Telemetry, Tracking and Command (TT&C) Subsystem:

• These systems are partly on the satellite and partly at the control earth station. They support the functions of the spacecraft management. The main functions of a TTC system are
• To monitor the performance of all satellite subsystems and transmit the monitored data to the satellite control center via a separate Telemetry link.
• To support the determination of orbital parameters.
• To provide a source to earth station for tracking.
• To receive commands from the control center for performing various functions of the satellite.

• Typical functions include:

  a) To correct the position and attitude of the satellite.

  b) To control the antenna pointing and communication system configuration to suit current traffic requirements. To operate switches on the spacecraft.

Telemetry:

• It collects data from all sensors on the satellite and send to the controlling earth station.
• The sighting device is used to maintain space craft altitudes are also monitored by telemetry.
• At a controlling earth station using computer telemetry data can be monitored and decode.
• And status of any system on satellite can be determined and can be controlled from earth station

Tracking:

• By using velocity and acceleration sensors, on spacecraft the orbital position of satellite can be detect from earth station.
• For accurate and precise result number of earth stations can be used

Propulsion sub-system:

• Propulsion sub-system is the reaction control sub-system carried by the satellite in the geostationary orbit so as to generate forces on it whenever needed.
• It moves satellite to its assigned position in orbit, to maintain in that position (station keeping) and to maintain the direction of spin axis and attitude control.
• Usually propulsion subsystem has three units. i) Low thrust (10-3 to 20N) actuators (Reaction control system, RCS) ii) High thrust (400 to 50,000 N) motor (Apogee kick motor: AKM or Apogee Boost Motor (ABM) which provides velocity increment) to inject satellite into geostationary orbit from transfer orbit apogee. iii) Perigee kick motor (PKM) which provides velocity increments required to inject the satellite into the transfer orbit.
• Low thrust actuators (RCS) are of much importance as these are responsible for keeping the satellite in orbit with its perfect attitude till its life end. They are either chemical or electrical thrusters.

Antenna Sub-system:

• Antenna on board serves as an interface between the earth on the ground and various satellite subsystems

Power Subsystem:

• This system provides the necessary DC power to the satellite. All communication satellites derive their electrical power from solar cells. There is also a battery backup facility used during launch and eclipses.
• The batteries are of sealed Nickel Cadmium type and have good reliability and long life.

Communication Subsystems :

• It is a major component of the communication satellite, and the remainder of the spacecraft is there solely to support it.

• It consists of:

  i. Microwave antennas and

  ii. As set of receiver and transmitter units referred to as Transponders

• The antenna system is used to receive signals from and transmit signals to the ground stations in the coverage area.

• The antenna used range from dipole type antennas where Omni directional characteristics are required to the highly directional antennas (the paraboloidal reflector being the most common) required for telecommunication purposes and TV relay and broadcast.
• The transponders amplify and retransmit the incoming signals.

Attitude and Orbit Control System (ACOS) :

• This subsystem provides stabilization of the satellite and controls its orbit. It fires jet thrusters to perform attitude adjustments and station keeping man oeuvres that keep the satellite in its original orbital position with correct orientation