Page 58 - Electronic Navigation Cyber Book

for a few hours each day. For military operations, the ground track repeat
can be used to ensure good coverage in combat zones.
As of December 2012, there are 32 satellites in the GPS constellation.
The additional satellites improve the precision of GPS receiver
calculations by providing redundant measurements. With the increased
number of satellites, the constellation was changed to a nonuniform
arrangement. Such an arrangement was shown to improve reliability and
availability of the system, relative to a uniform system, when multiple
satellites fail. About nine satellites are visible from any point on the
ground at any one time (see animation at right), ensuring considerable
redundancy over the minimum four satellites needed for a position.
CONTROL SEGMENT

The control segment is composed of:

1. a master control station (MCS),
2. an alternate master control station,
3. four dedicated ground antennas, and
4. six dedicated monitor stations.

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