| The laser communication has the advantages of higher bandwidth, higher capacity, lower energy consumption, and better security compared with the microwave communication. Moreover, its termination is much smaller and lighter, and thus using it as a vehicle of information became the tendency of the development of satellite-to-ground laser communication. There are a lot of countries and areas such as America, Japan, Europe which have already done many satellite-to-ground communication experiments, and they are working actively in order to make it more practical and commercial. In the future the establishment of a global coverage network of satellite laser communication is its development tendency.As an essential part of the satellite-to-ground laser communication, the laser links are influenced by atmospheric turbulence and atmospheric refractivity. The atmospheric turbulence makes the light beams drift, expand or even breaks the light spots, and thus changes the intensity of the light signals and brings a lot of difficulties to acquisition, pointing and tracking from both terminals of the communication. Therefore, we have to know the characteristics of the transmission of light beam in atmospheric turbulence in order to analyze the performance of communication. The atmospheric refractivity changes according to the altitude, which curves the transmission path of the light beams and leads to deviation in the pointing from both terminals. The research on the deviation caused by the atmospheric refractivity can supply the reference for the design of the communication termination. There is a "maximal zenith angle" in the satellite-to-ground communication links, which restricts the overlay area of each single satellite. The "maximal zenith angle" is determined by the atmospheric condition and the performance of the communication termination. It is the one of the most important factors to determine the performance and cost of satellite, and needs to be considered carefully in the satellite constellation design.This dissertation improved the simulation method of light beam transmission in atmospheric turbulence and made a conclusion by comparing the result of simulation with the actual result of the experimentation. At the same time, the dissertation discussed the influence of the atmospheric refractivity to the satellite-to-ground links, and analyzed the characteristics of the maximal zenith angel in the satellite-to-ground laser communication downlinks. The main research work and innovation of this dissertation are as follows:(1) By summarizing the multi-phase screen simulation method of light beam transmission in atmospheric turbulence base on standard parabolic equation, this dissertation elucidated how to choose grid parameters when discretized the turbulence and optical field. By the numerical analysis of the practical links, we found out that the existing methods could not guarantee the reliability when the divergence angle was larger than 100μrad. For such kind of condition, the characteristics of the maximum spatial sampling rate was discussed, and an improved method using multi-phase screen was developed with the examples of divergence angles of 100urad and 1.35mrad in horizontal links. The results showed that by using the improved method, the characteristics of the light field and the theoretical expectation as well as the experimental result were almost the same, which proved the reliability and practicability of the new method.(2) When the light beam divergence angle is relatively large, with the links grow longer, the simulation needs very large memory of the computer, sometimes even larger than the capability of the experimental computer. By analyzing respectively the relationship of accuracy of simulation with the value of grid parameters and the conditions of different links, this dissertation not only proposed an optimal grid parameters solution, but also fitted out an empirical formula of the relationship of the optimal grid parameters with the divergence angles and the lengths of the links, which helped a lot to determine the value of grid parameter and lowed down the memory requirement while guaranteeing the reliability during the procedures of simulation.(3) For the design of the acquisition, pointing, tracking (APT) system, the influence of atmospheric refractivity on APT and pointing in advance. A formula based on the the wavelength of light, air temperature and atmospheric pressure, considering the air temperature and atmospheric pressure as functions of the altitude, an atmospheric refractivity model affected by altitude and wavelength of the light was induced by segregate approximation of the atmospheric refractivity. By defining the pointing error angles in the acquisition and communication process, the relationship of the pointing error angles and the zenith angel of the links was analysed by simulation.(4) Focusing on the demand of the constellation design of the satellite-to-ground laser communication system, a model of the maximal zenith angle was induced. Considering the links which used on-off-keying (OOK) modulation and direct detection of avalanche photodiode detector (APD), the factors including the transmitted power of the satellite borne laser, the aiming error angle and the background light power were analyzed,which all affected the maximal zenith angle. Relationship among the maximal zenith angel, the intensity of atmosphere turbulent close to ground, and the bit error rate was obtained, which could be the reference for the constellation design of the satellite-to-ground laser communication system.
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