Satellite visibility in Northern Europe based on digital maps

This paper presents the calculation of satellite visibility and system availability based on digital elevation maps. New models for satellite visibility are derived for both non‐geostationary orbiting satellite constellations and geostationary satellites. Regression models for satellite visibility and average number of visible satellites based on easily available surface roughness variables are developed and tested. Cumulative distributions for the percentage of time that Iridium satellites are visible, taking terrain obstacles into account, are calculated and the relation between satellite visibility and system availability is discussed. Copyright © 2000 John Wiley & Sons, Ltd.     

Satellite selection scheme for reducing handover attempts in LEO satellite communication systems

Low earth orbit (LEO) satellite communication systems perform frequent intersatellite handovers for both fixed and mobile users. This paper proposes a satellite selection scheme for new/handover call requests when two or more satellites can be seen simultaneously. Each satellite in this scheme has a non-uniform transmitter antenna gain according to its relative position inside the coverage area. The antenna gain is proportional to the residual distance in the satellite's direction of movement and it compensates for the difference in path losses between satellite links. The residual distance distribution of the selected satellite and the mean number of intersatellite handovers during a call connection are calculated and compared with the results based on conventional methods. The proposed scheme can reduce the intersatellite handover call attempt rate without increasing system load and terminal complexity. Furthermore, this scheme can be extended to reduce both intersatellite and interbeam handover call attempt rates in a multiple spot beam environment. Especially, the average number of intersatellite and interbeam handovers during a call can be significantly reduced by using a hybrid algorithm with the proposed non-uniform power transmission scheme. © 1998 John Wiley & Sons, Ltd.     

Single and multiple satellite visibility statistics of first-generation non-GEO constellations for personal communications

The first generation of global satellite systems for personal and mobile communications is preparing to start commercial services. Service quality and availability are paramount to the user's acceptance of the service. For satellite systems, service quality and availability build up from satellite visibility, propagation conditions and equipment availability. This paper discusses the impact of elevation angle on visibility and propagation, that of azimuth separation between visible satellites, and compares multiple visibility statistics of all first-generation non-geostationary communications systems, namely Iridium, Globalstar, ICO, Ellipso and ECCO. © 1998 John Wiley & Sons, Ltd.     

基于M-Z光纤干涉仪测量极化聚合物的电光系数

提出了采用Mach Zehnder(M Z)光纤干涉仪的电光聚合物电光系数测量方法,对其测量原理进行了分析,研究了光纤干涉仪的臂长差、样品的插入损耗对系统可见度的影响,讨论了通过合理设置耦合器的耦合系数提高系统测量性能的方法。系统具有较好的稳定性和较高的灵敏度,既适合于聚合物薄膜也适合于聚合物波导的测量,系统干涉可见度为0.86,可分辨的最小相位变化约为6×10-6 rad。该系统可以同时测量2个独立的电光系数γ13和γ33,使用该系统测得一种有机/无机杂化材料ASD/SiO2 TiO2 极化后的电光系数为γ13=0.72±0.01 pm/V和γ33=2.8±0.01 pm/V。     

基于耦合分光可见度的光纤温度传感器

基于光纤耦合原理,利用耦合分光可见度概念,提出了一种新型的光纤温度传感器。其传感元件是由2根常规通信单模光纤熔融拉锥而成的光纤耦合器,将传感探头置于温度场中,传感器周围温度变化改变包层材料折射率,从而引起渐逝场分布变化导致能量在耦合器两臂中的重新分配,使输出耦合分光比（CR）相应发生变化。为提高传感器的性能,采用所定义的耦合分光可见度Vis来取代耦合CR,对CR和Vis的特性进行了分析、比较,验证了利用Vis作为光纤传感器的输出具有更高的灵敏度及信噪比。对该温度传感器在20～45℃的温度进行了测试,实验结果与理论分析具有良好的一致性。     

促安全目标下建筑集群全景敞视作用量化研究

全景敞视作用是能够高效提升城市安全的途径之一,本文以全景敞视理论为依托,以促进城市安全为目标,通过综合分析,将城市中建筑集群对外部空间的全景敞视作用归纳为两种类型:场地全景敞视和界面全景敞视.采用空间可视度分析和空间视域分析作为研究方法,构建建筑集群全景敞视模型,实现对建筑集群中全景敞视作用的量化,从而为科学评价城市安全提供新角度,为进一步确定公共安全薄弱区域提供新途径.     

融合空域与频域特征的能见度监督学习

大气能见度不仅影响海路空交通运输和居民出行,而且是空气质量检测的主要指标。现有基于图像处理的能见度评测方法存在采用固定公式计算、稳定性差、对适用环境要求苛刻等问题。能见度监督学习测量方法直接从观测场景图像中提取空域边缘相关特征与频域能量分布特征构成高维特征向量,无需人工设立目标物或对观测场景建模。它通过k最近邻(kNN)选取与待测图像相似样本进行支持向量回归机(SVR)的训练,动态构建图像特征与能见度之间的学习模型,将各种能见度影响因子隐藏于模型内。对自然场景的测量实验结果表明,该方法的测量正确率最高可达96.29%,且具有良好的稳定性和实时性,操作简单,便于大规模推广。     

基于暗原色先验的煤矿井下退化图像复原算法

针对煤矿井下因水雾和煤尘散射作用引起的图像退化问题,结合煤矿井下无线多媒体节点采集的图像特点,提出一种正则化拉普拉斯矩阵的暗原色先验去雾尘模型。根据暗原色先验理论对来自煤矿井下无雾图像数据库进行统计,建立了煤矿井下图像成像的物理模型,利用该模型估算介质传播函数和井下光线照度,再由去雾尘模型复原得到清晰化的图像。试验结果表明,该算法有效恢复了场景的对比度,明显提高了图像的视见度。     

改进的LIP偏微分方程图像去噪方法

针对对数图像处理-全变分(LIP_TV)去噪模型存在的不足,提出一种改进的LIP偏微分方程去噪方法。首先基于LIP数学理论,在LIP梯度算子中,引入四方向导数信息,得到改进的LIP梯度算子以全面客观地度量图像信息,更好地控制扩散过程。然后利用人类视觉系统的结构化特性,用噪声可见度函数构造新的保真项系数,进一步保持了图像的边缘细节并避免了人为估计噪声水平。理论分析和实验结果表明,该改进方法能够更好地去除噪声和保持图像边缘细节特征,在视觉效果和客观评价指标上都明显优于LIP_TV方法。     

基于时间序列分析和卡尔曼滤波的霾预报技术

为了提高霾预报的准确率,解决时序模型的预测延时和准确率不高的问题,提出了一种基于时间序列分析和卡尔曼滤波相结合的混合霾预报算法。首先,利用图检验法和单位根检验法（ADF）检验时间序列的平稳性,通过差分运算将非平稳序列转化成平稳序列,对转化后的平稳序列进行建模;然后,将得到的模型方程作为卡尔曼滤波的状态方程和观测方程,依靠卡尔曼滤波递推性进行预报。实验结果表明,采用时间序列分析和卡尔曼滤波相结合的混合霾客观预报订正方法能有效提高霾预测精度。