Most satellites have an electrical system, a communications system, and a computer system. Some have a control system and a life-support system as well.
The active life of a satellite is determined by how long its electric power lasts because all of the other systems require electricity. Most satellites have a combination of storage batteries and solar cells to provide electricity. The satellite draws its power from the rechargeable storage batteries. The solar cells are photoelectric devices that convert sunlight into electric current, which is used to recharge the storage batteries.
This system includes equipment to receive command signals from the earth, to emit radio signals used in tracking the satellite, and to transmit data from instruments carried by the satellite. The radio transmission of instrument readings is called telemetry. The telemetry conveys information not only from scientific instruments but also from instruments that measure voltages, temperatures, vibrations, and other indicators of how the satellite and its equipment are operating. Some types of satellites require additional communications equipment. For example, satellites that obtain images of the earth require equipment to convert optical images into electrical signals and transmit these signals to the earth as radio waves.
On-board computers in satellites provide the commands by which the satellite carries out its mission. The computers are programmed prior to lift-off. Updated information is usually fed into them later through radio communication with ground-based computers. In addition, the computers may store data acquired during experiments, for later transmission to earth.
Some satellites can be maneuvered while in space. Common maneuvers include changing orbits and holding the satellite steady (keeping it from rolling or tumbling) with respect to a reference body such as the earth or the sun. The maneuvers are made with small computer-controlled rocket motors that can be turned on and off as needed. Included in the control system are the rocket motors themselves, fuel and oxidizer tanks, and pumps.
Some satellites are set spinning to keep them from tumbling; the axis of rotation tends to remain pointing in one direction. A spin-stabilized satellite's antennas are despunthat is, they are mounted on a motor-driven platform that keeps them directed toward the earth.