Magellan GPS Tracker - Manual (Page 84)

74 Magellan GPS Tracker
What is GPS?
What is GPS?
GPS is a constellation of navigation satellites that orbit the earth. The
precise time and position information transmitted by these satellites is used
by a GPS receiver to triangulate a position fix.
GPS was developed by the United States Department of Defense to provide
consistent, reliable navigation information that is unaffected by rough terrain
and bad weather, and is highly resistant to multipath errors and interference.
The DoD continues to administer and control the Global Positioning
System.
Although GPS was developed as a military navigation system, its civilian and
commercial uses were recognized. The satellites transmit two codes, a
military-only encrypted code (PPS) and a civilian-access, Standard Position-
ing Service (SPS) code. All commercial and consumer GPS receivers are SPS
receivers.
How Does GPS Work?
Each GPS satellite transmits its precise location (position and elevation) and
the start time of the transmission. A GPS receiver acquires the signal, then
measures the interval between transmission and receipt of the signal to
determine the distance between the receiver and the satellite, a process called
ranging. Once the receiver has computed range for at least three satellites, its
location on the surface of the earth can be determined.
Each satellite transmits two types of data, almanac and ephemeris. Almanac
data is general information on the location and health of each satellite in the
constellation. Since it contains general information, an almanac can be
collected from any satellite. A receiver with a current almanac in its memory
knows to look for satellites, given its last position and the time of day.
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