In this paper, we consider the coordinated attitude control problem without velocity measurements. Based on the recently introduced unit quaternion output feedback for the attitude tracking of a rigid body, we present a class of decentralized coordinated control laws to solve the alignment problem for a group of spacecraft within a formation without velocity measurements. The approach consists of introducing an auxiliary system for each spacecraft and for each pair of spacecraft with a communication link. The vector parts of the unit quaternion, representing the discrepancies between the output of the auxiliary systems and the attitude tracking error as well as the relative attitude errors between spacecraft, are used in the control law instead of the angular velocity and the relative angular velocity vectors. The spacecraft attitudes are guaranteed to converge to a desired attitude (possibly time-varying), while keeping the flight formation during the transient. Simulation results of a scenario of four spacecraft are provided to show the effectiveness of the proposed control scheme.
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