Global Positioning System - Magellan Trailblazer Reference Manual

WHAT IS GPS?
GPS is a constellation of navigation satellites that orbit the earth. The precise
ti me and position information transmitted by these satellites is used by a GPS
receiver to triangulate a position fix.
The system is expected to be completed during 1994, and will provide
continuous, 24-hour 3D (position plus elevation) coverage anywhere on the
earth.
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 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 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: this is ranging.
Once the receiver has computed range for at least three satellites, its location
lite. A receiver with a current almanac in its memory knows where in the sky
to look for satellites, given its last known position and the time of day.
Ephemeris data is the precise satellite positioning information that is used for
ranging. Each satellite transmits its own ephemeris data.
30
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 in-
formation, an almanac can
be collected from any satel-