0191 -9067/82/040337-16$03.00/0 Copyright c 1983 SUNSAT Energy Council AN INVESTIGATION OF QUASI-INERTIAL ATTITUDE CONTROL FOR A SOLAR POWER SATELLITE JER-NAN JUANG* Guidance and Control Section Martin Marietta Denver Aerospace P.O. Box 179 Denver, Colorado 80201, USA SHYH JONG WANG Jet Propulsion Laboratory California Institute of Technology Pasadena, California 91109, USA Abstract — The concept of using space satellites to collect solar energy for Earth's use was first proposed in the late 1960s. This paper presents an efficient means, namely, quasi- inertial attitude mode, for maintaining the normal solar orientation for power collection in the synchronous orbit. Some formulae are introduced to establish the basic parametric properties for ideal quasi-inertial attitude and phasing. However, it is a fact that energy dissipation in those widely oscillating flexible bodies produces an instability of the quasi- inertial attitude in the sense that the spacecraft will tumble at the orbit rate. An active control system is thus necessary to compensate for the energy loss. A fixed terminal time and state optimal control problem is formulated and an algorithm for determining the optimal control as a means for the periodical attitude and phase compensation is proposed. The controller design is specialized in maintaining the vehicle orientation affected by internal disturbance (structural flexibility) and external disturbances (e.g., drag force). INTRODUCTION Some form of attitude determination and control is required for nearly all spacecraft, particularly for such a large spacecraft as the Solar Power Satellite (SPS) which consists of a huge solar collector platform of planar structure and two transmitting antennae supported by yoke structures. Attitude control for SPS is essentially the process of pointing directional antennae and solar panels for power generation. The dominant environmental torques that affect the attitude of SPS moving in synchronous orbit are the gravity-gradient torque caused by the small difference in gravitational attraction force from one end to the other (the same differential force which produces tides), and the magnetic torque caused by the interaction between the spacecraft magnetic field and Earth's magnetic field. The gravity-gradient torque is both space- and time-dependent when acting on large flexible components such as the SPS solar arrays; when large, the structural flexibility induced internal torque may become significant and its interaction with the gravity-gradient torque thus contributes to the attitude motion. However, the main influence of flexibility is not the ripple in the motion due to flexing, but rather it is the energy that is dissipated in This paper was presented at the NCKU/AAS Symposium on Engineering Sciences and Mechanics, National Cheng Kung University, Tainan, Taiwan. *Currently with NASA Langley Research Center, Hampton, VA 23665.
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