Reserach On Beacon Spot Imaging Algorithm In Satellite-ground Optical Communication

A laser beam is regarded as the carrier of information for satellite optical communication technology. With the wide frequency band, high transmission rate, good security, low power consumption, satellite laser communication, which will enjoy extensive development prospects in the modern society, has become a research hot topic of all countries. In the satellite optical communication system, the precise localization of light and fast acquisition will directly affect the establishment of laser communication link. Therefore, the research on high precision spot location algorithm is of significance.In satellite laser communication system, pointing, acquisition, targeting system(PAT system) is the core technology; capturing beacon by the PAT system is a prerequisite for the establishment and maintenance of laser communication link. To realize the fast acquisition and stable tracking of the beacon light, the center position of the laser spots in detector is required to be calculated quickly and accurately. The gray centroid algorithm is the most common spot location algorithm which is widely used because the calculation is simple, fast, etc. The influence of atmospheric turbulence in satellite-to-ground laser links result in distortion of imaging spots, which make it difficult to realize beacon acquisition. Accordingly, a new algorithm of spot location, which is named as neighborhood pixel gray fitting method, is proposed in this paper. The paper presents the basic principle and the specific calculation formula of the method.The pixel neighborhood fitting method is used to find the spot position information through gray correlations of adjacent pixel of a spot image. When gray values of the spot fluctuate dramatically, the corresponding correlation coefficient of the pixel will be smaller. Consequently, the pixel neighborhood fitting method can effectively reduce the interference of atmospheric turbulence on the spot location. Particularly, for the spots affected by the atmosphere which has high intensity distribution around the edges, this new algorithm is able to improve positing accuracy of laser spots compared with the traditional centroid algorithm.In this paper, analyze the transmission characteristics of far field in the theory and conduct a simulation by Matlab. Based on Zernike polynomials, the atmospheric turbulence phase screen is generated and laser imaging spots of passing through atmosphere channel is simulated as well. The centroid of spot is calculated according to neighborhood pixel gray fitting so as to verify the accuracy and reliability of the algorithm.An 11.16 km long-distance laser link experiment is established. The method is used to accurately calculate the centroid of beacon spot and verify its own effectiveness.