一种基于粒子滤波的双目视觉惯导SLAM技术

针对智能无人系统的定位与地图构建问题,提出一种基于粒子滤波的双目视觉惯导SLAM（即时定位与地图构建）方法。算法基于传统粒子滤波思想设计实现,后端处理时仅对位姿状态量进行滤波,有效解决了传统算法的维度爆炸问题,减少计算量的同时保证一定的精确度,兼顾SLAM算法精确性和即时性的要求。实验结果表明,方法整体定位精度相对误差低于5%,在光照条件适宜的小型场景效果更佳,误差低于3%,能够达到分米级精度,证明了SLAM方法能够完成SLAM系统的要求,实现即时定位与地图构建功能,具有一定的准确性、稳定性和鲁棒性。粒子滤波能够应用于视觉惯导SLAM领域并达到较高的精度要求。     

分布式接收宽带多目标信号盲检测迭代处理方法

随着软硬件技术的飞速发展和宽带接收机的广泛使用,频谱检测向着高瞬时带宽的方向发展,传统基于信道化处理的频谱检测方法存在搜索速度慢、处理效率低下的问题。文章提出了 1种新的分布式接收宽带多目标信号盲检测迭代处理方法,在无须预先知道信号数目及信号频谱位置的情况下,能够实现特定虚警概率多信号盲检测,具备较高的灵活性和稳健性。首先,在对信号特征进行分析的基础上,通过构造线性模型,将分布式接收多目标信号检测转化为线性模型求解问题进行处理;然后,基于贝叶斯多参数联合求解模型,在对未知参数先验分布进行合理假设的基础上,推导了各未知参数变分分布及信号检测门限的解析表达式,采用变分分布软信息迭代的方式实现多传感器信号、多参数联合估计,并利用每次迭代参数估计结果,对信号检测门限进行更新,通过置零操作实现预设虚警概率下的多信号盲检测;最后,通过仿真实验对所提方法性能进行了分析,并与相关方法进行了对比。仿真结果表明,所提方法能够有效利用多路接收信号信息,实现宽带未知多目标信号的盲检测,有效提升短数据下的算法处理效能,与现有方法相比,在接收单元数目较多以及信噪比较低时具有明显优势。     

Shadow play

ABSTRACT

Despite its visibility but in spite of its lack of materiality (or perhaps because of it), shadow has been a rich source of metaphor, both verbal and visual. Figurative extensions to the visual realms have been less explored; here, we present and analyze some figurative uses of shadow in a sample of visual jokes, cartoons, photographs, and paintings for insights into the nuanced senses of the concept. The visual domains allow for rich expressions of meaning not easily expressed in words.     

Satellite availability and positioning performance of uncombined precise point positioning using BeiDou-2 and BeiDou-3 multi-frequency signals

To realize the smooth transition from regional BeiDou Navigation Satellite System (BDS-2) to the global one (BDS-3), the integration of BDS-2 and BDS-3 is important for providing continuous, stable and reliable positioning, navigation and timing (PNT) services for global users. This work used 154 globally distributed multi-GNSS (Global Navigation Satellite System) experiment stations spanning 30 days to analyze the satellite availability and positioning performance of uncombined precise point positioning (UC-PPP) under current BDS-2 and BDS-3 constellations. We focused on three issues: the influence of BDS-3 receiver tracking abilities, the positioning performance among different areas, and the benefit of multi-frequency observations. The results show that the elliptical zone caused by poor BDS-2 satellite visibility is disappeared when the evenly distributed BDS-3 medium earth orbit satellites are introduced. When BDS-3 are integrated with BDS-2, the area with the Position Dilution of Precision (PDOP) less than 2 can be expanded to 75° S-75° N and 30° E-150° W. The positioning performance of BDS-3 and BDS-2/BDS-3 UC-PPP are seriously affected by the receiver tracking abilities of BDS-3 signals. When the maximum pseudo-random noise sequences (PRNs) of BDS-3 satellites tracked by stations are within 30 or 37, the positioning accuracy of static UC-PPP can be improved by 22.94% or 8.27% due to the integration of BDS-2 and BDS-3. Besides, the most improvement of BDS-2 and BDS-3 integration is achieved in Asia-Pacific regions, especially for the kinematic UC-PPP or the poor receiver tracking abilities of BDS-3. Similar to the multi-frequency BDS-2 UC-PPP, the benefit of multi-frequency signals for BDS-3 or BDS-2/BDS-3 UC-PPP is also non-vital. The three-dimensional positioning accuracy of BDS-2/BDS-3 multi-frequency UC-PPP in static mode and kinematic mode are 2.24 cm and 5.39 cm, while the corresponding convergence time are 49.62 min and 73.80 min, respectively. Compared with BDS-2, both the positioning accuracy and the convergence time of BDS-2/BDS-3 joint UC-PPP are improved by approximately over 50%, which indicates that BDS-3 has a great potential to provide high-quality PNT services as other global navigation satellite systems.     

民机复合材料目视检查中的眼动与疲劳检测

疲劳是导致民机复合材料结构目视检查差错的重要诱因，疲劳检测对于减少人的差错，保障飞行安全具有重要意义。对基于眼动行为的疲劳度量与检测方法进行了研究，建立了民机复合材料结构目视检查实验场景，利用Tobii眼动仪提取了正常状态和疲劳状态下的目视检查眼动数据，分析了瞳孔直径、平均注视时间、平均注视频率、平均眼跳时间、平均眼跳频率、注视热点与轨迹和扫视速度等眼动行为与疲劳的关系，进而提取了能表征疲劳的瞳孔直径、平均注视时间和扫视速度3种眼动指标，以该指标构建特征向量，利用支持向量机（SVM）方法构建了目视检查疲劳检测模型。研究发现疲劳状态下的目视检查平均注视时间更长，扫视速度更慢、瞳孔直径减小，右瞳孔减小程度更大，核函数为径向基函数和高斯函数的SVM方法对疲劳的检测效果好。研究结果表明，利用SVM方法训练由瞳孔直径、平均注视时间和扫视速度构成的眼动特征向量能有效检测目视检查中的疲劳状态。     

基于GCF-SB视觉注意模型的红外目标检测算法

针对复杂背景、低对比度条件下的红外目标检测，提出了一种基于灰度对比度特征 相似性贝叶斯(GCF SB)模型的红外显著性目标检测算法.建立了一种灰度对比度特征(GCF)模型，该模型利用两个通道分别提取红外图像的灰度特征和对比度特征，然后通过特征融合获得初级显著图；建立了一种基于相似性的贝叶斯(SB)模型，该模型根据初级特征图分别计算目标和背景的先验概率和似然函数，然后利用贝叶斯公式获得最终显著图，进而实现红外图像的显著性目标检测.实验结果表明，所提出算法能够有效抑制复杂背景、低对比度红外图像的噪声，增强对比度，具有较高的检测精度和鲁棒性.     

导航卫星多数表决恒包络技术研究

为了提高频带的利用率,现代卫星导航系统往往在一个信道内发送多路信号.直接的线性组合信号不是恒包络的信号,在通过功率放大器时会产生失真,影响信号的性能.为解决这一问题,引入了恒包络生成技术.多数表决算法是针对二进制信号的一种恒包络算法,目前已经成功应用于深空通信中的信号处理;其恒包络特性允许信号通过效率较高的C类功率放大...     

宽带LFM信号同时极化测量性能研究

针对雷达动目标的全极化高分辨信息的获取问题,建立宽带极化雷达的信号模型,分析对应的匹配滤波处理算法.对时/频域加窗的效果进行比较,提出了实用的窗函数选择方法.分析正负线性调频信号对高速运动目标的同时极化测量性能,提出对高速运动目标的极化--一维距离像联合识别方法.仿真结果证明了所此方法的有效性.     

可视化仿真在六自由度运动平台设计中的应用

在传统的六自由度运动平台的设计过程中,机械设计人员很难直观、准确地确定各个机械零部件的极限运动范围以防止发生机械干涉.为了解决这一问题,运用VRML - Java技术,基于运动学反解以及机械实体的静态、动态装配约束关系,建立六自由度电动运动平台的可视化仿真模型,利用该模型对其运动状态(包括极限位姿)进行可视化仿真,通过...     

Monitoring coastal sea level using reflected GNSS signals

A continuous monitoring of coastal sea level changes is important for human society since it is predicted that up to 332 million people in coastal and low-lying areas will be directly affected by flooding from sea level rise by the end of the 21st century. The traditional way to observe sea level is using tide gauges that give measurements relative to the Earth’s crust. However, in order to improve the understanding of the sea level change processes it is necessary to separate the measurements into land surface height changes and sea surface height changes. These measurements should then be relative to a global reference frame. This can be done with satellite techniques, and thus a GNSS-based tide gauge is proposed. The GNSS-based tide gauge makes use of both GNSS signals that are directly received and GNSS signals that are reflected from the sea surface. An experimental installation at the Onsala Space Observatory (OSO) shows that the reflected GNSS signals have only about 3 dB less signal-to-noise-ratio than the directly received GNSS signals. Furthermore, a comparison of local sea level observations from the GNSS-based tide gauge with two stilling well gauges, located approximately 18 and 33 km away from OSO, gives a pairwise root-mean-square agreement on the order of 4 cm. This indicates that the GNSS-based tide gauge gives valuable results for sea level monitoring.