Q 2.19 What are the functions of satellite command system?

A communication satellite consists of two functional units – payload and bus.

The function of the payload is to fulfill the primary goal of the mission – i.e., communication, remote-sensing, business goals, etc. The bus provides electrical and mechanical support to the payload. The payload is made up of antenna and repeater sub-systems. The antenna sub-system captures signals transmitted from the ground stations dispersed in the service area and transmits them back towards the service area. The repeater processes the received signals and routes the signals to the antenna after amplification. The bus, (sometimes called platform) consists of several sub-systems. The attitude and orbit control sub-system (AOCS) stabilizes the spacecraft and controls its orbit. The propulsion sub-system provides the necessary velocity increments and torques to the AOCS. The telemetry, tracking and command (TT & C) sub-system transmits the status of various sub-systems of the satellite (its ‘health’) to the satellite control center and accepts commands from the control center for performing instructed functions on the spacecraft. The TT&C transmissions are also used by ground stations for tracking. The electrical power supply sub-system provides DC power to the spacecraft. The thermal control sub-system maintains the temperature of sub-systems within tolerable limits. The structure provides the necessary mechanical support during all the phases of the mission.