High Precision Beam Deviation Angle Measurement Based On CCD In Space Laser Communication

Compared with microwave communication,space laser communication uses laser beam as carrier and has great advantages in communication rate,system volume,mass,power consumption,and safety.In the process of establishing and maintaining the communication link between the terminals of the space laser communication system,it is necessary to quickly and accurately acquire,track and point the laser beam.Accordingly,the deviation angle measurement of the incident beam relative to the receiver antenna plays a decisive role in acquiring efficiency and tracking accuracy.When the laser beam travels a long distance,the spot image formed on the imaging detector has low signal-to-noise ratio and irregular spot shape,it is necessary to design an appropriate spot positioning algorithm to ensure sufficient measurement accuracy of the beam deviation angle.Firstly,the paper analyzes the relationship between the beam deviation angle and the position of the spot on the detection plane,and derives the minimum signal-to-noise ratio that the spot can be detected by the detector.Subsequently,the positioning algorithm of the light spot is discussed.An improved window centroid method is proposed by using the spot boundary information and spot energy characteristics to calculate the spot center.This algorithm can improve the signal-to-noise ratio of the spot image,suppress the noise out of the window,and reduce the computation complexity.Compared with the traditional gray-scale centroid algorithm,it can significantly improve the positioning accuracy.Then,in order to meet the requirement of the real-time angle measurement in the acquiring and tracking phase,the image processing and centroid calculation are implemented based on FPGA(Field Programmable Gate Array).The image data stream from CCD(Charge Coupled Device)is transferred to FPGA directly and processed in the real-time pipeline operation.The time of FPGA processing single frame image is 2.056 ms,which meets the real-time requirement of beam acquiring and coarse tracking with the sampling frame rate of 100 Hz.Finally,the centroid error is measured and analyzed.The single-point non-uniformity correction method is used to correct the collected images and remove the dead pixels.The positioning error of the gray-scale centroid method and the improved window centroid method calculation is 0.0805 pixels and 0.0526 pixels,respectively.The improved window centroid method demonstrates higher positioning accuracy than the traditional gray centroid algorithm.