GPS System Components (Space Segment, Control Segment, User Segment) (Especially for GATE-Geospatial 2022)

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The GPS system consists of three segments

  • The Space Segment: comprising the satellites and the transmitted signals.
  • The Control Segment: the ground facilities carrying out the task of satellite tracking, orbit computations, telemetry and supervision necessary for the daily control of the space segment.
  • The User Segment: the entire spectrum of applications equipment and Computational techniques those are available to the users.

The Space Segment

  • Consists of the constellation of spacecraft and the signals broadcast by them which allow users to determine position, velocity and time.
  • The basic functions of the satellites are to:
    • Receive and store data transmitted by the Control Segment stations.
    • Maintain accurate time by means of several on-board atomic clocks.
    • Transmit information and signals to users on two L-band frequencies.
    • Provide a stable platform and orbit for the L-band transmitters.
  • Satellite signals can be received anywhere within a satellite՚s effective range. The effective range (shaded area) of a satellite located directly above the equator/zero meridian intersection.
  • The distribution of the 28 satellites at any given time can be seen. It is due to this ingenious pattern of distribution and to the great height at which they orbit that communication with at least 4 satellites is ensured at all times anywhere in the world

Fig: Position of the 28 GPS satellites at 12: 00 hrs UTC on 14 April 2001

Control Segment

  • The control segment (Operational Control System OCS) consists of a Master Control Station located in the state of Colorado, five monitor stations equipped with atomic clocks that are spread around the globe in the vicinity of the equator, and three ground control stations that transmit information to the satellites.
  • The most important tasks of the control segment are:
    • Observing the movement of the satellites and computing orbital data (ephemeris)
    • Monitoring the satellite clocks and predicting their behaviour
    • Synchronizing on board satellite time
    • Relaying precise orbital data received from satellites in communication
    • Relaying the approximate orbital data of all satellites (almanac)
    • Relaying further information, including satellite health, clock errors etc.
  • The control segment also oversees the artificial distortion of signals (SA, Selective Availability) , in order to degrade the system՚s positional accuracy for civil use. System accuracy had been intentionally degraded up until May 2000 for political and tactical reasons by the U. S. Department of Defence (DOD) , the satellite operators.
  • It was shut down in May 2000, but it can be started up again, if necessary, either on a global or regional basis.

User Segment

  • The signals transmitted by the satellites take approx. 67 milliseconds to reach a receiver. As the signals travel at the speed of light, their transit time depends on the distance between the satellites and the user.
  • Four different signals are generated in the receiver having the same structure as those received from the 4 satellites.
  • By synchronizing the signals generated in the receiver with those from the satellites, the four satellite signal time shifts t are measured as a timing mark.

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