Stabilization of attitude alignment control for spacecraft formation flying without velocity

Attitude alignment control in spacecraft flying without the measurement of angular velocity is proposed with actuator saturation. First an auxiliary quaternion dynamical system is defined, and then we presented a nonlinear feedback control law within real and estimated error of unit-quaternion and the relative error unit-quaternion between each spacecraft. In this approach, an output saturation function of the estimated quaternion error is used to provide small tracking error and short settling time. The detailed proof of the stability of the tracking error and formation-keeping system is also presented. The efficiency of the suggested control is verified by numerical simulation on attitude tracking and alignment among a group of spacecrafts under a bidirection ring topology.