- The function of a Telemetry sub system is to monitor various satellite/aircraft parameters and to transmit the measured values to the satellite control centre
- It refers to the overall operation of generating an electrical signal proportional to the quantity being measured and encoding and transmitting this to a distant station, which for the satellite is one of the earth stations
- The telemetry data is analysed at the control centre, and is used for routine operation and failure diagnostic purposes
The most commonly monitored parameters are:
1) Voltage, current and Temperatures of all major systems
2) Switch status of communications transponders.
3) Pressure of propulsion tanks
4) Output from attitude sensors
5) Reaction wheel speed
6) Environmental information such as the magnetic field intensity and direction, the frequency of meteorite impact.
The telemetry signal is commonly used as a beacon by ground stations for tracking purpose.
A Telemetry system
- The satellite movement is tracked, and correction signals are sent to the satellite
- This system is located at the earth station providing information on elevation and azimuth angles of the satellite
- Tracking is important during the transfer and drift orbital phases of the satellite launch. Once it is on station, the position of a geostationary satellite tends to be shifted as a result of the various disturbing forces. Therefore, it is necessary to be able to track the satellite’s movement and send correction signals as required
- Tracking beacons may be transmitted in the telemetry channel or by pilot carriers at frequencies in one of the main communication channels, or by special tracking antennas. Satellite range from the ground station is also required from time to time. This can be determined by measurement of the propagation delay of signals especially transmitted for ranging purposes.
- The command subsystem receives commands from the satellite control centre, verifies this reception, and then executes received commands
- Telemetry and command may be thought of as complementary functions. The telemetry subsystem transmits information about the satellite, while the command subsystem receives command signals from the earth station, often in response to the telemetered information
- It is important to prevent unauthorized commands from being received and decoded, and for this reason, the command signals are often encrypted.
Examples of common commands are:
1) Transponder switching
2) Switch matrix reconfiguration.
3) Antenna pointing control.
4) Controlling direction and speed of solar drive array.
5) Battery reconditioning
6) Beacon switching
7) Thrusters firing
8) Switching heaters of various subsystem
A Command system
- The receiving antennas used are omni-directional antennas to maintain contact for all orientation of the satellite, so that the satellite can receive the signals during launch, orbit transfer, and other periods prior to attitude stabilization
- The receiver then converts RF signal to base band signal, and command decoder decodes the command
- Then verification is done, which involves transmitting decoded commands to satellite control centre via telemetry carrier
- The command receiver also provides baseband ranging tone which is modulated on beacon telemetry and sent to satellite control centre
- The antennas used for telemetry and command signals are parabolic reflectors.