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These Custom Map Files (file format *.lcm) can also be shared be-
tween Lowrance GPS or sonar/GPS units and personal computers. This
unit automatically reads Custom Map Files directly from the MMC or
SDC. To use a custom map, all you need to do is slide an MMC contain-
ing a map into the GlobalMap.
Introduction to GPS and WAAS
Well, now you know the basics of how the unit does its work. You might
be ready to jump ahead to Section 2, Installation & Accessories, on page
11, so you can mount your GlobalMap 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 8. 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 available at first, but it was less accurate because
the military scrambled the signal somewhat, using a process called Se-
lective 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 pre-
cisely surveyed locations) controls the satellites and monitors their ex-
act locations in the sky.
Each satellite broadcasts a low-power signal that identifies the satellite
and its position above the earth. Three of these satellites are spares,
unused until needed. The rest virtually guarantee that at least four
satellites are in view nearly anywhere on Earth at all times.
6
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