Telemetry, Tracking, Command Monitoring Sub-system (TTC & M)
Telemetry System
TTC&M subsystems are located partly on satellite and partly on earth station.
The telemetry link sends the data about satellite’s health to the earth station.
At the ground station the tracking system provides the information about the range, azimuth and elevation angles of the satellite.
By repeated measurement of these data, the changes in the orbit can be optimized.
The telemetry data from satellite and orbital data from earth station are used to correct the position and attitude of the satellite and also to control the antenna pointing and communication system configuration.
Large no. of sensors located within the satellite sends data to the controlled computer at the earth station used to monitor, store and decode the telemetry data.
The sensors usually monitor pressure in the fuel tanks, voltage and current in the power conditioning unit, current drawn by each subsystem, critical voltages and currents in the communication circuits, temperature rise in different subsystems.
Telemetry system used during the launching and also when the satellite in GEO orbit, a highly directive antenna and PSK modulated and TDM multiplex digital data are sent to the earth station.
Low data rate used such that the receiver bandwidth is a narrow bandwidth and high carrier to noise ration.
Tracking System:
Tracking system in the TTC&M subsystem is used to determine the satellite’s orbit during launching and then to track the satellite. It determines the current orbit of a satellite.
Various techniques for tracking of a satellite.
Velocity and acceleration sensors on the satellites can be used to establish the change in orbit. Doppler shift of the telemetry carrier from the earth station or beacon transmitter may be measured to determine the rate at which the range is changing. Ranging tones may be used for measurement.
Active determination can be achieved by sending pulse or sequence of pulses and measure their delay at the earth station.
Triangular methods may be apply when sufficient no of earth station are observing, which can precisely determine the position accurately even within 10 meters.
Command System
Command system is used to make changes in attitude and corrections to the orbit and to control the communication system.
It is also used to spin-stabilized satellite and also to extend the solar sails and antennas of a three axis stabilized satellite.
Encrypted command and responses used to provide security sends from the controlled computer of the TTC&M system.
Usually earth-coverage horn antennas are used as TTC & M antenna in the 6/4 GHz system.
A back-up TTC&M system is used during the launching and injection into the geo-stationary orbit of the satellite, which control the apogee kick motor, the attitude control system and orbit control thrusters, the solar sail deployment mechanism and the power conditioning unit.
The back-up system is used to keep the satellite on station in the event of failure of the main TTC & M system. It is also used to eject the satellite from GEO orbit and to switch off all transmitters when the satellite reaches the end of life
