The term “Artificial
Satellites” refers to man made satellites orbiting the Earth. Objects which travel far beyond the Earth
to orbit or land on the moon or other planets are usually distinguished as “Space Probes”.
Much of the early stimulus to devise and orbit Artificial
Satellites was scientific in nature. But there were political and military reasons as well, as both the United States and the Soviet Union set out to win the “space race” in order to demonstrate their technological skill.
A modern artificial
Satellite is made up of a space craft ‘bus’ and a pay load. The payload sections include all the equipment needed to carryout the primary objectives of the mission, while the spacecraft bus provides supporting services to the payload and consists of several subsystems. The structural subsystem is the mechanical framework of the satellite on which all other components are mounted. It must be sturdy enough to purview the violent shaking which occurs during launching of a satellite in the orbit. The attitude control subsystem stabilizes the orientation of the satellite, and permits the reorientation maneuvers.
The satellite orientation depends upon the combination of star trackers, sun sensors and /or magnetrons. Stabilizing and attitude maneuvers can be accomplished by thruster – jet firings, gyroscopic control, interaction with the earth’s magnetic field, or by techniques involving radiation pressure or gravity gradients.
The power subsystem is responsible for controlling, distributing and monitoring the electricity needed to operate the satellite. The source of electric power in the satellite comes from the photo electric solar array, by fuel cells or by nuclear power devices. In addition batteries may be required as a storage purpose, especially on satellites with solar arrays, which spend a significant amount of time blocked from sunlight while in the earth’s shadow.
The communications and data handling subsystem provides a means for ground and on-board control ground stations can be carried out immediately or stored for execution at a later time. Data generated by the payload and sensors monitoring the status of the space craft are recorded on on-board magnetic tapes. These are then played back on command at high speed so that the satellite data can be transmitted to tape recorders at the ground stations.
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