How Does Gps Work - Magellan Trailblazer XL Reference Manual

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 trian-
gulate a location fix.
The system is now officially declared "initial operational", and provides continuous,
24-hour 3D (location plus elevation) coverage anywhere on the earth.
GPS was developed by the United States Department of Defense to provide consis-
tent, 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 commer-
cial uses were recognized. The satellites therefore transmit two codes, a military-only
encrypted code (PPS) and a civilian-access, Standard Positioning Service (SPS) code. All
commercial and consumer GPS receivers are SPS receivers.

HOW DOES GPS WORK?

Each GPS satellite transmits its precise position (location and elevation) and the start
time of the transmission. A GPS receiver acquires the signal, then measures the inter-
val between transmission and receipt of the signal to determine the distance between
the receiver and the satellite: this is ranging. Once the receiver has computed range
for at least three satellites, its location on the surface of the earth can be determined.
and the time of day. Ephemeris data is the precise satellite locating information that is
used for ranging. Each satellite transmits its own ephemeris data.
Global Positioning System
Every satellite transmits two types
of data, almanac and ephemeris. Al-
manac data is general information
on the location and health of each
satellite in the consteliation. Since
it contains general information, an
almanac can be collected from any
satellite. A receiver with a current
almanac in its memory knows
where in the sky to look for satel-
lites, given its last known location
61