NOTE
GPS will become the primary reference
navigation system for surface ships, submarines,
and aircraft. Refer to the DOD policy statement
under the LORAN and OMEGA section of this
chapter for specific details on this important
transition.
GROUND CONTROL
The ground control segment tracks the satellites,
monitors and controls satellite orbits, and updates the
satellite navigation data message. The ground control
system consists of unmanned monitor stations and a
manned control center. Monitor stations, located
throughout the world, use GPS receivers to track each
satellite.
Tracking information gathered by the
monitor stations is sent to the control center, where a
precise position and a clock error for each satellite are
calculated. The control center also calculates satellite
positioning for the group of satellites. Positioning
data for a single satellite is called ephemeris data;
data for a group of satellites is called almanac data.
Once each 24 hours, the control center transmits the
ephemeris and almanac data to each satellite to update
the navigation data message.
SATELLITES
There are 21 active operational and 3 active spare
satellites in circular orbits, with a 55-degree
inclination to the earth. These satellites provide
navigation data to the navigation sets. The satellites
are arranged in six concentric rings that allow them to
orbit the earth twice a day and provide world-wide
continuous coverage. Each satellite broadcasts two
Figure 1-11.—NAVSTAR GPS major elements.
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