The Space Laser Communication Link Error Correction Techniques Based On Image Detection

As consequences of the high frequencies and the high directivity,compared with conventional radio frequency systems,there are several advantages offered by free space optic systems,such as potential increase in modulation bandwidth,smaller size of the antenna,lower inter-channel crosstalk,and stronger confidentiality.Applications,such as space links,could benefit from free space optic connections.However,the high directivity of the transmitted beam increases the pointing difficulties.Inaccurate beam pointing can result in large signal fades at the receiving site,which would severely degrade system performance.Therefore,a acquisition,tracking and pointing(ATP)subsystem is required restrain the signal loss caused by pointing error.Based on the beacon imaging signal,the pointing error can be corrected to ensure the stability of the link and the reliability of the communication.The communication link errors caused by various factors will lead to the difference between the actual laser images and the ideal laser image.The cause of the error of communication link includes platform vibration,relative motion between terminals,atmospheric effect,and the optical aberration of the optical system,wherein the platform vibration and atmospheric effects is the main reason for introducing dynamic random error to the communication link,and the first order tilt aberration is the main dynamic random aberration.The optical aberration of the optical system is the main reason for introducing static error,and the low order primary aberration is the main static aberration.Compared to other reasons,the atmospheric turbulence effects on the laser propagation is more complex,and because the beam loses its power mainly during the propagation process,the thesis analyses of the laser propagation in free space atmospheric channel.The effects of measurable parameters on the beam characteristics(angle of arrival fluctuation,beam wander),link performance(tracking error,receiving efficiency)and imaging characteristics have been discussed.Therefore a system optimization scheme is proposed by changing ther parameters of the transmit laser.In order to deal with the dynamic random tracking error caused by the relative motion between terminals,the vibration of the platform and the atmospheric effect,and get a more accurate target state,this thesis introduces the Kalman filering technique,which is the optimal estimation technique for random signal processing,into the free space laser communication target location to deal with the random disturbance.To overcome the difficulty of choosing the dynamic model and giving the strength of noise that applying the Kalman filter will face,this thesis presents a modified algorithm,based on the characteristic of laser point targets.To better fit the dynamics of a video point target,the two-stage dynamic model,which can changes its form with regard to the value of correlation time constant,has been chosen as the state transition model for Kalman filtering.Moreover,the process noise is estimated according to the dynamic model and the observation data.Meanwhile,the observation noise is estimated according to the grey value distribution in each image.Then the process noise and the observation noise are adaptive.According to the outfield experimental result,the methods we proposed could effectively ensure the tracking accuracy.In order to deal with the static error caused by optical aberration of the optical system,the effects of various primary aberrations on the link performance and communication performance are analyzed,based on the Zernike polynomial expansion.Afterwards,the causes and distribution of the errors in the optical system of the telescope are analyzed.According to the error distribution analysis,under the existing technical conditions,reducing the error of telescope is an effective method to realize static aberration correction.In order to correct the static aberration,a new method estimating and correcting the error based on the morphology of image directly is developed.By introducing a new image operator to measure the imaging quality,this method connected the value of this operator and the value of alignment error.By taking Cassegrain telescope system as an example,through simulations and experiments,a mathematical model which can express the correspondence between the secondary mirror error and the measure function was represented.Experiments showed that the alignment error could be corrected automatically based on this model,and based on the model,an effective method for automatic correction of telescope static aberration is proposed.