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ate software. These MapCreate custom maps contain much greater de-
tail than the basic background map. These Custom Map Files (file
format *.lcm) not only may be shared between the LMS-334c iGPS and
339cDF iGPS, but also with other Lowrance GPS and sonar/GPS units
as well as personal computers. (For example, the exact same MMC,
custom map files and GPS data files can be used interchangeably be-
tween your gimbal-mounted unit and the hand-held iFINDER
GPS
receiver.)
Your unit automatically reads Custom removable (like a floppy disk or a
cassette tape), you can store these GPS Data Files on a personal com-
puter equipped with an MMC card reader. (Or store them on a pocketful
of Map Files directly from the MMC or SD card. To use a custom map,
all you need to do is slide an MMC containing a map into the unit.
Introduction to GPS and WAAS
Well, now you know the basics of how your unit does its work. You
might be ready to jump ahead to Section 2, Installation & Accessories,
on page 13, so you can mount your unit and plug in the power. Or you
might want to see how our text formatting makes the manual tutorials
easy to skim. If that's the case, move on to "How to Use This Manual"
on page 11. But, if you want to understand the current state of satellite
navigation, look over this segment describing how GPS and its new
companion WAAS work together to get you where you're going.
The Global Positioning System (GPS) was launched July 17, 1995 by
the United States Department of Defense. It was designed as a 24-
hour-a-day, 365-days-a-year, all weather global navigation system for
the armed forces of the U.S. and its allies. Civilian use was also avail-
able at first, but it was less accurate because the military scrambled
the signal somewhat, using a process called Selective Availability (SA.)
GPS proved so useful for civilian navigation that the federal govern-
ment discontinued SA on May 2, 2000, after the military developed
other methods to deny GPS service to enemy forces. Reliable accuracy
for civilian users jumped from 100 meters (330 feet) under SA to the
present level of 10 to 20 meters (about 30 to 60 feet.)
Twenty-four satellites orbit 10,900 nautical miles above the Earth,
passing overhead twice daily. A series of ground stations (with precisely
surveyed locations) controls the satellites and monitors their exact loca-
tions in the sky. Each satellite broadcasts a low-power signal that iden-
tifies the satellite and its position above the earth.