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One of the tasks of the GPS control segment is:
  • A
    to calculate the accuracy of GPS.
  • B
    to calculate and transmit differential corrections to users.
  • C
    to detect the malfunctioning of satellites.
  • D
    to control continuously the motion of GPS-satellites.
The Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services. This system consists of three segments: the space segment, the control segment, and the user segment..(see annex)

Control Segment
GPS satellites are under surveillance of the Control Segment. 5 monitoring ground stations constantly track the satellite’s transmissions, clocks, status and orbits.Time keeping is the most essential function of the control segment: a Composite Clock (a paper clock) consisting of all Monitor Stations and Satellite frequency standards gives GPS time. GPS time in turn is referenced to a Master Clock and steered to UTC from which system time will not deviate more then 1 microsecond.

The exact difference is given in the navigation message in the form of two constants for time difference and rate of change against UTC. The system uses Time-Of Arrival (TOA) measurements for the determination of user position.
A precisely timed clock is not essential for the user because time is obtained in addition to position by the measurement of TOA of four satellites simultaneously in view.

Another function is the Navigation messag: it collected data is send by data link to the Master Control Station, which calculates the orbital parameters of the satellites. These calculations as well as corrections of satellite time are included in the navigation message.

Satellite upload stations provide the satellites daily, via S- band radio uplink, with the latest version of the navigation message.
Each satellite then stores the navigation message until the next upload.

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