Magellan NAV 5000, User Manual

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THE GPS SYSTEM
The Global Positioning System (GPS) is a highly accurate, worldwide
navigation and positioning system that can be used 24 hours a day.
Designed at the impetus of the US Department of Defense and primari-
ly for military use, GPS is available to a variety of users worldwide,
including recreational boaters, fishing and shipping fleets, general and
commercial aviation, surveyors, and engineers.
The system is based on a constellation of satellites that, when com-
plete, will consist of 21 satellites and 3 working spares orbiting the
Earth twice a day in six orbital planes. Each satellite is in a fixed orbit
approximately 10,900 nautical miles above the Earth, and inclined at
55 degrees from the equator.
Data Transmission
Each satellite continuously transmits two types of orbit data: Almanac
and Ephemeris. Almanac data contains the health and approximate
location of every satellite in the system. Ephemeris data contains the
precise orbital parameters of each satellite. A GPS receiver gathers
Almanac data from any available satellite; using information from the
Almanac, the receiver then determines which set of satellites will give
the best geometries for a position fix.
The satellites also transmit two codes. The encrypted code (P-Code) is
the more accurate of the two, and is reserved for military use.
The unprotected code (Coarse Acquisition, or C/A Code) is intended
for public access. lt is also used to determine the precise range of the
user from each satellite, which is the first step in calculating a position
fix.
All transmissions from the satellites are in real time.
How a Position Fix is Obtained
Navigation with the Global Positioning System and a well-designed
GPS receiver is very simple. The receiver uses data collected from
three or four satellites to solve a fundamental geometric equation and
presents it in navigation displays.
First, the unit determines which satellites to use to obtain the position
fix and then the receiver obtains Ephemeris data from those satellites.
(Three satellites are used in 2D; a fourth satellite is used in 3D to
determine altitude.)