Monte Carlo Simulations Of Radiative Transfer For Space-borne Lidar

Laser radar is a kind of active remote sensing technology.It is a new kind of advanced detection technology including the advantages of the laser technology and the radar technology.Besides the characteristics of high brightness,high monochrome and high coherence of laser beam,it also including the ability of long-range detection of radar.Space-borne Lidar is a laser radar on the satellite,it has a high orbital height,a wide range of measurement,can provide a global distribution of detection data.As the laser beam from high altitude into the atmosphere directly,can obtain high-quality measurement data.To understand the radiation performance of laser radar light source,to evaluate the characteristics of laser beam,and to analyze the propagation of laser beam in atmosphere can provide guidance for the measurement and detection of laser radar.The interaction between the laser beam and the transmission medium is studied,and the transmission characteristics and laws of the laser beam are revealed by the study of the optical properties of the transmission medium.Firstly,the paper introduces the structure and working principle of the laser radar,including the GLAS system,and investigates the current situation of the laser radar radiative transfer at home and abroad,describes the theory of radiative transfer and the characteristics of atmospheric media,and then simulates the laser transmitting system,the laser transmission process and the receiving of the laser echo signal by Monte Carlo method.A space-borne laser radar radiative transfer model(MCSbLRT)is established,which can be used to simulate the receiving of laser radar echo signals and provide reference for the use of Space-borne laser radar.The main works of the thesis are as follows:1.The working principle of space-borne laser radar and the characteristics of laser transmission in atmosphere are studied.Based on the Monte Carlo method,an appropriate radiative transfer algorithm for Space-borne laser radar is established,and the possible problems of simulating space-borne laser radar radiative transfer are analyzed.As the local estimation method and the Russian roulette technique are improved,the signal receiving efficiency is improved and the variance is reduced.2.The radiative Transfer Model(MCSbLRT)of Space-borne laser radar is established by using the scientific software development method.3.The use of VC++ language,the development of a friendly visual software interface,so that the function of the software MCSbLRT and the required parameters are more intuitive and convenient;4.Using OPAC software,optical parameters such as absorption coefficient,extinction coefficient,scattering coefficient,the single scattering albedo and asymmetric factor of aerosol and cloud particles needed in the simulation process are obtained,and the optical parameters of 0-10km heights are solved according to the formula of aerosol optical parameters changing with height.In order to describe the effect of relative humidity on the optical parameters of aerosol particles from visible to infrared wavelengths in details,the wavelengths between 0.40um and 18um(visible to infrared),49 wavelengths,the scattering characteristic parameters of spherical particles for four type aerosols WASO(soluble aerosol),SSAM(sea salt aerosol),SSCM,SUSO(sulfate)in 8(0%,50%,70%,80%,90%,95%,98%,99%)in the small scale range under the condition of relative humidity are computed,including extinction coefficient,absorption coefficient,scattering coefficient,asymmetric factor and the single scattering albedo.5.Using the established Space-borne laser radar radiative transfer model(MCSbLRT)to analyze the influence factors of laser radar transmission,it is found that the echo signal received more and more with the increase of the single scattering albedo of the aerosol;The scattering energy of laser radar is mainly concentrated in single scattering;The increase of simulated photon number is linearly related to the simulation time under homogeneous atmosphere.6.Using the established Space-borne laser radar radiative transfer model(MCSbLRT),the simulations were carried out.Different simulation results were obtained by setting different aerosol single scattering albedo and receiver position,and the influence of various parameters on the transmission results of laser radar was analyzed.7.We developed a new analytic expression based on the fundamental theory of electromagnetic scattering and radiation transmission,the analytic expression of the single scattering phase function is studied.We compared the Henyey-Greenstein phase function,the Henyey-Greenstein*phase function and the new phase function for different asymmetry factors,and found the new phase function provides a more realistic description for the scattering of unpolarized light by small particles.Furthermore,the calculated value for ratio of scattering intensity at 90 degree to the scattered intensity in the backward direction is more reasonable.We also investigated the effectiveness in approximating scattering from polydispersed particles by comparing the new phase function,the Henyey-Greenstein*phase function and the Mie-scattering phase function for three of Derimendjian's polydispersions.Results show that the new phase function fits the Mie-scattering phase function much better than the Henyey-Greenstein*phase function.For the new phase function,the RMSE is small for 73.3%data.On the contrast,only 26.7%data fit the Mie-scattering phase function well for the Henyey-Greenstein*phase function.At the same time,the application of the analytic formula in the scattering of non spherical particles is analyzed,and the results of the T-matrix method are compared,and found the new empirical expression provided a more realistic description for the scattering of non-spherical particles.Furthermore,the calculated value for ratio of scattering intensity at 90 degree to the scattered intensity in the backward direction is more reasonable when the ratio of the horizontal to rotational axes and the diameter-to-length ratio is larger than 0.5.We also investigated the effectiveness in approximating scattering from polydispersed particles by comparison between the new empirical expression,the Henyey-Greenstein*phase function and the T-matrix method for four of the log normal distribution polydispersions.The results show that the new empirical expression fits the T-matrix method much better than the Henyey-Greenstein*phase function.For the new empirical expression,the RMSE is small for 100%data except for the ellipsoidal oceanic aerosol at the wavelength of 633 nm.Similarly,the effectiveness of the new empirical expression is significant when calculating the ratio of scattered intensity at 90 degree to the scattered intensity in the backward direction of non-spherical aerosol.