A-1
Appendix
ProMARK X
Appendix 1
GLOBAL POSITIONING SYSTEM
The Global Positioning System (GPS) is a constellation of
satellites that orbit the earth twice a day, transmitting precise
time and positioning information to anywhere on the globe, 24
hours a day.
The GPS constellation consists of 24 satellites orbiting the earth
in six fixed planes that are inclined at 60
°
from the equator. Each
satellite is 11,000 nautical miles above the earth, and orbits the
earth twice a day.
The system was developed and deployed by the U.S. Depart-
ment of Defense primarily to provide continuous, worldwide
positioning and navigation data to U.S. and allied military forces
around the globe. GPS also has broad civilian and commercial
applications, ranging from navigation and surveying to explo-
ration and tracking.
MONITORING AND CONTROLLING GPS
GPS is operated by the U.S. Air Force from a master contol
station in Colorado, USA. The facility is equipped for satellite
monitoring, telemetry, tracking, command and control, data
uploading, and navigation message generation.
Monitor stations and ground antennas throughout the world
passively track the GPS satellites and relay data to the master
control station. Exact satellite position and signal data accuracy
can therefore be constantly updated and maintained. Minor
discrepancies between where the satellite “thinks” it is and
where the monitor station “knows” it is can also be adjusted
when needed.
If any satellite emits erroneous data or is otherwise not operat-
ing properly, a ground station marks it “unhealthy.” The
affected satellite broadcasts its status to the GPS receiver, which
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