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LORAN TD System
LORAN C is a radio navigation aid operated and maintained in the United States by the United
States Coast Guard. The name LORAN is an acronym for “LOng RAnge Navigation”. The LORAN
system covers the entire United States and the U.S. Coastal Confl uence Zone. From the perspective of
a mariner, the system is used for ocean and coastal navigation. It can be used as a supplemental system
for harbor and harbor approach navigation, and it is used for inland navigation by recreational vehicles.
LORAN TD Feature
The LORAN TD (Time Delay) feature eases the transition from using LORAN to using GPS. The
GPSMAP unit automatically converts GPS coordinates to LORAN TDs for those who have a collec-
tion of LORAN fi xes for favorite fi shing spots and other waypoints recorded as TDs. You can display
your position as a TD or enter waypoints as TDs. The accuracy to be expected from this conversion is
approximately thirty meters. When the unit is placed in the LORAN TD format mode, it simulates the
operation of a LORAN receiver. Position coordinates may be displayed as TDs, and all navigation func-
tions may be used as if the unit was actually receiving LORAN signals.
Using the LORAN TD Format
When creating new waypoints using LORAN TD coordinates, you must set the correct LORAN
chain number and secondary stations in the Setup TD fi eld before storing the waypoint. After the
waypoint is stored in unit memory, it will always reference the LORAN chain number and secondary
stations currently selected in the Setup TD fi eld. If you enter a different LORAN chain number, change
the secondary stations or offsets in the Setup TD fi eld, the active waypoint information will refl ect those
changes. Since the GPSMAP 176 does not rely on the LORAN signal for navigation, it can reference a
different GRI chain and/or secondary stations and still navigate to the location stored in memory.
The LORAN Position Format fi eld is located under the Units tab in the Main Menu. The ‘LORAN
TD Setup’ window contains the fi elds to select the Loran GRI-Chain Number, Primary and Secondary
Stations, and TD Offsets.
LORAN TD Setup
Appendix G
190-00185-10_0A.indd 66 6/18/2002, 10:01:15 AM