August 6, 2003
Many parameters, such as temperature, voltages and attitude, are too complex or too laborious to be controlled permanently from the ground. The satellite's onboard computer is therefore programmed to perform these tasks, handling operations automatically in response to readings from a range of sensors.
Depending on its orbit, the satellite may be in contact with the ground almost all of the time, sending data via its antennas so that the control centre can keep track of operations. In turn, ground controllers regularly uplink commands to reconfigure the onboard computer or equipment.
To accomplish their mission, get enough power and communicate with Earth, the satellite’s instruments and antennas must maintain a precise orientation in space, which is obtained by rotating the bus. Satellite positioning and stationkeeping operations also involve performing orbital manoeuvres.
For this purpose, the satellite has its own thrusters that can be fired automatically or activated by ground controllers. Most thrusters today use chemical propulsion. As a result, a satellite’s service lifetime is dictated mainly by the amount of propellant on board, since it is inoperable once it can no longer be controlled. However, new and better forms of propulsion using ion thrusters or plasma thrusters are increasingly being used.
|Le saviez-vous ?|
Turn left at the star|
Seen from the ground, a satellite is just a dot in the sky. It is impossible to determine its attitude, or orientation, directly. For this reason, it is equipped with star sensors or star trackers able to measure their position with respect to the Sun, certain stars or a point on Earth. On the basis of this positional information, we can calculate the satellite’s orientation.