Lowrance LMS-350A, Инструкция по установке и эксплуатации

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GPS - HOW IT WORKS
The Global Positioning System (GPS) is the best approach to navigation
that has ever been devised. Conceived by the Department of Defense
(DOD) and the United States military, the GPS system is an answer to
their needs of 24 hour global positioning, 365 days a year.
Basically, the system works by using a constellation of satellites orbiting
Earth 11,000 miles in space. There will be 21 satellites in orbit when the
system is fully operational. Three more satellites will act as spares, for a
total of 24. When all satellites are in place, at least four of them will be in
view nearly anywhere on Earth twenty-four hours a day. The GPS receiver
requires at least three satellites to give a “2D” fix. (A 2D fix is your position
in latitude/longitude. A 3D fix is your latitude/longitude plus altitude.) When
it locks on to at least four satellites, it displays a 3D fix.
As the receiver locks on to each satellite, it calculates the distance from
the satellite by measuring the length of time it takes the radio signal to
reach it. Each satellite has an extremely accurate clock that tells the
receiver when the radio transmission started. The receiver compares that
time against its own clock, thus it knows how long it took the radio signal
(travelling at the speed of light!) to reach it. If you know time and speed,
then you can calculate distance. Once you have this from three satellites,
then the receiver can determine position.
It takes three satellites to determine position.