Lowrance 5150c 操作指南

6 

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 5150c 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 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. 

 

The system requires signal reception from three satellites in order to 

determine a position. This is called a 2D fix. It takes four satellites to