Lowrance electronic Lowrance GlobalMap 3500C - Manual (Page 12)

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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
containing a map into the GlobalMap 3500C.
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 3500C and plug in the power. Or
you might want to see how our text formatting makes the manual tuto-
rials easy to skim. If that's the case, move on to "How to Use This Man-
ual" on page 8. But, if you want to understand the current state of sat-
ellite 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 scram-
bled 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, pass-
ing overhead twice daily. A series of ground stations (with precisely sur-
veyed locations) controls the satellites and monitors their exact 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.