within
300 feet
(100 meters) of your intended destination. Obviously,
uncorrected GPS—with variations of up to 300 feet in displayed posi-
tion—wouldn’t be much help in getting you through a narrow channel.
Since most loran chains were configured for optimum coastline naviga-
tion, it’s easy to see why loran’s
repeatable
accuracy has served harbor-
masters and fishermen well for the last 25 years.
loran