. A further relaxation, to 1.0°, applies adjustment are usually mounted on the rotating body of the satellite, to certain old satellites, launched before 1987 (ITU, 19900). Con- and these thrusters accordingly must be operated in a pulsed mode, straints on broadcasting satellites operating at 12GHz are at least as synchronised with the rotation of the body. severe, and they are unconditional (ITU, 1990g). Body stabilised designs generally employ an internal momentum North-south excursions of satellites due to inclination of the wheel with its axis perpendicular to the plane of the orbit. Control orbital plane can also be corrected by the on board thrusters, but the of attitude about the pitch axis is obtained by varying the speed of amount of fuel that is required to maintain low inclination is quite rotation of the wheel. Hydrazine thrusters are used occasionally to significant; for a 10-year lifetime the mass of hydrazine required for dump momentum from the wheel, so avoiding an unacceptable this purpose is about 20% of the total mass of the satellite at start of build up of rate of rotation. Control about the yaw and roll axes may life. For this reason, bi-propellant thruster systems and electrically be obtained by gimbaling the wheel or by the use of hydrazine powered ion engines which use less payload mass are coming into thrusters. Figure 51.4 shows INTELSAT V, a typical large body use (Hayn, et al., 1978; Free, 1980). No regulatory limit is applied stabilised satellite (Fuenzalida et al., 1977). to excursions north and south of the equatorial plane, but satellites The ITU Radio Regulations require geostationary satellites in with inclinations of more than 5° have not been regarded as geosta- general to be capable of maintaining their antenna beams within tionary for regulatory purposes. 10% of the half power beamwidth, or +0.3°, of the nominal direc- tion, whichever requirement is less stringent (ITU, 1990p). The 51.3.4 Attitude stabilisation frequency assignment plans for satellite broadcasting at 12GHz r