Study On Beacon Beam Precision Control Of Space Optical Communication

With rapid progress of space technology, microwave communication has been unable to meet the growing demand of human society for high speed data transmission bandwidth at real time, while the space optical communication, as the best way, is to solve the bottleneck problem of this technology. Since space optical communication has potentially good prospect of application, it has become research focus of the world’s scientific and technological powers. However, its theory and technology are still not mature enough so far. As the extremely critical technology of space optical communication system, the PAT (the abbreviated form of Pointing, Acquisition and Tracking) technology is to establish space optical communication links and to maintain an important guarantee for its performance directly related to the success or failure of the entire communication system. The composite axis control system which is adopted by PAT, consisting of the coarse tracking assemble, the fine tracking assemble and point-ahead mechanism, can accomplish the large-scale, high-accuracy and long-range tracking mission. The performance of the whole PAT system largely depends on the fine tracking system. According to the experimental platform of space optical communication ground demonstration system, this thesis has made an intensive study of several key technologies in the fine tracking system. And it can provide technical support and practical experience of the fine tracking assemble for China’s future space optical communication system.The study of this issue was partly supported by National Defense Preliminary Research Project. Above all, this dissertation represents the basic concepts, key technologies, both the foreign and domestic status and development trend of research on laser communication and PAT technique. Secondly it briefly gives analysis of space optical communication system links, and then illustrates principles of work, components selection of PAT system and sources of systematic error. We use CMOS (Complementary Metal-Oxide-Semiconductor) camera with high frame rate as the photoelectric detectors and also selsect PZT (piezoelectric ceramic) as the executing component of FSM (fast steering mirror) in our fine tracking demo assemble developed by Laser Communication Laboratory at Wuhan University. In accordance with the requirements, PAT system takes advantage of composite-axis control structure to point, acquire and track laser beacon beam in real time. And we also analyze and investigate the effect of the line-of-sight (LOS) alignment error on the receiving power of the optical signal coupled into the single mode fiber simultaneously.In respect of laser spot identified in the beacon positioning under complex background and low SNR (Signal-to-Noise Ratio) conditions, the performance of several prevalent thresholding image segmentation and positioning algorithms were comprehensively compared in their calculation time and effectiveness. The adaptive threshold segmentation and the improved centroid method of the beacon spot were used to realize precise positioning with the help of variable window technology. In the control strategy studies, PID control and robust control are discussed and compared individually in analysis and design of control system. Taking the uncertainty of fine tracking control system into account, we proposed robust control algorithm. And experimental results show that the system’s dynamic performance and property index improved significantly.First of all, experimental results of the fine tracking system are analyzed and discussed in detail under different conditions. According to prior knowledge of external interference, by making use of the signal generator to simulate different types and frequency of disturbance as beacon beam jitter on indoor demo platform, our fine tracking system can capture and track laser beacon, and also compensate beam jitter to verify the performance and effectiveness of PID controller and robust controller. Furthermore its tracking bandwidth can significantly achieve up to 50Hz. Moreover, we have successfully completed the individual 1.2km and 16km away from point to point ground space optical communication experiments. And the accuracy of the fine tracking system can achieve 3urad and lOurad respectively. The contents of this dissertation can provide an important reference value for China’s future theoretical research and practical engineering design of the PAT system.