The Journey of Flight.pdf - Valkyrie Cadet

In 1974, the Air Force was directed to oversee the creation of a new global navigation system. It
would combine the best features of the previous programs. The new system was called NAVSTAR
Global Positioning System. All the military services would participate in the program, assisted by the
Defense Mapping Agency. The first NAVSTAR launched in 1978.
The NAVSTAR Global Positioning System (GPS) is a space-based radio-positioning system managed
by the NAVSTAR GPS Joint Program Office at the Space and Missile Systems Center, Los
Angeles Air Force Base, California. It provides navigation and timing information to both civilian and
military users worldwide. Position, velocity and time can be precisely determined by GPS users. GPS
satellites emit continuous navigation signals as they orbit Earth every 12 hours. The GPS system
provides positioning accuracy for military users to 16 meters nominally, while accuracy for civilian
users is nominally 100 meters.
The system consists of a constelation
of 24 orbiting satellites, a
worldwide satellite control network
and GPS receiver units that pick up
signals from these satellites and translate
them into position information.
The satellites act as precise
reference points and continuously
broadcast position and time data.
These satellites are spaced in orbit
NAVSTAR Global Positioning System Satellite (GPS)
so that a minimum of six will be in
view at any one time.
The control network is made up of ground stations operated by the US Air Force; the master
control station is in Colorado. Five monitoring stations and three ground antennas located throughout
the world all work together to track the satellites and send the data to the master control station. This
data is used to adjust the satellite positions if that should
be necessary.
Every user of the system has a receiver, each one of
which uses the broadcast signals to calculate its position.
The entire system is based on ranging the satellite,
a process of determining how far away it is. The
satellite broadcasts its location and the time that the
signal was sent. The receiver then measures how long it took to
receive the signal, thus determining relative distance.
Highly accurate atomic clocks are the key to the NAVSTAR
GPS satellite system. On board each satellite are three
of these clocks, which lose or gain only one second in 36,000
years. Any receiver with a minimum of three GPS satellites in
view and an atomic clock can calculate a three-dimensional posi-
tion, which is given in latitude, longitude and altitude. If no atomic
clock is available, a fourth satellite must be in view in order to
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Backpack and Hand-held GPS /
Receivers