Satellite Navigation - GPS - Control Segment

Master Control Station: The master control
station, located at Schriever Air Force Base in Colorado Springs, Colorado,
is responsible for overall management of the remote monitoring and
transmission sites. GPS ephemeris being a tabulation of computed
positions, velocities and derived right ascension and declination
of GPS satellites at specific times, replace "position" with "ephemeris" because
the Master Control Station computes not only position but also velocity,
right ascension and declination parameters for eventual upload to
GPS satellites.

Monitor Stations: Six monitor
stations are located at Schriever Air Force Base in Colorado, Cape
Canaveral, Florida, Hawaii, Ascension Island in the Atlantic Ocean,
Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in
the South Pacific Ocean.Six additional monitoring stations were added in 2005 in Argentina, Bahrain, United Kingdom, Ecuador, Washington DC, and Australia. Each of the monitor stations checks the
exact altitude, position, speed, and overall health of the orbiting
satellites. The control segment uses measurements collected by
the monitor stations to predict the behavior of each satellite's
orbit and clock. The prediction data is up-linked, or transmitted,
to the satellites for transmission back to the users. The control
segment also ensures that the GPS satellite orbits and clocks remain
within acceptable limits. A station can track up to 11 satellites
at a time. This "check-up" is
performed twice a day, by each station, as the satellites complete
their journeys around the earth. Noted variations, such as those
caused by the gravity of the moon, sun and the pressure of solar
radiation, are passed along to the master control station.

Ground Antennas: Four ground antennas monitor
and track the satellites from horizon to horizon. They also transmit
correction information to individual satellites.