TELEMETRY, TRACKING AND COMMAND (TT&C) SYSTEM
It is the most essential subsystem of a satellite, which performs several routine functions aboard the spacecraft. The block diagram of the TT&C system is shown in the figure below.
Telemetry means remote Sensing
Tracking means: Follow up
Command means: Instructions to be obeyed.
 |
| FIGURE: Block diagram of the TT&C system |
Telemetry
This is the system which collects data as many as 100 sensors located within the spacecraft and sends these data to the controlling earth stations. When the satellite is in the transfer orbit, it sends telemetry data through the omnidirectional antennas mounted on the satellite. The telemetry data are usually digitized and transmitted as FSK or PSK of a low power telemetry carrier using time division techniques. The block diagram of the Telemetry system is given in the figure below.
 |
| FIGURE 2: Block Diagram of the Telemetry system |
Tracking System
This system is used to determine the satellite's orbit during launching and then to track it. In the Doppler shift method, a single pulse or a sequence of pulses are transmitted towards the satellite and then a time delay is measured from the received pulse. When a sufficient number of earth stations are observing the satellite, it's position can be established by triangulation form and the earth station look angles are determined by simulations range measurements. The block diagram of the Tracking system is given below. |
FIGURE 3: Block Diagram of Tracking systemCommand System
This is a set of instructions of controlling earth stations for the spacecraft. During the time of launching, the command signal can control the firing of apogee boost motor. When the satellite has reached the proper orbit, solar panel deployment and the action of spinner operation starts after getting the command signal. The command signal can be used to make changes in attitude and thus makes a correction in the orbit. The block diagram of the command system is given below.
|
 |
| FIGURE 4: Block Diagram of the Command system |
From the above discussion, it is clear that the telemetry, tracking and command functions are complex operations which required special facilities of the ground stations in addition to the TT&C subsystems aboard the satellite.
