Simulation of the infrared radiation characteristics of high-temperature exhaust plume including particles using the backward Monte Carlo method

The analysis of the infrared radiation characteristics of high-temperature free-stream flow including particles is very significant for the field of target detection, combustion diagnosis and temperature measurement of flame. In this paper, the infrared radiation characteristics of high-temperature free-stream flow are calculated and analyzed using the backward Monte Carlo method, considering the effect of the directional radiation heat flux due to the particle scattering and the different boundary conditions. The calculation results of emitting, absorbing and anisotropically scattering media are compared with the forward Monte Carlo and finite-volume methods results, which shows the superiority on computational efficiency with the backward Monte Carlo method.     

Infrared radiative transfer modelling in a 3D scattering cloudy atmosphere: Application to limb sounding measurements of cirrus

The Monte Carlo cloud scattering forward model (McClouds_FM) has been developed to simulate limb radiative transfer in the presence of cirrus clouds, for the purposes of simulating cloud contaminated measurements made by an infrared limb sounding instrument, e.g. the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). A reverse method three-dimensional Monte Carlo transfer model is combined with a line-by-line model for radiative transfer through the non-cloudy atmosphere to explicitly account for the effects of multiple scattering by the clouds. The ice cloud microphysics are characterised by a size distribution of randomly oriented ice crystals, with the single scattering properties of the distribution determined by accurate calculations accounting for non-spherical habit.A comparison of McClouds_FM simulations and real MIPAS spectra of cirrus shows good agreement. Of particular interest are several noticeable spectral features (i.e. H₂O absorption lines) in the data that are replicated in the simulations: these can only be explained by upwelling tropospheric radiation scattered into the line-of-sight by the cloud ice particles.     

Physical model of acoustic forward scattering by cylindrical shell and its experimental validation

Research on the underwater target scattering can provide important theoretical support for target detection. The scattering model of cylindrical shell is established in this paper. It is found that the forward target strength is much stronger and varies with angles of incident wave less significantly than backward target strength. The received forward signal strength fluctuates with the target moving due to the interference between direct signal and scattering signal, which is most significant when target approaches the baseline. An experiment is carried out in an anechoic tank to validate the scattering model. The method of acquisiting forward scattering in the tank is proposed. The forward and the backward target strengths are achieved by using the pulse compression technology, and they are about 3dB less than the modeling results. The forward scattering phenomena of quiescent and moving target are measured, which are similar to modeling results with different target types.     

激光在准球形边界卷云中传输的衰减和透过特性

高空卷云主要由各种不同形状的冰晶粒子组成,是地空链路上激光信号传输的重要影响因素。依据高空卷云中冰晶粒子的分布特征和散射特性,采用C版本的离散纵标法（CDISORT）,充分考虑地球球形曲率及云层冰晶粒子多次散射影响因素,研究准球形边界云层的激光透过率和衰减特性,并比较了太阳天顶角不同时平面平行模式和准球面模式下卷云大气激光透过率的差异,数值计算了三种激光波长（0.65,1.06和3.8 μm）在卷云中传输时的衰减和透过特性。计算结果表明：较小太阳天顶角（小于80°）入射时,两种模式下卷云大气激光透过率相对误差很小,其中0.65 μm激光波长入射时两种模式下的相对误差仅为1.72%,较大太阳天顶角（大于80°）入射时,两种模式下卷云大气激光透过率相对误差明显增大,0.65 μm激光波长入射时两种模式下的相对误差最大达到69%;卷云粒子单次散射时,激光在云层的衰减与卷云粒子有效半径、传输距离、光学厚度及激光波长等因素有关,随光学厚度的增加,云层的激光透过率减少,1.06 μm激光波长入射时透过率最大,3.8 μm激光波长入射时透过率最小;0.65和1.06 μm激光波长入射时,随云层粒子有效半径的增加激光透过率逐渐增加,而3.8 μm波长激光,随云层粒子有效半径的增加激光透过率逐渐减少,随相对方位角的增加,云层的激光透过率减少,且不同卷云传输模型对激光透过率也存在不同的影响。该研究工作将为开展地空链路星载、机载激光通信、激光雷达探测等工程系统中的激光信号云层传输特性的应用提供理论支持,同时也可进一步拓展为地空链路激光遥感、制导和预警等应用提供预先理论研究基础。     

Computation of Near Sea Target Scattering from Complex Background Radiation

The background radiation contribution is an important component of target scattering characteristics. A study is made on target scattering characteristics from complex background radiation, with contributions of the sun and the sea and sky background and that among each components of target. In this paper, the shadowing function of sea surface is numerically calculated by using the Z-BUFFER shadowing elimination algorithm, and the scattering of each facet of sea surface and target from the sun and the sky background infrared radiation is computed based on the rough surface scattering model as well as the infrared self-radiation of the sea surface. Finally, a numerical calculation of the forward and backward radiance of a near-sea circular cylinder scattering from the sun, the sea and sky background infrared radiation of near sea target and the infrared self-radiation of the target skin have been made. The results show the important influences of each contribution on target scattering from the sea and sky background radiation in 3-5 m and 8-12 m bands.     

云的红外辐射模型综述

介绍了云的分类和组成,以及云的红外辐射模型(包括二流近似、灰体近似和黑体近似模型),测量了云的红外光谱辐射。分析表明:二流近似模型可用于云层对太阳和大气中红外波段的反射、透射和云自身的辐射。黑体近似模型和灰体近似模型主要用于对云层较粗略的估算,灰体模型可近似描述8～12μm波段的辐射,且形式上比较简单,是一种常用的简化模型。     

迎头方向隐身导弹红外辐射特性分析北大核心CSCD

隐身导弹应用红外隐身技术将降低导弹被红外探测系统的发现概率。通过以AGM-158C导弹为参考,对迎头观测方向目标的红外辐射特性,包括目标蒙皮、目标蒙皮反射周围背景和目标尾焰的红外辐射强度进行估算,并与典型亚音速导弹红外辐射特性的数据进行对比分析。分析结果表明:红外隐身技术可以明显降低导弹的中、长波红外辐射,尤其是中波波段的红外辐射;红外探测系统设计时尾焰探测通常选用中波红外探测器,弹体探测通常选用长波红外探测器;隐身导弹采用红外隐身技术将提高导弹的突防能力。     

Scattering of Radiation of the Sun and the Sky from Two Dimensional Rough Sea Surfaces

The scattering of optical wave from two dimensional rough sea surfaces is studied first with method of facets. The sea surface is divided into many facets, and each facet is treated as a surface with small roughness instead of a smooth plane, therefore more practical and effective. In addition the shadowing function of sea surfaces for arbitrary incident and scattering angles is numerically calculated with the Z-BUFFER method, which applies to any kinks of rough surfaces compared with the methods available. Finally the spectral irradiance of the sun and the spectral radiance of the sky for different time at sea level with fine weather are obtained with the software of Lowtran7, and the scattering of the radiation of the sun and the sky from two dimensional rough sea surfaces for different time, waveband and wind speed is studied, which is of great reference value for reducing the interference to the infrared detector due to the scattering of the radiation of the sun and the sky from sea surfaces.     

Scattering of Solar and Atmospheric Background Radiation from a Target

Light scattering property of environment is very important in theoretical study and application of the remote sensing. What's more, it is valuable for infrared radiation, imaging, and the detection of target and tracking. In this paper, solar and atmospheric background radiation, and their scattering property from target are discussed. BRDF (Biodirectional Reflectance Distribution Function) is a very important quantity that shows the radiation and reflection feature of target. According to electromagnetic radiant and scattering theories, the relationship between laser radar scattering cross section (LRCS) and BRDF is introduced. LOWTRAN model is an effective method of calculating the spectral distribution of solar and atmospheric radiation. Here it is applied to compute solar and atmospheric background radiation scattered from a target. The relative equations are deduced. Thus, the spatial and spectral distribution of scattering light is given. As a special example, for the Lambert's surface, the equations are simplified. As a result, the spatial and spectral distributions scattering radiation of solar and atmospheric background from a rough painted surface are present. The scattering of solar radiation plays a primary role in MIR region, but scattering of atmospheric background radiation is higher in LIR region. At the same time, there is obviously specular reflectance for solar radiation due to coherent scattering from rough surface.     

基于MODIS数据的卫星辐射特性研究

本文提出了一种结合MODIS数据产品和MODTRAN模型的典型地形条件空间目标红外辐射特性计算方法.考虑了地形条件和大气环境对空间目标的红外辐射特性影响,应用MODIS 3级陆地,海洋,冰雪圈数据产品得到全球温度分布,地形特征分布,将温度及地形条件代入MODTRAN计算得到地表-目标的辐亮度,进而计算目标的外热流以及表...     

基于光学厚度的云红外辐射计算

 为了研究不同物理厚度和不同波段下云的红外辐射特性,提出了基于光学厚度的云红外辐射计算方法。综合考虑云的红外辐射的各个因素,建立了较为完善的云的红外辐射模型,引入光学厚度经验计算公式,并根据光学厚度针对中波和长波红外分别计算了云的发射率、反射率和透过率,进而得到云的红外辐射亮度分布。利用该计算方法,计算了中波和长波红外云的辐射亮度数值,计算结果表明：随着光学厚度增大,云的发射率和反射率增大,探测器接收到的云红外辐射增强。比较发现,该计算结果与实测数据有较好的一致性,该计算模型可以为云背景的红外特性分析、探测及仿真提供参考数据。     

Distribution characteristic of scattering field for an ellipsoidal target irradiated by an electromagnetic wave from an arbitrary direction

It is of great importance for engineering applications to obtain the expression of scattering field for an ellipsoidal target irradiated by an electromagnetic wave from an arbitrary direction. Literature relevant to this problem is seldom found. In this paper, the scattering field for an ellipsoidal target is presented by utilizing the scale transformation of electromagnetic field and the rotation of coordinate system, with an electromagnetic wave projecting on the target from an arbitrary direction. The obtained result is in good agreement with the solution available from the literature if we consider the scale factors to be unity. Taking a conducting ellipsoidal target for sample, we perform the partial simulations of the ellipsoidal model and a plant leaf model by choosing different scale factors. The obtained results show that the distribution characteristic of scattering field is sensitively affected by the polarization of the incident wave and varies not much with the incident wave angle but changes with the observation point. At some points the scattering energy arrives at its maximum.     

Radiative transfer calculations through an aerosol cloud

A radiative transfer calculation of the EM radiation through an aerosol cloud is presented. The effects taken into account are elastic and inelastic scattering of the incident radiation and self-emission by the aerosol cloud. The elastic scattering effects include multiple scattering of the incident photons. We describe the model and calculational procedure based on the multiple scattering approach. The results of the present model are compared with other models for the two given aerosol clouds.     

Multiple-scattering in radar systems: A review

Although extensively studied within the lidar community, the multiple scattering phenomenon has always been considered a rare curiosity by radar meteorologists. Up to few years ago its appearance has only been associated with two- or three-body-scattering features (e.g. hail flares and mirror images) involving highly reflective surfaces.Recent atmospheric research aimed at better understanding of the water cycle and the role played by clouds and precipitation in affecting the Earth's climate has driven the deployment of high frequency radars in space. Examples are the TRMM 13.5 GHz, the CloudSat 94 GHz, the upcoming EarthCARE 94 GHz, and the GPM dual 13-35 GHz radars. These systems are able to detect the vertical distribution of hydrometeors and thus provide crucial feedbacks for radiation and climate studies. The shift towards higher frequencies increases the sensitivity to hydrometeors, improves the spatial resolution and reduces the size and weight of the radar systems. On the other hand, higher frequency radars are affected by stronger extinction, especially in the presence of large precipitating particles (e.g. raindrops or hail particles), which may eventually drive the signal below the minimum detection threshold. In such circumstances the interpretation of the radar equation via the single scattering approximation may be problematic. Errors will be large when the radiation emitted from the radar after interacting more than once with the medium still contributes substantially to the received power. This is the case if the transport mean-free-path becomes comparable with the instrument footprint (determined by the antenna beam-width and the platform altitude).This situation resembles to what has already been experienced in lidar observations, but with a predominance of wide- versus small-angle scattering events. At millimeter wavelengths, hydrometeors diffuse radiation rather isotropically compared to the visible or near infrared region where scattering is predominantly in the forward direction. A complete understanding of radiation transport modeling and data analysis methods under wide-angle multiple scattering conditions is mandatory for a correct interpretation of echoes observed by space-borne millimeter radars.This paper reviews the status of research in this field. Different numerical techniques currently implemented to account for higher order scattering are reviewed and their weaknesses and strengths highlighted. Examples of simulated radar backscattering profiles are provided with particular emphasis given to situations in which the multiple scattering contributions become comparable or overwhelm the single scattering signal. We show evidences of multiple scattering effects from air-borne and from CloudSat observations, i.e. unique signatures which cannot be explained by single scattering theory. Ideas how to identify and tackle the multiple scattering effects are discussed. Finally perspectives and suggestions for future work are outlined.This work represents a reference-guide for studies focused at modeling the radiation transport and at interpreting data from high frequency space-borne radar systems that probe highly opaque scattering media such as thick ice clouds or precipitating clouds.     

Anomalous sea surface reverberation scale model experiments

Low frequency sea surface sound backscattering from approximately 100 Hz to a few kHz observed from the 1960s broadband measurements using explosive charges to the Critical Sea Test measurements conducted in the 1990 s is substantially higher than explained by rough sea surface scattering theory. Alternative theories for explaining this difference range from scattering by bubble plumes/clouds formed by breaking waves to stochastic scattering from fluctuating bubble layers near the sea surface. In each case, theories focus on reverberation in the absence of the large-scale surface wave height fluctuations that are characteristic of a sea that produces bubble clouds and plumes. At shallow grazing angles, shadowing of bubble plumes and clouds caused by surface wave height fluctuations may induce first order changes in the backscattered signal strength. To understand the magnitude of shadowing effects under controlled and repeatable conditions, scale model experiments were performed in a 3 m x 1.5 m x 1.5 m tank at the Technical University of Denmark. The experiments used a 1 MHz transducer as the source and receiver, a computer controlled data acquisition system, a scale model target, and a surface wave generator. The scattered signal strength fluctuations observed at shallow angles are characteristic of the predicted ocean environment. These experiments demonstrate that shadowing has a first order impact on bubble plume and cloud scattering strength and emphasize the usefulness of model scale experiments for studying underwater acoustic events under controlled conditions.     

舰船目标红外图像特性研究

针对成像型反舰导弹所获红外图像的特点,分析了舰船目标和海天背景的红外辐射特性,得知舰船目标红外图像有一重要特征-海天线,它可以确定目标的所在区域。为了舰船目标的跟踪定位,对舰船目标红外图像中的海天线特征进行研究,提出一种基于预处理和直线拟合法提取海天线特征的算法,首先对图像进行边缘检测得到梯度图像,进而二值化并细化,最后利用Hough变换提取海天线。结果表明,在低对比度和有云团干扰的情况下,该算法与改进的直线拟合法相比,能够有效地、准确地提取舰船目标红外图像中海天线特征。     

基于三维重建理论的目标光谱散射特性研究

根据三维重建理论,基于目标的多角度视图,重建了目标表面的三维点云。利用德洛奈三角剖分法结合可见性原理,得到了目标的曲面和曲面面元的法线方向。根据粗糙面散射理论和目标表面的双向反射分布函数(BRDF),结合大气传输软件Modtran计算的某时间、地点的背景光谱辐射亮度,数值分析了目标光谱散射亮度分布特性。以覆盖车衣的汽车为例,重建的三维几何模型误差为4.11%,数值计算了目标在三个波段的光谱散射亮度分布。上述方法可以进一步用于卫星和其他空间目标的光谱辐射、散射特性研究。     

隐身飞机尾焰的红外辐射特性

隐身飞机尾焰的红外辐射是隐身飞机探测的主要辐射源.本文提出了一种新的隐身飞机尾焰红外辐射特性计算模型.该模型以普通飞机尾焰红外辐射特性计算模型为基础,进而考虑红外隐身措施的影响,间接实现隐身飞机尾焰的红外辐射特性的计算.计算模型分别考虑了隐身飞机的二元喷口、引射技术、红外遮蔽云以及遮挡对尾焰辐射的影响.计算结果得出,添加隐身措施后尾焰辐射强度仅为添加前辐射强度的5.8%.针对隐身飞机尾焰红外辐射特性很难获取的问题,将计算结果与喷灯燃烧航空煤油的光谱峰位数据进行了比较,实验结果显示隐身前后辐射能量量级变化与国外文献相同,表明该模型可以用于隐身飞机尾焰红外辐射特性计算.     

红外区磷化稼散射强度的计算与分析

根据Mie散射理论,对磷化稼粒子光散射特性进行了数值计算与理论分析,得到了散射强度与散射角、入射波长以及偏振度与散射角的关系。研究表明,红外波段光散射很小,前向散射占有优势,粒子半径越大,前向散射越强,并且在散射角900方向上能观测到线偏振光,对研究GaP红外光学特性提供了理论参考。     

Comparison of single and multiple scattering approaches for the simulation of limb-emission observations in the mid-IR

The validity of single scattering radiative transfer calculations for simulation of limb-emission measurements of clouds in the mid-infrared spectral region was investigated by comparison with a multiple scattering model. For in limb direction optically thin clouds, like polar stratospheric clouds, errors of the single scattering scheme range below 3%. For optically thick clouds deviations are below 3% in case of low single scattering albedo (ω₀=0.24) increasing up to 10-30% for ω₀=0.84. Clouds which are optically thick in limb, but thin in nadir direction, can cause limb radiances which are by a factor of 1.7 higher than the blackbody radiance at cloud altitude.