satellite as a vehicle and as a complicated piece of communications chapter. equipment. The optimum choice of orbit for a satellite depends primarily on The signal channels between satellite and control earth stations the mission. The geostationary satellite orbit (GSO) has several required for these functions are carried by two radio subsystems. important advantages over other orbits that might be used for One subsystem operates at low microwave frequencies allocated to communication systems. A geostationary satellite, being stationary the space operations service, using low gain satellite antennas; this in the sky as seen from the Earth, is constantly in sight of a fixed subsystem is used during the launch phase, when TT&C may be earth station. The variation with time of the transmission delay is required, for example between the launch site and the satellite and zero for an ideal geostationary satellite and it is small where good the attitude of the satellite is not normal. The other subsystem standards of satellite station keeping are maintained. Satellites in operates in a narrow band within the frequency bands appropriate other orbits, such as those illustrated in Figure 51.6, move across the to the mission of the satellite and uses the mission satellite antennas, sky, so earth station antennas, if they are directional, must track the often of high gain; this subsystem is used during the normal oper- satellite movement. ation of the satellite. The communication chain 51.4.1 The chain in outline Satellite circular polar orbit Satellite incined in eliptical Figure 51.5 represents the communication chain in satellite com- munication in its simplest form. At one earth station, signals from a source modulate a carrier (M), the carrier is up-converted (U/C) to a radio frequency suitably for the up-link, amplified in a transmitter (Te) and radiated to the satellite receiving antenna. At the satellite, 35,800 KMs- the received carrier is amplified (Rs), changed to the down-link frequency (F/C), amplified in a trans