Lowrance electronic IFINDER PRO - Manual (Page 14)

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iFINDER automatically reads Custom 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 iFINDER does its work. You
might be ready to jump ahead to Section 2, Installation & Accessories,
on page 9, so you can install the batteries. 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 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 identi-
fies 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.
A minimum of three satellites are required to determine a 2D fix.