Question

Illustrate how telemetry tracking and command system used in satellite communication.

Answer 1

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.

Telemetry System:

1. The telemetry system is used to transmit information like temperature, pressure, voltage, etc. or data regarding the status of the on board subsystems to the ground station at all times. The telemetry system consists of various electronic sensors for the measurement of quantities as voltage, current, temperature, pressure, radiation level, power supply, status of switches and solenoids

2. The telemetry measurements can run into hundreds thus necessitating time division multiplexing to combine different data into a single stream for downlink transmission. In all modern satellites, pulse code modulation is used. After modulation, the transmitter sends the telemetry data back to the earth station where the processing equipment in the TT&C earth station recovers this telemetry information and monitors it.

3. With this information the ground station is then able to determine the operational status of the satellite at all time

Tracking:

 1. Another important function of the TT&C system is measurement of satellite position.

2. Beacon transmitters are usually provided on the spacecraft for tracking during the launch and operation. This transmitter can also carry telemetry signals and range signal, turn around and command verifications.

3. Angular measurements are done by conventional terrestrial methods using large antennas and mono pulse or conical scanning system is used.

4. Ranging is done by sending uplink frequency which is modulated by a tone frequency by the earth station to the satellite. The received uplink frequency is demodulated in the command receiver, the tone re-modulated and transmitted back to the earth station on the telemetry carrier (downlink frequency). The precise range is obtained by measuring at TT&C station the time delay between the transmitted and received pulses

5. In this way the orbital parameters are obtained by tracking the communication satellite from the ground and measuring angular position and range of the satellite

Command:

1. The computers on the ground station generate the command signals which are sent to the satellite on the command uplink. The TT&C receiver accepts the commands and decodes these signals and sends verification signal back to the earth station.

2. On reception of the verification signal, the ground station sends back an execute pulse to the satellite. Then the satellite executes these commands.

 

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