Publisher Summary Satellites are used for a variety of purposes from providing links for long-distancetelecommunications, to direct broadcast television and radio, gathering weather and geological information, as well as a host of other facilities. They must be capable of operating in extreme conditions while still maintaining the highest standards of reliability because they cannot be retrieved for maintenance or repair. Satellites also contain a number of systems used for what is called station keeping. These systems are used to maintain the satellite in the correct orbit and position. The satellite tends to drift away from its correct position over a period of time. To correct this, small thrusters are used. When a satellite orbits the earth, either in a circular or elliptical orbit it forms a plane. This passes through the centre of gravity of the earth or the geocenter. Geostationary satellites are a special case as they appear directly over the same point of the earth all the time and accordingly their groundtrack consists of a single point on the earth's equator. For satellites with equatorial orbits the groundtrack is along the equator.