Research On Highly Stable Acquisition And Tracking Technologies Of Space Two-axis Optoelectronic Gimbals

After decades of research, the space optical communication terminals have gone through two generations of development. Several countries have accomplished a series of on-orbit optical communication experiments, and have made glorious achievements. Due to the strong directionality of laser, there is a need for the specialized Acquisition, Tracking and Pointing(abbreviated to ATP) systems to establish and maintain a high-precision alignment of the optical link. In space optical communications, the link availability and bit error rate make their requests for the acquisition stability and the tracking & pointing accuracy, respectively, of the ATP systems. At the same time, the ATP systems face the challenges from the space environment and space platforms. Therefore, ATP technique is one of the key technologies of space optical communications.This dissertation is founded by the Knowledge Innovation Project of the Chinese Academy of Sciences. In the context of the satellite-borne ATP system development for the satellite-ground quantum communications, this dissertation studies the highly stable acquisition and tracking technologies of the space two-axis optoelectronic gimbals. In this dissertation, the ATP workflow, system structure and operating principles are firstly introduced, as well as the hardware configuration and software algorithms of the two-axis optoelectronic gimbal. Then the common test methods of the acquisition and tracking performances are summarized, and the basic performances of our system are tested. The impacts of the external environment constraints on the system performance are analyzed, including the atmospheric channel characteristics, the satellite platform disturbances, as well as the influences of the space thermal environment. Aiming at solving the practical problems, the acquisition methods and acquisition sensitivity are analyzed, several optimization algorithms are made, and the acquisition system stability is improved. At last, the tracking error of the two-axis optoelectronic gimbal is comprehensively analyzed, the factors which constrain the system’s ultimate tracking performance are discussed, and some methods to improve the tracking accuracy are pointed out.The main innovations of this dissertation are as follows:1) Considering the demands for rapid recovery after the link interruptions in optical communications, a fast reacquisition approach based on predictive filtering ofIV angular information is proposed. According to the pointing angle characteristics in satellite-ground optical communications, an optimized finite memory filter is designed. Simulation results show that the algorithm prediction accuracy is better than 0.05? in 3 seconds, and is better than 0.1? in 5 seconds. Experimental results show that, after about 5 seconds of dark period, the system can complete reacquisition within 1 second. In contrast, the scan acquisition method typically takes several tens of seconds, so that the approach can effectively improve the link availability of optical communications.2) For the low-light robustness problem, an optimization algorithm based on variable velocity constraint of the acquisition system is proposed. The satellite-ground relative velocity is predicted from the light spot centroids and angle measurement data, and is used as the reference for the velocity control, realizing a variable limitation of the acquisition velocity. The simulation and experimental results show that the optimization enables an enhancement of the acquisition sensitivity by about 5d B. This method has been applied to the ATP system of a satellite-ground quantum communication experiment, and ground tests showed that the beacon acquisition process stability was significantly improved. In addition, for power-limited deep-space optical communication links, the method can also improve the success rate of the link establishment.3) The tracking performance of the two-axis optoelectronic gimbal is comprehensively analyzed from systematic perspective. The results show that the gimbal tracking accuracy is mostly affected by the sampling delay error due to the limited detector frame rate, and the system’s ultimate bandwidth is limited by both the mechanical resonance frequency and the detector frame rate. The method that uses a window-adjusting CMOS detector is pointed out as one way to improve the tracking accuracy. Theoretical analysis shows that the window-adjusting tracking method can upgrade the ultimate tracking bandwidth of the two-axis gimbal from 4Hz to 15 Hz,while the tracking accuracy can be correspondingly improved from 20~30urad to 10 urad order.The results of this dissertation play a role in promoting the highly stable acquisition and tracking technologies for space optical communications. Integrating the several approaches and ideas presented in this dissertation, it helps a lot to realize a miniaturized, non-compound-axis ATP terminal, which is based on the two-axis optoelectronic gimbal structure. Due to its simplified structure and high stability, it will have practical value for space optical communications with relatively short range and medium data rate.