Key Technology Research Of Coarse Pointing Mechanism For Laser Communication Based On Risley Prisms

Space laser communication is a communication method that uses laser as a carrier to transmit information in space.Benefit from the excellent characteristics of laser,laser communication has the advantages of high communication capacity,high security,long communication distance,and lightweight terminal,which is an important technology to complement microwave communication in specific scenarios and achieve ultra-high speed and high capacity communication.The extremely small divergence angle of the laser and the long communication distance require a high-precision,stable and reliable PAT(Pointing Acquisition Tracking)system to achieve stable communication.The development trend of miniaturization has put forward higher requirements on the volume and quality of communication terminals.Compared with the traditional universal turntable,Risley Prisms as coarse pointing mechanism have the advantages of small size,light weight,low power consumption,compact structure and strong antivibration capability,which is very suitable for small satellite communication,airborne communication and other application scenarios.The use of Risley prisms as coarse pointing mechanisms for laser communications requires the solution of two key problems.Firstly,the effect of systematic errors on pointing accuracy.Secondly,the influence of the characteristics of the Risley prisms on the capture performance.This thesis focuses on the two key issues,and the main research contents are as follows:1.The current application status and research progress of Risley Prisms are investigated,the existing theoretical models and inverse solution methods of Risley Prisms are summarized and compared.The widely used non-paraxial ray tracing method and the two-step method are selected as the forward and inverse solution methods according to the requirements of laser communication application scenarios.The mathematical model of Risley Prisms is derived by the vector refraction law.2.The first key issue,the effect of systematic errors on pointing accuracy,was investigated.Firstly,the system error sources are analyzed,the vector representation of the directional system error is defined,an error model of the Risley Prisms is established,the pointing error caused by each system error and the influence on the light motion trajectory are numerically simulated by the error model.After comparing the existing forward solution error correction algorithms,the error parameter identification method is selected as the forward solution error correction algorithm,and simulation experiments are conducted for the forward solution correction.The inverse solution error of the two-step method is studied for the first time.Based on the results of the forward solution error analysis,the causes of the inverse solution error of the two-step method are explained.The correction algorithm based on pointing field transformation is proposed to correct the inverse solution error.The results of the simulation experiments showed that the maximum value of the inverse solution error and the RMS value of the two models decreased by 82.58%,87.57% and 82.49%,87.22%respectively.3.The second key issue,the influence of nonlinear effect of the Risley prisms and changing the laser divergence angle on the capture performance are studied.Firstly,the capture mode and capture performance index of the space laser communication system are introduced.Secondly,the non-linear effect of the Risley Prisms is numerically analysed,and the variation law of the beam deceleration ratio in the pointing field when the light moves radially and tangentially is obtained.Third,the functionl relationship between the divergence angle of the outgoing beam and the deflection angle is derived.The influence of the change of the divergence angle of the outgoing light on the light intensity is analyzed.An elliptical Gaussian light scanning step model is established,an adaptive step formula is proposed and the optimal expansion factor is calculated,which can dynamically adjust the scanning step according to the position of the uncertain region to reduce the number of scanning steps.Fourth,in order to analyse the effect of singularity on the performance of different scanning methods,the spiral scan,rectangular scan and rectangular spiral scan are modelled respectively.The maximum prism rotation angles of the single-field scanning of the three scanning methods are obtained by numerical simulation when the uncertain region is located at different positions.Fifth,simulated capture experiments are conducted to analyze the capture performance of three scanning modes.4.An experimental platform is built and an angle measurement method that can measure a wide range of angles with high resolution is proposed.The forward and inverse solution error correction experiments were conducted.Based on the experimental data,the system errors were obtained by the error parameter identification method,and the correction effect of the pointing field transformation method on the inverse solution error was verified based on the actual error model.The results show that the maximum value and the RMS value of inverse solution error for the small range model and the large range model are reduced by 89.60%,83.52% and 60.42%,49.07%respectively,proving the effectiveness of the algorithm.Finally,simulated capture experiments were conducted to verify the simulation results on the capture performance of the system.