Lowrance 7200c 操作指南

5 

9, 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 7. 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 the federal government 

discontinued SA on May 2, 2000, after the military developed other 

methods to deny GPS service to enemy forces. Reliable accuracy for ci-

vilian 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. 
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 

determine both position and elevation (your height above sea level — 

also called altitude). This is called a 3D fix.