Lowrance LMS-522c iGPS [18/252] Introduction to gps and waas

tom map files and GPS data files can be used interchangeably between

your gimbal-mounted unit and the hand-held iFINDER™ GPS re-

ceiver.)

Your 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 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 10. 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, 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.