Near real-time PPP-based monitoring of the ionosphere using dual-frequency GPS/BDS/Galileo data

Ionosphere delay is very important to GNSS observations, since it is one of the main error sources which have to be mitigated even eliminated in order to determine reliable and precise positions. The ionosphere is a dispersive medium to radio signal, so the value of the group delay or phase advance of GNSS radio signal depends on the signal frequency. Ground-based GNSS stations have been used for ionosphere monitoring and modeling for a long time. In this paper we will introduce a novel approach suitable for single-receiver operation based on the precise point positioning (PPP) technique. One of the main characteristic is that only carrier-phase observations are used to avoid particular effects of pseudorange observations. The technique consists of introducing ionosphere ambiguity parameters obtained from PPP filter into the geometry-free combination of observations to estimate ionospheric delays. Observational data from stations that are capable of tracking the GPS/BDS/GALILEO from the International GNSS Service (IGS) Multi-GNSS Experiments (MGEX) network are processed. For the purpose of performance validation, ionospheric delays series derived from the novel approach are compared with the global ionospheric map (GIM) from Ionospheric Associate Analysis Centers (IAACs). The results are encouraging and offer potential solutions to the near real-time ionosphere monitoring.     

基于三参数幂函数的低周疲劳寿命预测方法研究

为了找到能够更好地预测材料低周疲劳寿命的方法,基于三参数幂函数公式,对引起材料疲劳损伤的原因进行分析,提出了描述损伤能密度与疲劳寿命关系的寿命预测模型。采用高温合金与钛合金等多种材料的低周疲劳试验数据对提出的模型进行分析验证,并将分析结果与传统的Masson-Coffin模型和三参数幂函数公式的分析结果进行对比。结果表明:所提出的损伤能密度模型对不同条件下的材料数据的拟合效果较好,七种不同条件下的材料数据拟合结果中,有六种的相关系数大于0.9,且所有条件下的寿命预测点都在2倍分散带以内,精度高于另两种模型,并且该模型能够确切地描述引起材料低循环疲劳损伤的原因,简洁明了。     

等离子体对GPS信号载噪比的影响

飞行器再入过程产生等离子体,等离子体中电子密度的变化对电磁波信号造成幅度衰减和相位偏移,严重时导致通信中断。理论分析了等离子体对GPS信号载噪比的影响;利用实验室环境下的等离子体模拟装置,设计了GPS二键相控(BPSK)调制信号透过等离子体的地面模拟方案,进行了等离子体对GPS接收机性能影响的实验研究,获得了不同电子密度下的衰减和导航信号载噪比的数据。实验结果与理论分析一致,不仅证明了等离子体下衰减与导航信号载噪比的变化是一致的结论,而且验证了实验方法的有效性。     

纳米金刚石改性LLDPE的非等温结晶动力学

采用X射线衍射(XRD)、偏光显微镜(POM)以及差示扫描量热(DSC)法,考察了纳米金刚石(Nano-diamond)对线型低密度聚乙烯(LLDPE)的结晶形态以及非等温结晶行为的影响。通过Jeziorny法和莫志深法研究了复合材料的非等温结晶动力学,并利用Kissinger方程计算了LLDPE/Nano-diamond复合材料的非等温结晶活化能。结果表明:Nano-diamond是一种有效的成核剂,为LLDPE结晶提供了更多的活化晶核,起到了异相成核的作用,既提高了LLDPE的结晶起始温度To、结晶峰值温度Tp和结晶速率,又降低了球晶的尺寸,使得球晶的分布更为均匀致密;添加Nano-diamond使得复合材料的结晶迁移活化能降低,当Nano-diamond含量为1%时,复合材料结晶活化能达到最小值。     

基于超光滑表面工艺过程的PSD评价技术研究

基于功率谱密度简化公式,通过数据拟合提取出PSD曲线的两个重要评价参数.研究PSD曲线变化与超光滑表面加工工艺相关性,提出基于超光滑表面工艺过程的PSD评价技术,以指导工艺改进和新工艺的开发.     

Electron density retrieval from occulting GNSS signals using a gradient-aided inversion technique

In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere.     

Peculiarities of the nighttime winter foF2 increase over Irkutsk

An analysis of properties and peculiarities of the nighttime winter foF2 increases (NWI) in the East Siberia is made on data of ionospheric station Irkutsk in the periods 1958–1992 and 2002–2009 and the empirical model of the F2 layer critical frequency under the geomagnetic quiet conditions deduced from these data (model Q-F2). It is revealed, that the NWI is the stable regularity of the quiet ionosphere over Irkutsk. The amplitude of the NWI (the difference between maximum and minimum foF2 values at night hours) is the greatest in December–January and nearly the same at low and middle solar activity. It is a peculiarity of the quiet ionosphere in the East Siberia. Maximum in night foF2 under quiet geomagnetic conditions is observed mainly after midnight (02-04 LT) and is shifted to predawn hours as solar activity increases. At low solar activity the quiet ionosphere at ∼02–04 LT shows the following properties: (a) the fluctuations of foF2 and hmF2 are in the reverse correlation but this dependence is weak; (b) very strong fluctuations of foF2 (|δfoF2| > 30%) occur seldom (∼4% of events) and almost all of them are positive; an example of very strong fluctuations of foF2 up to 60% can be an extreme increase in the foF2 on 19.12.2008; (c) the very strong enhancements of foF2 in the NWI maximum can be observed at the low geomagnetic activity, they occur more often during substorms but very seldom during geomagnetic storms. Possible reasons of these properties of NWI are discussed.     

Study of TEC changes during geomagnetic storms occurred near the crest of the equatorial ionospheric ionization anomaly in the Indian sector

GPS satellites data obtained at Bhopal (23.16° N, 77.36° E, geomagnetic latitude 14.23° N) India were analyzed to study the TEC changes during several geomagnetic storms (−300 nT < Dst < −50 nT) occurred in 2005–2007. We had segregated the storms according to the Dst value, i.e. moderate storms (−100 nT < Dst ? −50 nT), strong storms (−150 nT < Dst < −100 nT), and severe storms (Dst less than −150 nT). Total of 21 geomagnetic storms (10 moderate, 9 strong, 2 severe) are considered for the present study. Deviation in vertical total electron content (VTEC) during the main phase of the storm was found to be associated with the prompt penetration of electric field originated due to the under-shielding and over-shielding conditions for almost all geomagnetic storms discussed in this paper. For most of the storms VTEC shows the positive percentage deviation during the main phase while it shows positive as well as the negative deviation during the recovery phase of the storms. The −80% deviation in VTEC was found for geomagnetic storm occurred on July 17, 2005 and the negative trend continued for recovery phase of the storm. This was mainly due to the thermospheric composition changes by Joule heating effect at auroral latitudes that generate electric field disturbance at low latitudes. Traveling ionospheric disturbances (TIDs) were responsible for the formation of wave like nature in VTEC for the storms occurred on May 15, 2005, whereas it was not observed for storm occurred on August 24, 2005.     

基于预警卫星观测的弹道导弹运动状态估计算法

针对预警卫星仅有角度测量信息，目标运动可观测性很弱，而导致最大似然弹道估计条件病态严重的问题，提出基于提供目标状态估计和联合误差协方差的Levenbebg-Marquardt算法的修正最大似然估计（MML）算法。利用预警卫星初始观测时刻的目标高度和速度等部分先验信息作为伪测量量，推导出导弹运动状态参数估计的修正最大似然算法。最后，对算法进行了Monte—Carlo仿真研究，仿真结果表明估计误差同理论误差边界相当，验证了估计算法的一致性和有效性．     

基于自适应谱峰搜索的脉冲重复间隔最大似然估计

高精度的脉冲重复间隔（PRI）估计对于提高现代电子情报（ELINT）系统的辐射源识别能力具有重要意义。将观测的脉冲到达时间（TOA）序列建模成点过程信号,其周期图谱谱峰就对应于PRI的最大似然估计（MLE）。本文提出了一个“两步数值搜索”策略,可以确保搜索到谱峰而达到MLE。仿真表明,在低信噪比以及高脉冲丢失率条件下,本文算法性能远超已有算法。