Fig. 1. SPS control requirements in orbit. c. Constructions in Orbit In the SPS construction phase, the total system characteristics are greatly changed. Although the shape control and pointing accuracy stated in a. and b. are not required in this phase, the control system must take care of this great change in configuration. Modular and hierarchy control system architecture is suggested. CENTRALIZED VS. DECENTRALIZED CONTROL STRATEGY Among the SPS control requirements mentioned above, active attitude and shape control problems have been extensively discussed during this past decade. Almost all research work to date, however, treated centralized control. Centralized controllers are usually designed for the reduced order model (ROM) of vibrational equations in the modal space. Therefore it is pointed out that it suffers from the model mismatching due to mode truncations as well as uncertain modal parameters of structures. On the other hand, in the decentralized controller design, only some small-sized part of the total SPS dynamics models are required and local controllers can be designed based on the local information of the decoupled each subsystems. The total system's stability can be evaluated by aggregating these subsystems (3). From the practical point of view, decentralized controllers have the following additional advantages: 1) Small on-board processors can implement control laws, 2) a large amount of control power/signal transmission between distant locations is not essentially required, and 3) the fault-tolerance can be expected.
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