Comparison of three SeaWiFS atmospheric correction algorithms for turbid waters using AERONET-OC measurements

The use of satellites to monitor the color of the ocean requires effective removal of the atmospheric signal. This can be performed by extrapolating the aerosol optical properties in the visible from the near-infrared (NIR) spectral region assuming that the seawater is totally absorbant in this latter part of the spectrum. However, the non-negligible water-leaving radiance in the NIR which is characteristic of turbid waters may lead to an overestimate of the atmospheric radiance in the whole visible spectrum with increasing severity at shorter wavelengths. This may result in significant errors, if not complete failure, of various algorithms for the retrieval of chlorophyll-a concentration, inherent optical properties and biogeochemical parameters of surface waters.This paper presents results of an inter-comparison study of three methods that compensate for NIR water-leaving radiances and that are based on very different hypothesis: 1) the standard SeaWiFS algorithm (Stumpf et al., 2003; Bailey et al., 2010) based on a bio-optical model and an iterative process; 2) the algorithm developed by Ruddick et al. (2000) based on the spatial homogeneity of the NIR ratios of the aerosol and water-leaving radiances; and 3) the algorithm of Kuchinke et al. (2009) based on a fully coupled atmosphere-ocean spectral optimization inversion. They are compared using normalized water-leaving radiance nL_w in the visible. The reference source for comparison is ground-based measurements from three AERONET-Ocean Color sites, one in the Adriatic Sea and two in the East Coast of USA.Based on the matchup exercise, the best overall estimates of the nL_w are obtained with the latest SeaWiFS standard algorithm version with relative error varying from 14.97% to 35.27% for λ = 490 nm and λ = 670 nm respectively. The least accurate estimates are given by the algorithm of Ruddick, the relative errors being between 16.36% and 42.92% for λ = 490 nm and λ = 412 nm, respectively. The algorithm of Kuchinke appears to be the most accurate algorithm at 412 nm (30.02%), 510 (15.54%) and 670 nm (32.32%) using its default optimization and bio-optical model coefficient settings.Similar conclusions are obtained for the aerosol optical properties (aerosol optical thickness τ(865) and the Ångström exponent, α(510, 865)). Those parameters are retrieved more accurately with the SeaWiFS standard algorithm (relative error of 33% and 54.15% for τ(865) and α(510, 865)).A detailed analysis of the hypotheses of the methods is given for explaining the differences between the algorithms. The determination of the aerosol parameters is critical for the algorithm of Ruddick et al. (2000) while the bio-optical model is critical for the algorithm of Stumpf et al. (2003) utilized in the standard SeaWiFS atmospheric correction and both aerosol and bio-optical model for the coupled atmospheric-ocean algorithm of Kuchinke. The Kuchinke algorithm presents model aerosol-size distributions that differ from real aerosol-size distribution pertaining to the measurements. In conclusion, the results show that for the given atmospheric and oceanic conditions of this study, the SeaWiFS atmospheric correction algorithm is most appropriate for estimating the marine and aerosol parameters in the given turbid waters regions.     

Switching model predictive attitude control for a quadrotor helicopter subject to atmospheric disturbances

In this article a switching model predictive attitude controller for an unmanned quadrotor helicopter subject to atmospheric disturbances is presented. The proposed control scheme is computed based on a piecewise affine (PWA) model of the quadrotor's attitude dynamics, where the effects of the atmospheric turbulence are taken into consideration as additive disturbances. The switchings among the PWA models are ruled by the rate of the rotation angles and for each PWA system a corresponding model predictive controller is computed. The suggested algorithm is verified in experimental studies in the execution of sudden maneuvers subject to forcible wind disturbances. The quadrotor rejects the induced wind disturbances while performing accurate attitude tracking.     

舰载机逃逸过程动力学研究

为研究舰载机逃逸过程中的动力学特性,建立了舰载机的六自由度飞行动力学模型.为充分考虑复杂海洋环境的影响,引入了Dryden大气紊流模型、航母舰首紊流模型和航母运动模型,生成了工程化的模拟逃逸环境.在此基础上,分析了紊流速度和紊流速度梯度对飞气动参数的影响,并采用仿真方法模拟了舰载机逃逸的全过程.结果表明,海上复杂环境会影响舰载机正常飞行姿态,加大横向位移和滑跑距离,影响飞行安全.     

大气风对导弹作战效能影响的动力学仿真

大气风对导弹作战效能的影响表现为风影响导弹的动力学特性。基于导弹动力学仿真,分析大气风对导弹飞行特性的影响,能精确、直观地反映出风对导弹作战效能的影响情况。建立了理想条件下和风扰动条件下的导弹六自由度运动模型和导弹制导控制系统模型,并实现了整个导弹系统的仿真。基于导弹动力学仿真,提出了一种由飞行参数误差计算风影响度的方法,来定量评价风对导弹运动的影响。仿真结果表明,这种风影响度评价方法能较准确地反映出风对导弹的影响,可为用户在特定气象环境下是否发射导弹提供一种辅决策手段。     

遥感数字图像的大气辐射校正应用研究

卫星遥感数字图像成像过程中，由于电磁波受大气作用造成数据质量下降，影响遥感信息的提取及精度。介绍了大气辐射校正的一般原理和PCI软件ATCOR2模块的算法，研究了基于该模块的遥感数字图像大气辐射校正实现方法。根据研究区实际情况，选用1976年美国标准大气乡村气溶胶大气参数，对一景Landsat TM影像进行校正处理。对比处理前后图像和直方图，可知该方法增强了图像清晰度，提高了视觉效果，有利于遥感信息提取和专题解译。     

Comparison of land surface emissivity and radiometric temperature derived from MODIS and ASTER sensors

This study compared surface emissivity and radiometric temperature retrievals derived from data collected with the MODerate resolution Imaging Spectroradiometer (MODIS) and Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) sensors, onboard the NASA's Earth Observation System (EOS)-TERRA satellite. Two study sites were selected: a semi-arid area located in northern Chihuahuan desert, USA, and a Savannah landscape located in central Africa. Atmospheric corrections were performed using the MODTRAN 4 atmospheric radiative transfer code along with atmospheric profiles generated by the National Center for Environmental Predictions (NCEP). Atmospheric radiative properties were derived from MODTRAN 4 calculations according to the sensor swaths, which yielded different strategies from one sensor to the other. The MODIS estimates were then computed using a designed Temperature-Independent Spectral Indices of Emissivity (TISIE) method. The ASTER estimates were derived using the Temperature Emissivity Separation (TES) algorithm. The MODIS and ASTER radiometric temperature retrievals were in good agreement when the atmospheric corrections were similar, with differences lower than 0.9 K. The emissivity estimates were compared for MODIS/ASTER matching bands at 8.5 and 11 μm. It was shown that the retrievals agreed well, with RMSD ranging from 0.005 to 0.015, and biases ranging from −0.01 to 0.005. At 8.5 μm, the ranges of emissivities from both sensors were very similar. At 11 μm, however, the ranges of MODIS values were broader than those of the ASTER estimates. The larger MODIS values were ascribed to the gray body problem of the TES algorithm, whereas the lower MODIS values were not consistent with field references. Finally, we assessed the combined effects of spatial variability and sensor resolution. It was shown that for the study areas we considered, these effects were not critical.     

对消雷器的粗浅看法

本文从大气电学角度，讨论了有关消雷器的消雷原理、消灭向上闪电原理和限流功能。     

天然气大气式灶具的CO排放特性预测

CO是城市燃气燃烧过程中生成的主要污染物之一,必须对其排放量进行严格的控制。随着我国气源多样化格局的形成,不同组分的天然气在同一管网中运输,可能会引发末端设备的互换性问题,造成CO排放量的不稳定。为此,通过实验分析了天然气组分与大气式灶具CO排放特性之间的关系,并提出CO排放特性的预测公式。实验分为3个阶段:(1)在设计的大气式基准燃烧器上,测试了2 kPa条件下CH_4掺混单一组分时CO的排放特性,确定了C_2H_6、C_3H_8、C_4H_(10)、N_2和H_2各自对CO排放量的影响;(2)进行以CH_4为主要成分的多组分随机掺混配气CO的排放实验测试,并对实验结果进行多元线性回归分析,确定CO排放量与天然气各组分含量之间的数学关系;(3)选取3台结构型式具有代表性的实际天然气灶具,进行不同组分天然气的CO排放特性测试,验证提出的数学关系式的通用性。实验验证结果表明:实验回归得到的计算公式具有一定的通用性,可以很好地预测天然气组分变化时燃烧器的CO排放性能。     

The initiation mechanism of corrosion of zinc by sodium chloride particle deposition

This study details the mechanism of attack by sodium chloride particles on zinc. The dissolution and subsequent reaction of a single micro-particle of salt on zinc has shown explicitly the mechanism by which attack occurs. Following the initial formation of a hemispherical droplet when the particle wets in humid air, a secondary spreading effect occurs. The rate and potential associated with this spreading process has been measured using a Kelvin probe, and the chemistry of the compounds formed as a result of the corrosion mechanism have been determined using spectrographic techniques.