Research On Method For Precision Tracking Departure Angle In Satellite-ground Laser Links

In the satellite-ground laser communication systems, the laser beamwidth is very narrow. In order to quickly found and maintain a stable communication link, laser communication terminal must be with the ability of high-precision pointing and tracking. High-precision tracking departure angle is the important guarantee for the foundation and maintenance of laser link. Therefore research on acquisition for the method of high-precision tracking departure angle is very urgent and necessary.Atmospheric channel is a part of satellite-ground laser communication channel. As an information carrier, the beams through the atmosphere are affected by atmospheric turbulence. In satellite-ground laser communication systems, atmospheric turbulence will affect the light intensity distribution of the beacon laser beam received in tracking system receiving termina. It will lead to spot distortion, reducing acquisition and tracking accuracy, making the system detection accuracy decline. thus increasing aiming deviation, decreasing the received power, increasing the error rate, and even making the establishment of satellite laser links become difficult to found and maintain. So the method of acquisition for laser spot center, to effectively correct or reduce the effects of atmospheric turbulence, is very important for foundation and stable satellite-ground laser communications.This paper focuses on finding methods to get high-precision departure angles, by the intensity distribution of incident beacon beam images from the receiving laser terminal CMOS, and thus suppressing the effect on the detection accuracy of the tracking departure angle caused by atmospheric turbulence. This is one of the key issues for the foundation and maintenance of the satellite-ground laser communications.Firstly, the researches on the techniques of satellite–ground laser communication are summarized. The theory of detection mechanism of the tracking departure angle is introduced. The method of acquisition for the tracking departure angle is analyzed. On the basis of the analysis, further researches are carried out in this paper.The horizontal and vertical convex image geometric centroid regeneration method is proposed, by which the image geometric centroid is recalculated using grayscale distribution of incident images from the receiving laser terminal CMOS. In the process of eliminating image noise spot on images, if the euclidean distance between the pixel and the geometric centroid of the image spot is far than normal, the pixel is useless, and then this image is reconstructed into a single connected convex region in the horizontal and vertical direction. It makes the geometric centroid calculated by the regeneration image more closely to the ideal center of laser beam on the receiving CMOS. It improves the detection accuracy of tracking departure angle. Geometric meaning of the formula for the the horizontal and vertical convex image geometric centroid regeneration is also shown. The paper demonstrates how to choose the filter threshold of the distorted image, in order to filter the single point noise. The long memory dynamic optimization method of filter threshold is presented, combined the filter threshold of single point noise with the image effected by the atmospheric turbulence, so that the memory threshold optimizes filtering threshold at current frame.The iterative spectral phase distribution regeneration method for the higher-precision tracking departure angle is proposed. According to diffractive optical theory, spectral phase distribution can be iterative calculated. After phase correction, the incident iamge intensity distribution is obtained by convolution processing, using the receiving laser beam at the CMOS optical field distribution. And combined with the horizontal and vertical convex image geometric centroid regeneration method, higher-precision with the tracking departure angle can be attained. The formula of the tracking departure angle after phase correction is given. The iteration initial value of the iterative phase distribution regeneration method is represented. Combined with the initial value of iteration with the peak of the iamges, so the iteration initial value versus atmospheric turbulence is given. The convergence rate of the iterative phase distribution regeneration method is researched, and compared with the conventional method. The error of iterative phase distribution regeneration method is analysed. The correction factor is determined.According to the theory of atmospheric phase screen, the numerical simulation model for the long-distance distortion intensity distribution is established. And an11.2km urban laser link is established to demonstrate the long term experimentation of the laser beam tracking departure angle through atmospheric turbulence of the simulated ground-to-satellite laser communication. Images in static and dynamic links are researched with different scintillation.This paper proposes the horizontal and vertical convex image geometric centroid regeneration method and iterative phase distribution regeneration method. By suing intensity distribution of the received laser beam from the satellite-ground terminal CMOS, the influence of atmospheric turbulence on satellite-ground laser communications is reduced, the better precision of tracking departure angle is obtained. This will help to improve the performance of pointing、acquisition and tracking system, found and maintain a stable communication link quickly in satellite-ground laser communications. The research presents important theoretical significance and practical value for optimization of satellite-ground laser communication system design and the performance improvement of the communication system.