GPS System Segments - Understanding GPS: Principles and Applications - page 87

contracts and a brief planning period for the government, GPS III entered phase A
development with two contracts, again to Boeing and Lockheed Martin teams. Each
team completed a system requirements review in 2005. At the time of this writing,
the first GPS III satellite launch was planned for fiscal year 2013.

3.3 Control Segment

The control segment (CS) is responsible for monitoring, commanding, and control-
ling the GPS satellite constellation. Functionally, the CS monitors the downlink
L-band navigation signals, updates the navigation messages, and resolves satellite
anomalies. Additionally, the CS monitors each satellite's state of health, manages
tasks associated with satellite stationkeeping maneuvers and battery recharging,
and commands the satellite payloads, as required [11].
The major elements of the CS consist of the MCS, L-band monitor stations, and
S-band ground antennas. The primary CS functions are performed at the MCS,
under the operation of the U.S. Air Force Space Command, Second Space Operation
Squadron (2SOPS), located at Schriever Air Force Base (AFB) in Colorado Springs,
Colorado. It provides continuous GPS services, 24 hours per day, 7 days a week,
and serves as the mission control center for GPS operations. A backup MCS, located
at a contractor facility in Gaithersburg, Maryland, provides redundancy of the
MCS. The major elements of the CS and their functional allocation are shown in
Figure 3.14.

Operational Control System

Master Control Station

Resource allocation and scheduling
Navigation message generation
Satellite health and housekeeping
SV processors' load generation
Constellation synchronization steering
GPS system status/performance evaluation
and reporting

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Monitor Stations

Ground Antennas

Navigation signal tracking
Range and carrier measurement
Atmospheric data collection
Collect decoded navigation
data provided to the GPS user

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SV command transmissions
SV processor load transmissions
SV navigation upload transmissions
Collect SV telemetry

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Figure 3.14 CS overview.