Longitudinal variations of the ionospheric trough position

For the first time a comprehensive pattern of the longitudinal effect of the ionospheric trough position was obtained. We present new results with longitudinal variations of the winter trough position as a function of geomagnetic latitude for both hemispheres and conditions of high and low solar activity and all local time hours. We used a large observational data set obtained onboard the Kosmos-900, Interkosmos-19 and CHAMP satellites for quiet geomagnetic conditions. We found that a magnitude of the trough position longitudinal effect averaged for a fixed local time is greater in the daytime (6–8°) than in the nighttime (3–5°). The longitudinal effect magnitude reaches its maximum (16°) in the morning (at 08 LT) in the Southern hemisphere at high solar activity. But on certain days at any solar activity the longitudinal effect magnitude can reach 9–10° even at night. The shape of the longitudinal effect was found to differ significantly in two hemispheres. In the Northern hemisphere the trough is usually closest to the pole in the eastern (American) longitudinal sector, and in the Southern hemisphere the trough is closest in the western (Eurasian) longitudinal sector. The magnitude and shape of the longitudinal effect is also different during low and high solar activity. The Global Self-consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSM TIP) simulations demonstrate that during low solar activity, the longitudinal variations of the daytime trough position is mainly determined by longitudinal variations of the ionization function, formed due to the longitudinal variations in the solar zenith angle and the atomic oxygen density distribution. The longitudinal variations of the nighttime trough position is formed by the longitudinal variations in ionization of precipitating auroral particles, neutral atmosphere composition, and electric field.     

基于希尔伯特-黄变换的雷达数据误差修正处理技术

由于雷达跟踪测量数据总存在误差,而目前采用的误差处理算法修正效果并不理想,为了有效估计数据中的未知误差,提高数据处理的精度,提出了一种基于希尔伯特-黄算法的雷达误差残差诊断方法,利用得到的内蕴模态函数拟合出误差残差的特征,准确分离出设备的误差信息。通过对某雷达的实测数据分析证明该方法较其他方法对解决该类问题更为有效,且在事后高精度数据处理中具有较好的实用性。     

基于经验模态分解的地球同步轨道高能电子通量预报

在磁暴恢复相期间,大量相对论(高能)电子从磁层的外辐射带渗透到地球同步轨道区.其中> 2 MeV的高能电子能够穿透卫星表面并聚积在材料内部,导致卫星无法正常运行或完全损坏.磁暴期间的高能电子通量变化的非平稳与非线性特征十分明显.通过实验发现,经验模态分解法能够极大地降低高能电子通量非平稳性问题造成的预报影响.以2008-2009年的数据作为训练集,2010-2013年数据作为测试集.结果表明:2010-2013年的预报率约为0.84;在太阳活动较为复杂的2013年,预报率达到0.81.引入经验模态分解后预报效率得到显著提高.     

EMD滤波在捷联惯导晃动基座初始对准中的应用

为了实现晃动基座情况下光纤捷联惯性导航系统的高精度初始对准,提出了一种基于经验模态分解(EMD)法去噪的抗干扰初始对准算法。该算法在凝固惯性系下进行姿态更新,以反映载体在晃动干扰下的姿态变化,从而消除角运动干扰的影响;并针对EMD法的不足对其进行改进,然后对加速度计的输出进行去噪,以消除线运动干扰的影响。结果表明:改进后的EMD法具有良好的去噪效果,基于EMD法去噪的抗干扰初始对准算法具有角运动和线运动的干扰抑制能力,能够实现晃动基座下的高精度抗干扰初始对准。     

工艺参数对复合材料长桁成型载荷的影响

复合材料长桁机械成型过程中的成型载荷与成型工艺参数的关系会影响预制体成型过程的稳定性和长桁成型后的性能,利用自制复合材料L型长桁自动化成型机,搭建出一种长桁结构机械成型载荷测试系统,通过成型温度、成型速率、成型间距和铺层结构的研究,对成型载荷的变化规律进行测试和分析,探索出工艺参数与成型载荷之间的经验公式。结果表明:在合适的成型温度范围内,随着成型温度的逐渐升高,成型速率逐渐降低,成型间距逐渐增大,会使得成型载荷逐渐降低;铺层方式不同时,成型载荷变化情况为:[0°/45°/90°/–45°]ns>[0°/90°]ns>[±45°]ns;最后得到成型载荷与工艺参数关系的经验公式,在一定范围内对成型温度、成型速率、成型间距可具备实际指导能力。     

基于经验模态分解的非平稳信号趋势项消除

归纳概括了传统的趋势项消除方法,指出各类方法的优点和不足,提出了基于EMD（经验模态分解）的非线性、非平稳信号剔除方法。该方法通过数据驱动自适应构造基底函数IMF（本征模函数）,再由若干阶IMF分量和剩余分量的重组获得趋势项,避免了对复杂趋势项的数学建模和分析计算。仿真结果表明,EMD法能够有效地提取和剔除非平稳信号中的复杂趋势项成分,获得平滑的趋向性信号。     

外测数据处理中小波滤波的鲁棒性算法研究

为消除外测数据处理中异常值和噪声信号对处理结果的影响,结合数据处理的实际,给出一种基于小波变换的鲁棒性滤波算法。首先用移动中值滤波算法剔除原始数据中的异常值,然后采用小波系数去噪算法并结合经验维纳阈值滤波算法,抑制数据中的噪声。仿真计算及实际工程应用表明,该算法在保留特征段及有用信息的同时,有效地剔除了异常值,抑制了噪声,具有很好的鲁棒性。     

A global empirical orthogonal function model of plasmaspheric electron content

Based on the method for establishing a global plasmaspheric model using observations from COSMIC and MetOp-A orbit determination GNSS receivers, Chen et al. (2017) obtained a global plasmaspheric total electron content product with a spatial resolution of 2.5° × 5° and a time resolution of 4 h. In this paper, we use those global plasmaspheric electron content product in 2008, 2010, 2011, 2012, 2014 for 1446 days to establish a global plasmaspheric empirical model based on empirical orthogonal function (EOF). The model can well characterize the spatiotemporal variation of plasmaspheric electron content (PEC) and the influence of solar radiation on it. Only the first four orders of EOF sequences can characterize the 98.43% features of the original PEC dataset. The principal component coefficient Pk is decomposed twice during modeling, and the combination of trigonometric function and linear function is used to model Pk to characterize the solar cycle, annual cycle, semi-annual cycle and quarter-cycle variation. We compare the PEC model values with the actual observation data, the results show that the empirical PEC model values are highly correlated with the actual observations. The correlation between the two is above 0.96, and the RMS maximum of the difference between the PEC model values and the observed values are 0.70 TECU, and the average of the difference between the PEC model values and the observed values are −0.18 TECU, respectively. In addition, we validate the reliability of the global plasmaspheric model established by two empirical orthogonal function decomposition method using actual observation data, according to the global distribution of the differences between the PEC model values and the observed values in low solar activity and high solar activity, it can be seen that under low solar activity and high solar activity conditions, the model has good adaptability.     

Comparison of topside ionosphere scale height modeled by the Topside Sounder Model and incoherent scatter radar ionospheric model

The topside ionosphere scale height extracted from two empirical models are compared in the paper. The Topside Sounder Model (TSM) provides directly the scale height (H_T), while the incoherent scatter radar ionospheric model (ISRIM) provides electron density profiles and its scale height (H_R) is determined by the lowest gradient in the topside part of the profile. H_T and H_R are presented for 7 ISR locations along with their dependences on season, local time, solar flux F10.7, and geomagnetic index ap. Comparison reveals that H_T values are systematically lower than respective H_R values as the average offset for all 7 stations is 55 km. For the midlatitude stations Arecibo, Shigaraki, and Millstone Hill this difference is reduced to 43 km. The range of variations of H_R is much larger than that of H_T, as the H_T range overlaps the lower part of the H_R range. Dependences on ap, DoY and LT are much stronger in the ISRIM than in TSM. This results in much larger values of H_R at higher ap. Diurnal amplitude of H_R is much larger than that of H_T, with large maximum of H_R at night. The present comparison yields the conclusion that the ISR measurements provide steeper topside Ne profiles than that provided by the topside sounders.     

一种提高脉冲雷达多普勒相位精度的方法

针对低信噪比条件下脉冲雷达模糊多普勒相位精度较低,可能导致相位测距时不能正确解相位模糊问题,基于EMD（Empirical Mode Decomposition,经验模式分解）区间阈值去噪方法,提出了一种新的提高多普勒相位精度的方法：利用EMD分解后各层信号的频率特性和能量特性,选取合适的阈值,并对各层信号进行区间阈值化处理,在提高信号信噪比的同时保持了信号的连续性.分别在回波信号噪声为高斯白噪声和AR（2）相关噪声的情况下,以及不同信噪比条件下,对该方法进行验证.仿真结果表明：在低信噪比条件下,当回波信号噪声为白噪声和相关噪声时,EMD区间阈值去噪方法能将回波信号信噪比提高5 dB,去噪性能优于小波阈值去噪方法,其对应多普勒相位精度能提高1倍以上.