椭球雨滴群旋转轴取向对双线偏振多普勒雷达参量影响的计算分析

以椭球粒子的电磁波散射理论为基础, 主要研究椭球雨滴群旋转轴在空间的取向特征, 建立粒子旋转轴空间取向与风场、 雷达天线仰角间的关系。在考虑粒子传输效应的情况下, 分析了旋转轴取向对双线偏振雷达探测参数ZDR和LDR的影响, 为提高双线偏振多普勒天气雷达观测资料的精度提供了理论依据。     

A dual quaternion-based,closed-form pairwise registration algorithm for point clouds

The representation of similarity transformation in three-dimensional (3D) space, especially of orientation, is a crucial issue in navigation, geodesy, photogrammetry, robot arm manipulation, etc. Considering the large amount of computer resources required by iterative algorithms designed for spatial similarity transformation, the high dependence on initial values of unknown parameters, and the instability of solving transformation parameters for large-angle registration, a closed-form solution for pairwise light detection and ranging (LiDAR) point cloud registration is proposed. In this solution, dual-number quaternions are used to represent the 3D rotation. The relationship between the rotation matrix-based representation of similarity transformation and the dual quaternion-based representation is described first. Considering that the same features from two neighboring stations coincide after pairwise registration, a dual quaternion-based error norm, which is associated with the sum of the position errors, is constructed. Based on theory of least squares and by extreme value analysis of the error norm, detailed derivations of the model and the main formulas are obtained. Once the similarities between the same features from the two neighboring LiDAR stations are constructed, the rotation matrix, the scale parameter, and the translation vector are simultaneously derived. Two experiments are conducted to verify the feasibility and effectiveness of the proposed algorithm. The proposed algorithm has the advantages of simplicity and ease of implementation, making it better than the traditional methods that use matrices to describe spatial rotation. Moreover, it solves the transformation parameters without the initial estimates of unknown parameters, making it better than iterative algorithms. Most importantly, in contrast to unit quaternion-based algorithms, the proposed algorithm solves seven unknown parameters simultaneously. Therefore, it effectively avoids the accumulation of introduced error in calculation and the negative impact from the inappropriate choice of initial values.     

A performance comparison of single and dual frequency GPS ambiguity resolution strategies

The impact of observation selection, observation combination and model parameterization on GPS carrier phase ambiguity resolution and position accuracy under operational conditions is investigated. The impact of an ionospheric bias for a generic linear combination of L1 and L2 measurements is assessed and the results are used to clearly outline the desirable characteristics for improving ambiguity resolution versus positioning accuracy performance. Ambiguity resolution performance and position accuracy are shown for widelane (WL), L1-only, and ionospheric-free (IF) combinations. Several techniques for dealing with the ionospheric bias are also presented and compared, including stochastic ionospheric modelling. Multiple carrier phase combination solutions estimated in the same filter are also compared. The concept of an optimal processing strategy—in terms of both reliable ambiguity resolution and high accuracy positions—is presented. In total, eight strategies, which vary in observables and parameters, are tested on several datasets ranging from 13 km to 43 km.     

Spatial and temporal mapping of water in soil by magnetic resonance imaging

Experiments on model and real soil blocks designed to assess the feasibility of using magnetic resonance imaging for three-dimensional mapping of the time-varying spatial distribution of water in structured soils are reported. The results show that, notwithstanding inherent problems in imaging natural soils with a significant iron content, experimental parameters can be identified which allow satisfactory images to be obtained. Magnetic resonance imaging may therefore provide important information on soil structure and water movement in dual porosity soils, with attendant benefits for the calibration of models of non-Darcian flow in such soils.     

双能光子吸收法提高CT衬度(密度)分辨率的研究

目的：提高CT衬度的分辨率，30年来，医学CT经历了原始的第一代CT发展至目前第5代高分辨率CT和高速电子束CT，其成像的空间分辨率和时间分辨率均有明显提高，但CT成像的衬度（密度）分辨率提高并不显著，而衬度分辨率与图像的灰度等级（灰阶）密切相关，是图像清晰度的重要参数。本文对应用双能量光子扫描吸收法以提高CT衬度（密度）分辨率的原理进行分析，并探讨其实际应用的可行性。     

C波段双线偏振雷达退偏振因子的应用潜力

退偏振因子是双线偏振雷达可以获得的一个很重要的云不的物理量。从理论上计算了液态和简单的冰相降水粒子对空间取向和尺度大小等因素的关系。     

双线偏振雷达降雨估测分析

本文提出排序配对逼近法来拟合１９９３—１９９４年１３１４次雷达和雨量计的取样资料，这样既弥补了线性回归法的不足，又提高了区域降水估测的精度。由误差分析知道：估测１０００ｋｍ２以上区域的降水时，对双线偏振雷达而言，其估测精度将极有可能降低到１０％以下且比常规雷达有明显改善；双线偏振雷达的两种估测公式之间差异不大；按降水强弱分型处理可提高常规雷达估测精度，但对双线偏振雷达估测无有利影响     

双差分吸收激光雷达:-种能有效减小气溶胶对臭氧测量影响的方法

文中提出了一种新的激光雷达测量臭氧的方法：双差分吸收方法。理论分析和数值模拟表明这种方法可以有效减小气溶胶消光和后向散射对臭氧测量的影响，从而使激光雷达在气溶胶影响严重地区测量的臭氧精度比传统差分吸收激光雷达大大提高。利用（２８９，３１３；２７７．１，２９９．１ｎｍ）或（２６８．４，２８９；２７７．１，２９９．１ｎｍ）４波长进行双差分吸收可以用于对流层大气气溶胶含量丰富或分布不均匀地区臭氧的测量。利用（２９９．１，３４１．５；３０８，３５３ｎｍ）４波长进行双差分吸收可以对火山爆发后平流层臭氧进行较精确的测量。     

双线偏振雷达探测的云和地物回波的特性及其识别方法

双线偏振雷达在定量测量降雨量的过程中，经常受到地物回波的干扰和影响。研究云和地物回波的反射率因子ＺＨ、差反射率因子ＺＤＲ的取值大小及两者的关系、空间变化梯度的结果表明：云的ＺＤＲ大于零，且有明显的随ＺＤＲ大于零、且有明显的随ＺＨ增加而增大的趋势，这反映了在强降雨回波中大粒子占的比例较大；而地物回波的ＺＨ和ＺＤＲ无明显变化规律，ＺＤＲ可正可负；云回波的空间变化率也明显小于地物回波。     

Comparison of three models of dispersion in dual porosity media

The work presented in this paper shows that the probabilistic approach as well as the fractional Brownian motion model are reliable to simulate the dispersion of contaminant in the dual porosity media. Three models were tested on experimental data obtained by tracer tests in chalk characterized by a primary and a secondary porosity: a numerical model (MT3DMS), a probabilistic model based on the Bayesian approach and a fractional Brownian model. By using Einstein’s rule, it is demonstrated that the dispersion coefficient is time dependent and that the dispersion is ballistic which is a particular case of superdiffusion. A test of sensitivity was carried out and shows that the numerical model is slightly suitable to simulate the concentration in time and space for a given discharge starting from the physical parameters obtained by calibration for an other discharge because of the great sensitivity of its parameters according to the hydrodynamic conditions. On the other hand, the variability of the parameters of the two other models does not generate important errors on the values of simulated concentrations when other hydrodynamic conditions are considered.