Research On Acquisition And Tracking Technologies In Satellite Optical Communication

Satellite optical communication offers the potential advantages of large data rate and high immunity to interference, and it will play an important role in the field of communication in the future. However, the realization of satellite optical communication link with large capacity and high reliability depends on continuing alignment between two satellite optical communication terminals, which requires high performance for acquisition, tracking and pointing(ATP) subsystem of satellite optical communication terminal. Acquisition is the premise to establish and restore communication links, and the maintenance of the optical communications link needs to achieve high precision tracking. Based on these needs, in this paper, acquisition technology and high precision tracking technology are studied to establish a technical foundation for future development of ATP system.First of all, the basic principle and working process of ATP system are analyzed to lay a theoretical foundation for the study on acquisition and improving tracking accuracy. According to the structural composition of ATP system, the compound axis control of ATP system is analyzed. The closed-loop transfer function and error transfer function of the compound axis control system are derived, which show that the stability of compound axis control system is determined by coarse and fine tracking system together, while the final accuracy of compound axis control system is determined by fine tracking system. Furthermore, the process for ATP system to establish optical communication link and the link budget of ATP system are analyzed. According to the link equation, the requirement for tracking accuracy of ATP system can be evaluated, and then the design of ATP system can be conducted.The pointing error of ATP system is also studied to provide the basis for determination of acquisition and stable tracking strategy. The sources of pointing error are analyzed, and the platform vibration of satellite is the main disturbance source. The way how the platform vibration contributes to pointing error is analyzed, which provides the basis for studying the method to improve system accuracy and realize stable tracking. Pointing error includes dynamic random error and static bias error. To decrease the influence of pointing error on intersatellite optical communication systems, the mathematic statistical model of pointing error is developed, and the relation formula between pointing error and link reliability is derived. The divergence angle of laser beam is selected to optimize the link reliability in the case of zero bias error and nonzero bias error respectively, and the optimum beam divergence angle is obtained. Using the designed optimum divergence angle in satellite optical communication, a good system performance can be achieved with pointing error.To realize the lightweight and miniature design of satellite optical communication terminal, the beaconless acquisition is studied. Based on the estimation of beam divergence angle, the point ahead fine steering mirror in ATP subsystem is used to conduct multi-scan to acquire the optical communication terminal. Then the acquisition method, scan pattern, acquisition mode and other factors which affect acquisition time are analyzed and modeled, and the average acquisition time of parallel processing acquisition mode and serial processing acquisition mode are obtained respectively. The field of uncertainty is selected properly to minimize acquisition time, and the acquisition time is reduced to a certain extent. Then, simulation and experiment are conducted, the results of which are both consistent with theoretical analysis, showing that, when the point ahead fine steering mirror is used to conduct multi-scan to acquire the optical communication terminal, the serial processing acquisition mode take less time to acquire target than parallel processing acquisition mode, and that the probability that the target can be acquired within 5 times of scanning is more than 98%.The method of rejecting platform vibration is studied to realize high precision and stable tracking. An adaptive feedback control technique based on Youla-parameterization is proposed to be used in fine tracking system to compensate for vibration-induced optical jitter and improve tracking accuracy. The main idea is to use the well-known Youla parameterization formula to construct a feedback control scheme with the guaranteed closed loop stability, and the feedback controller is updated by a RLS(Recursive Least-squares) algorithm in realtime to improve the system ability of rejecting disturbance. An experimental testbed is developed and experiment is conducted to test the adaptive feedback control algorithm. The result of experiment shows that compared to classical PI controller, the jitter suppression of the adaptive Youla-parameterization controller is increased by 9.1d B. Besides, the jitter rejection ability of PI controller is limited by bandwidth of control system, while the adaptive Youla-parameterization controller can reject optical jitter to some extent almost in the whole frequency domain, so that the spectral range which thesystem can suppress is expanded. Moreover, the adaptive Youla-parameterization controller can eliminate spikes of vibration to make the power spectral density smoother.