Attitude and orbit control system(AOCS)
This subsystem consists of rocket motors that are used to move the satellite back to the correct orbit when external forces cause it to drift off station and gas jets or inertial devices that control the attitude of the satellite.
Telemetry, Tracking, Command, and Monitoring (TTC&M): These systems are partly on the satellite and partly at the controlling earth station. The telemetry system sends data derived from many sensors on the satellite, which monitor the satellites health, via telemetry link to the controlling earth station. The tracking system is located at this earth station and provides information on the range and the elevation and azimuth angles of the satellite.Based on telemetry data received from the satellite and orbital data obtained from the tracking system, the control system is used to correct the position and attitude of the satellite. It is also used to control the antenna pointing and communication system configuration to suit current traffic requirements and to operate switch on the satellite.
Power system: All communications satellite derive their electrical power from solar cells. The power is used by the communications system, mainly in its transmitters, and also by all other electrical systems on the satellite. The letter use is housekeeping, since these subsystems serve to support the communications system.
Communications systems:
The communication subsystem is the major part of the satellite. It is usually composed of one or more antennas, which receive and transmit over wide bandwidths at microwave frequencies, and a set of receivers and transmitters that amplify and retransmit the incoming signals. The receiver-transmitter units are known as transponders. Two types of transponder in use: the linear or bent pipe transponder that amplify the received signal and retransmits it at a different, usually lower, frequency, and the baseband processing transponder which is used only with signals, that converts the received signal to baseband processes it and then retransmits a digital signal.
Satellite Antennas: On large GEO satellites, the antenna systems are very complex and produce beams with shapes carefully tailored to match the areas on the earth’s surface served by the satellite. Most satellite antennas are designed to operate in a single frequency band, for example, C band or Ku band. A satellite which uses multiple frequency bands usually has four or more antennas.
