Design And Performance Analysis Of The Terminal Control System For Inter-Satellite Laser Communication

Satellite laser communication is superior to the traditional microwave communication thanks to its higher communication rate, larger capacity of data transmission, stronger anti-interference ability and higher security, and therefore is gradually becoming the focus of research in many countries. To achieve more reliable inter-satellite laser communications, three steps are required: fast acquisition, precise pointing and stable tracking, in which the terminal control system of inter-satellite laser communication plays a critical role. In this thesis, the terminal control system of intersatellite laser communication is designed, the proposed pointing error is thoroughly analyzed, and suppression or elimination algorithms are put forward to address for the major interfering factors:First, the APT system of inter-satellite communication is described in detail, including the structure of its terminal control: composite axis of coarse pointing subsystem and fine pointing subsystem. To build the terminal control system for intersatellite laser communication, the coarse pointing subsystem and fine pointing subsystem is completely designed respectively, and its system components and control system design is described. A coarse pointing control system is proposed based on double closed loops containing a position one and a speed one, and a fine pointing control system is presented based on double closed loop control structure comprising a piezoelectric displacement loop and a beam displacement loop.Secondly, the performance of the inter-satellite laser communication control system is analyzed, and pointing error is chosen as its performance index. The pointing error is analyzed from two aspects: the distribution of the received beam and the bit error rate of satellite laser communication, with the results showing that the pointing error has a huge negative impact on system performance. By investigating the origin of pointing error, the main factors are identified as vibration of the satellite platform and detector noise. Their causes, characteristics and types are analyzed, and their impact on system performance is studied.After that, vibration of the satellite platform, one origin of pointing error, is analyzed in detail. Common approaches for suppressing the satellite platform vibration are introduced. A scheme is proposed using the design data of SILEX platform or the measured data of Landsat satellite to design a filter. The white Gaussian noise is filtered to obtain the simulated signal of satellite platform vibration, which then undergoes Fourier transformation to obtain the simulated vibration power spectrum. Therefore, the effectiveness of the algorithm is verified. The feed-forward vibration suppression algorithm is analyzed, and, to address its limitation, a vibration suppression algorithm is put forward, which was based on the improved LMS adaptive control algorithm. Simulation results show that it can suppress vibration satisfyingly.Finally, detector noise, another origin of pointing error, is analyzed in detail. According to their features and inducing factors, the types of detector noise are divided into two categories: the small mixed noise and the stellar background noise. Then their effects on the location of spot center are analyzed respectively. To remove most of the small mixed noise, a hybrid filtering algorithm is put forward based on the median-mean filtering algorithm. For the stellar background noise, it is proposed to first carry out threshold segmentation using OTSU method, and then to separate the effective spot from the stellar background noise based on Two-pass connected region labeling. The filtering result is shown to be satisfactory.