equire beam pointing within +0.1° (ITU, 1990g). Stabilisation of a communication satellite's attitude relative to the Earth is necessary, in order that the gain of directive satellite 51.3.5 Electrical power supply in space antennas may be used to use satellite power efficiently and to permit geographical re-use of spectrum. Spinning body stabilisation has Silicon solar cells are used as the primary satellite power source in been widely used in the past but three axis body stabilisation is in normal operation. When the Sun, for the satellite, is eclipsed by the general use now. Earth, power is maintained by nickel cadmium or nickel hydrogen In a spinning body satellite the satellite body rotates at 30rpm to secondary batteries. Full shadow eclipse for geostationary satellites 100rpm about the axis which is perpendicular to the plane of the occurs on 44 nights in spring and 44 nights in the autumn, the orbit. Antennas are usually mounted on a rotatable platform which longest eclipses occurring at the equinoxes and giving 65 minutes is 'de-spun' relative to the Earth and is, accordingly, stabilised in all of full shadow. Research on new types of storage cell, such as the three axes. Areference to enable the on board control system to keep silver hydrogen cell, seeking longer life, lower mass and higher the antenna platform continually pointing towards the Earth is efficiency, is in progress. Spacecraft technology 51/9 TRANSFER ORBIT OSA THERMAL SHIELDS SOLAR AARAY UMBILICAL CONNECTION ROLL THRUSTERS SOLAR ARRAY DRIVE ASSEMBLY OPTICAL SOLAR REFLECTORS SOLAR ARRAY RELEASE MECHANISM WEST THERMAL BLANKET 11/14 GH2 EAST SPOT ANTENNA WEST THRUSTER CLUSTER SUPPORT STRUCTURE 6 GHI HEMIZZONE RECEIVE REFLECTOR 4 GHI HEMIZZONE REFLECTOR FORWARD INSULATION SHIELD INSULATION REMOVED FROM FEED SUPPORT STRUCTURE FOR CLARITY) HEMIZZONE TELEMETRY AND COMMAND ANTENNA 6 GH1 GLOBAL HEMIZZONE 6 GH2 11GH1 11/14 GHI GLOBAL BEACON WEST SPOT ANTENNA GEOSTATIONARY IA SENSORS Figure 51.4 INTELSAT V satellite configuration Spinning body satel