3-axes satellite attitude control based on biased angular momentum

Satellites are used for telecommunications, telephotography and scientific experiments. In almost all of these cases, the exact orientation of satellite antennas toward the earth station is crucial. In this paper the attitude control system of the satellite will be designed in such a way that one specific point of the satellite will be always accurately oriented towards the earth station. In this way, to have the required quantity of the angular momentum, the use of momentum wheel actuator and magnetic torque generators are proposed. Three distinct modes are defined for a full positioning of the satellite to the desired direction; detumbling mode, spin-up of momentum wheel mode and the normal mode. In the first, the target is to decrease the angles phi and psi and their time rate phidot and dotpsidot. This will be done by designing a PD controller in roll and yaw directions. The control torque is applied to the satellite body to achieve the desired control. In the second mode, the momentum wheel speed will be increased so high that the changes in phi, thetas and psi and their time rate phidot thetasdotdot and dotpsidot are minimized and this gives the requested stability in the pitch direction. In the normal mode, the satellite should be oriented towards earth and hold its direction, during which phi, thetas and psi should be zero.