Attitude control of a satellite with a rotating solar array

Abstract

This paper describes the dynamic modeling and fine pointing control system design for the SPOT French Earth observation satellites. The dynamic model of the vehicle includes a representation of the flexible solar array by effective mass technique. An onboard computer processes the attitude rate information provided by a gyro package and, possibly, the measurement of torque around boom axis to provide a very high angular rate accuracy with the help of reaction wheels and solar array drive motor. Several control design techniques are investigated for this multivariable control problem. Performances, including sensitivity to modal data, are checked by extensive simulations. I. Introduction E ARTH observation satellites differ mainly from other satellites (scientific, telecommunications, television) in the strong requirements on attitude rates of the spacecraft during the operational observation mode. A great deal of the control design is then directed by the short term torque disturbances; the dynamic reactions of the payload itself and of the flexible solar array give major contributions. Moreover, this array is mounted dissymmetrically on the spacecraft in order to avoid payload interaction. The control of a satellite with flexible appendages often has been investigated, the concern centering on the stability of the closed-loop control laws; in the Earth observation case, a problem of performance appears, particularly when harmonic disturbances are considered. The solar pointing mechanism of the power generator induces such disturbances and the present paper is devoted to a detailed investigation of these problems and their solutions in the frame of the SPOT French Earth observation project.1