|Originally established by the Department of Defense, the global positioning
system (GPS) is a satellite navigation system. It consists of approximately
24 satellites in orbit around the Earth, several ground tracking stations,
and a receiver in the aircraft, other vehicle, or held by an individual.
The ground control sites watch where the satellites are in orbit and continually
correct their reported location and time-of-day signals. This is done so
that when the satellite communicates with a receiver, it gives the best
possible position it can to help navigate.
The GPS receiver converts the signals coming from the satellites into
position coordinates. It can give a pilot his latitude, longitude, elevation,
and current time as long as the receiver can "see" any four
of the 24 satellites.
Position accuracy is very important to military operations. As of the
year 2000, GPS signals are encoded so that only U.S. military forces can
use it. This is called the Precise Positioning System (PPS). So that the
civilian community can also use GPS, the GPS satellites send out an additional
signal for position data that is accessible by non-military users. This
is called the Standard Positioning System (SPS).
Global Positioning System