Attitude stabilisation

Stabilisation of a communication satellite's attitude relative to the Earth is necessary, in order that the gain of directive satellite antennas may be used to use satellite power efficiently and to permit geographical re-use of spectrum. Spinning body stabilisation has been widely used in the past but three axis body stabilisation is in general use now.

In a spinning body satellite the satellite body rotates at 30rpm to l00rpm about the axis which is perpendicular to the plane of the orbit. Antennas are usually mounted on a rotatable platform which is 'de-spun' relative to the Earth and is, accordingly, stabilised in all three axes. A reference to enable the on board control system to keep the antenna platform continually pointing towards the Earth is usually obtained from infra red Earth sensors, supplemented by Sun sensors. Antenna pointing accuracy of +_0.2° or better is obtained with such systems. The thrusters which are used for east-west orbit adjustment are usually mounted on the rotating body of the satellite, and these thrusters accordingly must be operated in a pulsed mode, synchronised with the rotation of the body.

Body stabilised designs generally employ an internal momentum wheel with its axis perpendicular to the plane of the orbit. Control of attitude about the pitch axis is obtained by varying the speed of rotation of the wheel. Hydrazine thrusters are used occasionally to dump momentum from the wheel, so avoiding an unacceptable build up of rate of rotation. Control about the yaw and roll axes may be obtained by gimbaling the wheel or by the use of hydrazine thrusters. Figure 51.4 shows INTELSAT V, a typical large body stabilised satellite (Fuenzalida et al., 1977).

The ITU Radio Regulations require geostationary satellites in general to be capable of maintaining their antenna beams within 10% of the half power beamwidth, or ±0.3°, of the nominal direc­tion, whichever requirement is less stringent (ITU, 1990p). The frequency assignment plans for satellite broadcasting at 12GHz require beam pointing within ±0.1° (ITU, 1990g).

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