GPS Surveying: Ground Control Segment and User Segment

CONTROL SEGMENT

Control segment is the vital link in GPS technology. Main functions of the control segment.

-   Monitoring and controlling the satellite system continuously

-         Determine GPS system time

-   Predict the satellite ephemeris and the behavior of each satellite clock.

-         Update periodically the navigation message for each particular satellite.

For continuos monitoring and controlling GPS satellites a master control stations (MCS), several monitor stations (MS) and ground antennas (GA) are located around the world (Fig. 9). The operational control segment (OCS) consists of MCS near Colorado springs (USA), three MS and GA in Kwajaleian Ascension and Diego Garcia and two more MS at Colorado Spring and Hawai.

GROUND CONTROL SEGMENT

The monitor station receives all visible satellite signals and determines their pseudorages and then transmits the range data along with the localmeteorological data via data link to the master control stations. MCS then precomputes satellite ephemeris and the behaviour of the satellite clocks and formulates the navigation data. The navigation message data are transmitted to the ground antennas and via S-band it links to the satellites in view. Fig. 9 shows this process schematically. Due to systematic global distribution of upload antennas, it is possible to have atleast three contacts per day between the control segment and each satellite.

USER SEGMENT

Appropriate GPS receivers are required to receive signal from GPS satellites for the purpose of navigation or positioning. Since, GPS is still in its development phase, many rapid advancements have completely eliminated bulky first generation user equipments and now miniature powerful models are frequently appearing in the market.