An improved approach to model regional ionosphere and accelerate convergence for precise point positioning

Given the severe effects of the ionosphere on global navigation satellite system (GNSS) signals, single-frequency (SF) precise point positioning (PPP) users can only achieve decimeter-level positioning results. Ionosphere-free combinations can eliminate the majority of ionospheric delay, but increase observation noise and slow down dual-frequency (DF) PPP convergence. In this paper, we develop a regional ionosphere modeling and rapid convergence approach to improve SF PPP (SFPPP) accuracy and accelerate DF PPP (DFPPP) convergence speed. Instead of area model, ionospheric delay is modeled for each satellite to be used as a priori correction. With the ionospheric, wide-lane uncalibrated phase delay (UPD) and residuals satellite DCBs product, the wide-lane observations for DF users change to be high-precision pseudorange observations. The validation of a continuously operating reference station (CORS) network was analyzed. The experimental results confirm that the approach considerably improves the accuracy of SFPPP. For DF users, convergence time is substantially reduced.     

中低纬度Pc3-4地磁脉动特性研究——子午工程数据分析初步结果

利用新建成的子午工程地磁台站数据,对比分析了地磁平静期间(2011年3月20-27日)和磁暴期间(2011年9月25日至10月1日)Pc3-4地磁脉动的时空分布特征及其对行星际条件的响应.数据分析结果表明,中低纬度(1.3<L<2.3,L为磁壳参数)的Pc3-4地磁脉动在这两个时期内的分布存在明显的晨昏不对称性,在昼侧前出现明显的Pc3-4地磁脉动并与行星际上游波动密切相关,其振幅增强可能与太阳风动压脉冲相关,高速太阳风更易导致Pc3-4地磁脉动;而对于近赤道低纬(L<1.3)区域,无论是在地磁平静期还是磁暴期均未能观测到Pc3-4地磁脉动,Pc3-4地磁脉动存在明显的纬度效应.     

北极黄河站极光亚暴期间低热层大气中性风研究

极光亚暴是地球空间基本的能量输入、耦合及耗散过程,其对低热层大气中性风的影响不容忽视,对这一问题进行深入研究具有重要意义.2010年11月,中国北极黄河站(78.92°N,11.93°E)安装了一台自主研制的全天空法布里-珀罗干涉仪(FPI)并开始进行正常观测,为中国首次获得了极光亚暴期间的FPI观测数据.根据2012年越冬期间的观测情况,对2012年11月12-14日及12月09-11日两个极光亚暴事件期间的数据进行处理,计算得到了5级干涉环对应的风场.亚暴期间风场变化与地磁活动变化的对比分析表明,风速的剧烈变化可能是由地磁活动剧烈扰动造成的.针对2012年11月13日00:00UT-02:00UT和2012年12月10日05:00UT-07:30UT的亚暴事件,将全天空相机拍摄到的极光图像与FPI干涉图像对应的视线风场进行对比分析.结果表明在极光活动中,视线风速加强的方向与极光弧的方向垂直,而在极光弧的平行方向,风速相对较小.     

球形压头压入中摩擦对材料参数识别的影响

利用量纲分析原理和有限元方法,研究了球形压头压入中摩擦对于幂强化材料塑性材料参数识别的影响.通过定义一种基于能量的表征应变,根据识别的两个压入深度的表征应变和表征应力,得到材料的塑性特性;最后研究了摩擦对压入响应、表征应力及材料参数提取的影响.研究结果表明,摩擦对压入的影响与材料的应变硬化指数、弹性模量与屈服极限的比值及弹性功与总功的比值密切相关,弹性功与总功的比值越大,摩擦的影响越小;摩擦对材料参数提取的影响不可忽略,当摩擦系数未知时,对于压入过程中弹性功与总功的比值较大的材料,近似取摩擦系数为0.15,能得到较合理的材料参数识别的结果.      

基于Tophat变换和文字纹理的车牌定位算法

针对车牌自动识别系统的应用场景越来越广泛,提出一种新的基于Tophat滤波和文字纹理特征的车牌定位算法.首先,利用形态学Tophat滤波抑制背景、消除不均匀光照,然后对图像进行二值化操作和连通域分析;其次,扫描图像得到区域的垂直投影图(VPM, Vertical Projection Map),对VPM进行离散余弦变换(DCT, Discrete Cosine Transformation),利用中低频描绘子重构VPM,重构后的VPM更为平滑,不受噪声影响,其波峰波谷数量、统计量更能描绘区域纹理的本质属性;最后,结合部分中低频描绘子和统计量组成描述区域纹理的模式向量,输入支持向量机归类.实验表明,算法适用于自然场景中的车牌定位问题,具有较强的适应性.      

大气波动监测仪在岩石圈-大气层-电离层研究中的应用

针对北京大学空间物理与应用技术研究所研制的两台大气波动监测仪近三年的观测数据,对这一时段内所观测到的重力波和次声波周期尺度扰动的形态特征、谱结构特征及其在时间分布上的统计特征进行了分析. 给出了几例雷暴、地震等事件中观测到的大气扰动,并揭示了这些事件期间观测到的与地面大气扰动周期尺度相类似的电离层扰动. 结果表明,北京大学研制的大气波动监测仪可有效记录到地面的微弱大气扰动,观测数据可用于进行岩石圈-大气层-电离层之间的耦合研究.     

空间非合作目标双行轨道根数快速生成算法

针对空间目标TLE拟合过程中可能出现的奇点问题,提出了基于无奇异变换的空间目标双行轨道根数（TLE）生成算法. 为提高观测平台对空间目标状态估计效率,提出带有自适应遗忘因子的非线性最小二乘递推算法,利用最速下降法在线修正遗忘因子,使得估计值有较快的跟踪速度和较小的稳态误差. 仿真结果表明,该TLE生成算法的数据处理速度和轨道预报误差满足要求,可用于低轨目标的天基监视.     

Tropospheric wet refractivity tomography using multiplicative algebraic reconstruction technique

Algebraic reconstruction techniques (ART) have been successfully used to reconstruct the total electron content (TEC) of the ionosphere and in recent years be tentatively used in tropospheric wet refractivity and water vapor tomography in the ground-based GNSS technology. The previous research on ART used in tropospheric water vapor tomography focused on the convergence and relaxation parameters for various algebraic reconstruction techniques and rarely discussed the impact of Gaussian constraints and initial field on the iteration results. The existing accuracy evaluation parameters calculated from slant wet delay can only evaluate the resultant precision of the voxels penetrated by slant paths and cannot evaluate that of the voxels not penetrated by any slant path. The paper proposes two new statistical parameters Bias and RMS, calculated from wet refractivity of the total voxels, to improve the deficiencies of existing evaluation parameters and then discusses the effect of the Gaussian constraints and initial field on the convergence and tomography results in multiplicative algebraic reconstruction technique (MART) to reconstruct the 4D tropospheric wet refractivity field using simulation method.     

Effects of in-track maneuver on Sun illumination conditions of near-circular low Earth orbits

By developing approximate analytical models considering the J₂ perturbation, the effects of an in-track maneuver on the orbital Sun illumination conditions of near-circular low Earth orbits are analyzed. First, two approximate models for the variations in orbital sunshine angles are developed, one for variations at a given time and the other for variations at a given argument of latitude. Next, two approximate models for variations in orbital arc in Earth shadow are developed, one considers the small eccentricity and the other uses the zero eccentricity. Finally, the developed approximate models are applied to analyzing the Sun illumination conditions of a typical in-track maneuver mission on a near-circular low Earth orbit. From the results obtained, three major conclusions can be drawn. First, the variations in orbital sunshine angles at a given time may reach tens of degrees when the drifting time reaches hundreds of orbital periods, and the approximate model for that situation cannot effectively approach the numerical results. Second, the variations in orbital sunshine angles for any given argument of latitude are only a couple of degrees even when the drifting time reaches 500 orbital periods, and the approximation model developed can effectively approach the numerical results. Third, for variations in orbital arc in Earth shadow, the approximate model considering the small eccentricity has simple expressions and can effectively approach the numerical results; in contrast, the approximate model using the zero eccentricity has relatively worse precision.     

Simulation analysis for effects of bone loss on acceleration tolerance of human lumbar vertebra

The purpose of the present study was to analyze and predict the changes in acceleration tolerance of human vertebra as a result of bone loss caused by long-term space flight. A human L3–L4 vertebra FEM model was constructed, in which the cancellous bone was separated, and surrounding ligaments were also taken into account. The simulation results demonstrated that bone loss has more of an effect on the acceleration tolerance in x-direction. The results serve to aid in the creation of new acceleration tolerance standards, ensuring astronauts return home safely after long-term space flight. This study shows that more attention should be focused on the bone degradation of crew members and to create new protective designs for space capsules in the future.