Q 2.17 How are satellites maintained?
The Telemetry Tracking and Command (TT & C) system supports functions of satellite maintenance and management. These functions, separate to that of communication management, are vital for successful operation of a satellite. The TT & C system:a) transmits monitored performance of satellite sub-systems to satellite control center;
b) supports determination of orbital parameters;
c) receives, decodes, verifies and executes commands from satellite control center.
The monitored parameters include:
a) voltage, current and temperature of major sub-systems;
b) communication transponders settings;
c) pressure of propulsion tanks;
d) outputs from attitude sensors;
e) reaction wheel speed.
The telemetered data are analyzed at the control center and used for routine operational and failure diagnostic purposes. For example, the data can be used to provide information about the remaining fuel on the satellite.
The telemetry transmission is commonly used as a beacon by ground stations for tracking.
The command system receives commands transmitted from the satellite control center, verifies and executes them. Examples of common commands are:- switch transponder
– reconfigure satellite
– control antenna pointing
– control speed of solar array drive
– recondition battery
– fire a thruster
– switch one or more heaters
Hundreds of different commands are used on a communication satellite.
A fail-safe operation has to be achieved. A commonly used feature is to verify each command from the satellite control center before execution.
To reduce the impact of high bit error rate of the received command signals, coding and data repetition are employed. Further improvements are possible by combining outputs of two on-board receivers with a message being accepted when both outputs are the same.
The idea of in-orbit satellite repair or retrieve them back to the Earth for reuse are in their infancy. Several schemes are in research and development while some remarkable feats such as repair of Hubble telescope have been successful.