Inversion Of Fluctuation Characteristics And Turbulence Intensity For Loess Plateau

Atmospheric laser communication is a communication mode in which laser is used as information carrier to transmit information in the atmosphere.Atmospheric laser communication is a kind of technology it has become a perfect indispensable to construct the future communication network,it is also the most potential applications in the future of military and civilian communication mode,with its network mode is flexible,engineering installation and convenient maintenance,and is not subject to electromagnetic interference,it has a large bandwidth capacity,and has the advantages of high reliability and confidentiality,gradually become a global hot spot in recent years.However,in the process of data transmission by laser in atmospheric channel,as the signal is exposed to the external environment,when the laser communication link goes through the atmosphere,it will be affected by atmospheric turbulence,atmospheric attenuation and space loss,etc.,especially the atmospheric turbulence and atmospheric attenuation have the most significant impact on the beam in the transmission path.Atmospheric turbulence distorts the laser wavefront,causing beam drift,beam expansion and arrival angle fluctuation.At the same time,the laser communication is affected by atmospheric turbulence and atmospheric attenuation,which will cause the wireless optical signal to miss the target,giving rising to large fluctuation error and bit error rate,and will also appear intermittent communication interruption,which will seriously affect the stability and reliability of the communication system.At present,a large number of studies on atmospheric turbulence and attenuation characteristics in atmospheric laser communication show that different rules can be presented in different environments(such as different underlying surfaces under laser transmission channels or different meteorological conditions).Therefore,it is very meaningful to study the fluctuation laws of light intensity under different underlying surface,different weather and different time.However,the current research shows that the laser transmission experiments are mostly carried out at night,and from the perspective of geographical distribution,the laser transmission experiments are mostly carried out in the developed areas along the east coast.Therefore,the selection of the Loess Plateau region to carry out laser transmission experiments,which has the characteristics of semi-arid climate,and the underlying surface is not uniform,resulting in more complex turbulence characteristics than other regions in China.In addition,the region has a relatively high aerosol optical thickness compared with other regions,and the frequentoccurrence of dust storms and floating dust weather results in a significant difference in aerosol composition and distribution compare with the eastern coastal region.Therefore,an all-weather experimental study on the fluctuation of light intensity was carried out in this region.The main work of this paper is as follows:(1)Combined with the theory of light intensity fluctuation,a wireless optical link in length of 720 m is established in Loess Plateau area to investigate the light intensity distribution of Gaussian laser under different weather conditions.The spatial fluctuation characteristics of the light intensity of gaussian laser at different time are studied.The results show that the fluctuation of light intensity is the most drastic from 10 AM to 2 PM in 24 hours of a day,and the fluctuation of light intensity also varies greatly at the time of sunrise and sunset in the morning and evening.(2)during the daytime experiment,due to the interference of sunlight,the collected spot image has bright spots,also known as pepper and salt noise.The adaptive median filter is used to de-noising the spot image.The results show that the noise interference is reduced and the accuracy is improved after the median filter is used.(3)based on the machine learning theory,the atmospheric refractive index structure constant was estimated and the turbulence intensity was inverted.Successively adopted the PCA data dimension reduction method and mind evolutionary algorithm,the BP neural network is optimized,further improved the accuracy of the atmospheric refractive index structure constant estimation value and the error range is reduced,measured values are compared with the experiment,the results show that both the alignment is good,can show basic optical turbulence characteristics of the Loess Plateau region.The analysis results show that the correlation coefficient between the estimated value and the observed value is above66%.The error analysis of root-mean-square error,average absolute percentage error and average absolute error shows that the optimized BP neural network model can basically and accurately estimate the atmospheric refractive index structure constant in the loess plateau region,and this constant can accurately describe the change of atmospheric turbulence strong or weak activity in this region.The research results of this paper can provide some guidance for the development of laser communication in the Loess Plateau area.