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Free space optical communication is one of the best approaches in future communications. Laser beam's acquisition,
pointing and tracking are crucial technologies of free space optical communication. Fine tracking system is important
component of APT (acquisition, pointing and tracking) system. It cooperates with the coarse pointing system in
executing the APT mission. Satellite platform vibration and disturbance, which reduce received optical power, increase
bit error rate and affect seriously the natural performance of laser communication. For the characteristic of satellite
platform, an active disturbance rejection controller was designed to reduce the vibration and disturbance. There are three
major contributions in the paper. Firstly, the effects of vibration on the inter satellite optical communications were
analyzed, and the reasons and characters of vibration of the satellite platform were summarized. The
amplitude-frequency response of a filter was designed according to the power spectral density of platform vibration of
SILEX (Semiconductor Inter-satellite Laser Experiment), and then the signals of platform vibration were generated by
filtering white Gaussian noise using the filter. Secondly, the fast steering mirror is a key component of the fine tracking
system for optical communication. The mechanical design and model analysis was made to the tip/tilt mirror driven by
the piezoelectric actuator and transmitted by the flexure hinge. The transfer function of the fast steering mirror, camera,
D/A data acquisition card was established, and the theory model of transfer function of this system was further obtained.
Finally, an active disturbance rejection control method is developed, multiple parallel extended state observers were
designed for estimation of unknown dynamics and external disturbance, and the estimated states were used for nonlinear
feedback control and compensation to improve system performance. The simulation results show that the designed
controller not only accurately estimates and compensates the disturbances, but also realizes the robustness to estimation
of unknown dynamics. The controller can satisfy the requirement of fine tracking accuracy for free space optical
communication system.
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