 is that its attitude will change. This is of great importance because directive antennas or cameras are often used, and the satellite needs to be orientated in the correct direction for them. The basic method of gaining the correct orientation is to use the thrusters. However, the attitude will change comparatively quickly. The most common method to overcome this is to use the gyroscopic effect. Sometimes a large flywheel may be made to spin inside the satellite. This can be inefficient in its use of the weight of the satellite. To overcome this other cylindrical satellites actually rotate a portion of the body, often an inner cylindrical section SO that the antennas mounted on the outer section do not revolve. Electrical power is also required by the satellite for its electronic circuitry and other electrical systems. This is supplied by the large arrays of photo or solar cells that are used. Some cylindrical satellites have them positioned around the outer area on the cylinder SO that some part of the body is always exposed to sunlight. Others have large extending panels that are orientated to collect the maximum amount of light. Today these Figure 8.3 A typical satellite (courtesy NASA/JPL/Caltech) Satellites 227 panels are capable of producing the many kilowatts of power required for the high power output stages used in many transponders. The satellite also needs batteries that charge up when the satellite is in sunlight. These need to charge sufficiently from the solar cells SO that when the satellite passes out of the sunlight it can still remain operational. They should be sufficient to power the satellite for the full period of darkness. This naturally places an additional burden on the solar cells because they need to be able to power not only the satellite itself, but also charge the batteries. This can double the power they have to supply during periods of sunlight. Satellites must also be designed and manufactured to withstand the harsh environment encountered in outer space. Extremes of temperature 