A stellar gyroscope is an attitude sensor that is composed of a micro star sensor and a microelectro-mechanical system gyroscope. The attitude is the output by fusing the information of the two sensors. Due to the processing of the star image from the star sensor, a measurement delay is introduced and can affect the performance of the fusion of the two sensors. In this paper, a factor graph method that is based on a sliding window is investigated for dealing with the measurement delay. Temporary factors are calculated to temporarily occupy the places of the factors that are under processing, and sliding window incremental smoothing is only performed with a set of factors and states after the delayed measurement has arrived. To reduce the frequency of attitude calculation based on a star image, the keyframe strategy is adopted. Only in keyframes, the attitude is solved by using the star image and used as the measurement of the factor. In nonkeyframes, only the star positions in the star images are determined, with a robust star tracking method that is aided by the gyroscope. Moreover, a high-rate low-latency output framework that is by a gyroscope is proposed for accelerating the attitude output rate. According to the results of the experiments on simulated data, the proposed method can overcome the problem of measurement delay and is more effective than the compared methods. According to the results of the experiments on real data, the proposed method can be used in applications. INDEX TERMS Attitude estimation, factor graph, temporary factor, sliding window incremental smoothing. I. INTRODUCTION With an ever-widening range of space missions becoming feasible, attitude determination performance is critical for many spacecraft. Since a small spacecraft may have a limited payload, only compact, low-power and miniature sensors can be used on satellites. Using new sensor technologies, such as micro-electro mechanical system(MEMS) gyroscopes and active pixel sensor (APS) imaging devices, suitable attitude sensors, namely, stellar gyroscope, has been developed by many institutes, such as the Charles Stark Draper Laboratory, University of Kentucky and StarVision Technologies [1]-[3]. A stellar gyroscope is composed of a MEMS gyroscope and a micro star sensor and is small, lightweight and inexpensive [4]. Under typical operating conditions, a star sensor can output attitude information accurately with a low output rate. The output rate of the gyroscope is high; however, the angular velocity suffers due to long-term stability, namely, The associate editor coordinating the review of this manuscript and approving it for publication was Zheng Hong Zhu.