大气信道模型

光束消光Beam Extinction
☞基本概念 ☞基本规律 ☞分子消光 ☞气溶胶消光
基本概念和基本规律
概 念：激光束在含有原子、分子或颗粒的介质中
传播时产生衰减的现象。
起
因：吸收（分子）和散射（气溶胶）。
基本规律： 基本规律：朗伯定律——在物理状态保持稳定的情况
下，消光过程在辐射强度和物质数量上都 是线性的。
Total transmittance for a path of interest is determined by finding the transmittances for each homogeneous layer or region that composes the path, and then multiplying them together to get the resultant total transmittance.
分子消光
分子消光模型
——描述由分子吸收产生的透光比损失
FASCODE Model LOWTRAN Model MODTRAN Model
FASCODE Model（1）
FASCODE ——line-by-line model, determines the optical depth as a function of either wave number or wavelength. An absorption line has three primary characteristics: the central position of the line Vo; the strength or intensity of the line S; the shape function for the absorption line. The natural line shape (or Lorentz profile) is given by
气溶胶消光
气溶胶模型
——气溶胶颗粒数量的集中情况
Modified Gamma Distribution Junge Function Shettle and Fenn Model Navy Aerosol Model (NAM) Advanced Navy Aerosol Model (ANAM)
Shettle and Fenn Model（1）
Shettle and Fenn model include four models (rural, urban, maritime, and tropospheric). The tropospheric model contains only a single log-normal distribution, while the other three models combine two log-normal distributions .
σ 4 = 0.5, R4 = 8.0, N 4 = 10 A , A4 = −2.5 + 0.07v10 − 0.04h
4
h
——height above the water surface；
v10 —— wind speed measured at a 10m height.
小
T(s)是透光比：
Limitations——NAM was designed to determine extinction at a single height above the ocean surface, namely 10 m. In addition, it was designed to represent aerosol extinction conditions that occur over open-ocean conditions, and not necessarily the actual conditions.
MODTRAN&LOWTRAN Model
LOWTRAN Model——an initial low resolution transmittance code. evolve MODTRAN Model——a band model, calculates the spectrally averaged atmospheric transmittance, does not obey the Lambert law (except for the very narrow spectral windows over which an absorption coefficient can be considered to be constant, which is much less than a line width), the fundamental output of the MODTRAN model: the full-path transmittance, rather than the optical depth or an extinction coefficient.
σi
ρi —— the mode radius (radius for which the
distribution is maximized) σ i ——a distribution width parameter.
Shettle and Fenn Model（2）
The rural model——represent aerosols that have no urban or industrial sources. The aerosol is a mixture of water soluble components and "dust-like“ components. The complex refractive index is determined by the relative weighting of the components in the mixture. The urban model——modifies the aerosol mix by including combustion products and other industrial products. The maritime model——has two components: (I) a sea-salt component, and (2) a continental component. The continental component is identical with the mixture defined in the rural aerosol model except that the largest particles are eliminated due to assumed fallout by the time the air mass has transported out to the open ocean.
组
会 报 告 张 鑫 2011年11月17日
主要内容
★大气信道对自由空间光通信的影响
光束消光及其模型 大气湍流及其模型
★近期学习的其他内容 ★下一步工作
大气信道对自由空间光通信的影响
光束消光——激光光束在传播过程中强度的降
低，即衰减。
大气湍流——导致空间光强度波动、光束方向
的随机变化、接收角波动等。
This profile is most important in the lower atmosphere under conditions of relatively high pressure.
FASCODE Model（2）
The transmittance T of a region is the ratio of incident power to emergent power, and the relation between transmittance and optical depth is implicitly defined:
γ mod ——the mode radius (in um) that corresponds to
the radius at which c(r ) is a maximum； N——the total number density in particles per cm3 ； a, b, α , and γ ——four adjustable parameters that allow the function to be fit to a large variety of models.
大气湍流百度文库型
☞折射率结构参数 Cn2
——与波长、气温、气压有关，随地面高度h的变化而 变化
☞空间光功率谱密度 Φ n (κ ) ——用来描述 n( ρ ) ☞大气湍流模型 ——描述折射率结构参数
2 Cn 折射率结构参数
折射率结构方程 Dn ( ρ1 , ρ2 ) = n( ρ1 ) − n( ρ2 )
Advanced Navy Aerosol Model
It was developed to correct some of the limitations and deficiencies found in NAM. ANAM introduces a height dependence for aerosol particle concentrations from the surface up to several tens of meters.
模型简介
The aerosol particle number concentration c(r ) is defined as the number of particles per unit volume per radius increment:
dN c(r ) = dr
Modified Gamma Distribution
Navy Aerosol Model (NAM)
NAM is widely used to model the effects of aerosol extinction in marine environments. The aerosol distribution is assumed to be representable as a sum over four lognormally distributed modes:
结
由于光束消光产生的大气链路损耗：
β v 是总消光系数：
For typical clear air conditions, β v ≈ 0.1 ; for haze conditions (with a visibility of roughly 4 km), β v ≈ 1.0 ; in fog conditions, β v ≥ 10 ; for a dense fog with a visibility of 10m, β v ≈ 391.
Junge Function
dN c(r ) = = pr − v d log r
c(r) is now defined as the number of particles per unit volume and per logarithm of the radius. both p and v is dependent upon the particular characteristics of the aerosol. The exponent v can vary over the range 2 < v < 4, with the lower value v ≈ 2 typically found in fog conditions.
For locally homogeneous
2
and isotropic turbulence
Dn ( ρ1 , ρ 2 ) = Dn ( ρ ), ρ = ρ1 − ρ 2