基于MF-DFA的极低频瞬态事件多重分形分析

利用三个测站实测极低频全球闪电活动背景信号南北及东西方向磁场分量资料, 应用多重分形方法对极低频瞬态事件发生时前1s背景信号中的长程关联性及多重分形特征进行了分析. 发现极低频瞬态事件发生时, 最大脉冲前1s背景信号南北及东西方向磁场均具有长程关联性和多重分形特征. 其中南北方向最小标度指数0.15 < a_min< 0.22, 分形谱宽度1.09 < Δa < 1.51; 东西方向最小标度指数0.23 < a_min< 0.59, 分形谱宽度0.75 < Δa < 0.99. 多重分形谱参数Δa和a_min可以很好地自动识别Q-burst发生.      

基于瞬时相关频域检测的复合调制引信定距方法

为了实现混沌调相与线性调频复合调制无线电引信对不同散射特性的地面目标的精确定距功能，提出一种基于瞬时相关频域检测的复合调制引信定距方法。在时域瞬时相关处理之前，对本地预设混沌码与目标回波信号以码元宽度为采样周期进行同步采样；为减小目标回波幅值对定距方法的影响，以二维快速傅里叶变换（2D-FFT）提取目标的距离、速度信息，以相关窗位置、谐波包络主瓣位置、稳定的多普勒频率3个特征量作为定距依据，从频域角度进行精确定距。仿真结果表明:所提方法的判定结果只与目标信号信噪比有关，而与信号幅值具体大小无关；所提方法可在-28 dB的极低信噪比条件下实现复合调制引信对不同散射特性地面目标的精确定距功能。      

调频引信粗糙面目标与干扰信号识别

为有效识别调频（FM）引信地面目标回波信号和数字射频存储（DRFM）转发式干扰信号,提出了一种对地调频引信粗糙面目标与干扰信号识别方法。建立了对地调频引信粗糙面差频信号模型,利用二维距离-速度提取方法提取其差频频率和多普勒频率,采用差频频率峰值带宽和多普勒频率峰值带宽2个特征量识别地面目标回波信号和DRFM转发式干扰信号,并使用蒙特卡罗仿真和非参数假设统计检验对其进行了验证。结果表明:差频频率峰值带宽和多普勒频率峰值带宽的展宽幅度与引信天线照射范围内粗糙面的大小呈正相关,多普勒频率峰值带宽的展宽与载频成正比,利用峰值带宽特性可有效区分粗糙面目标回波信号和DRFM转发式干扰信号。      

“Fine structure” of the storm-substorm relationship: Ion injections during DST decrease

Following a long period of consensus on the storm-substorm relationship, a dispute on this topic has emerged in recent years. The importance of substorms for the buildup of the terrestrial ring current, which is the major element of magnetic storms, has been questioned in several studies. This paper is an effort to assess the “fine structure” of the storm-substorm relationship, by investigating the correlation between the changes in Dst and the substorm-associated O⁺ enhancements in the inner magnetosphere during the storm main phase. For this purpose we use energetic ion measurements from the Magnetospheric Ion Composition Spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES), and the newly produced high-resolution (5-min) Dst index for the intense storm of June 5, 1991. Substorm signatures from both MICS measurements and ground magnetometers correlate well with changes in the Dst decrease rate. This implies a significant influence of substorm occurrence on storm dynamics.     

Digital spacecraft ionosondes

In cooperation with RCA, Astro-Electronics Division, a digital spacecraft ionosonde has been developed and its prototype built by ULCAR. On-board use of the Direct Discrete Fourier Transform for signal-to-noise enhancement, suppression of unwanted echoes and the identification of echo properties permit several applications. In ionospheric topside satellites (>800 km above ground) the ionospheric profile over all parts of the earth will be measured with simple and new data compression techniques on board of the satellite. Digital data preprocessing overcomes the breakthrough of ground-based man-made interference and analyzes auroral and equatorial echoes. Because the ionosonde measures amplitude, phase, Doppler, range and polarization of a radio wave simultaneously, it will be a very useful tool for the Waves in Space Plasma program of the NASA Space Shuttle, especially for a mother-daughter configuration. With a synchronized transmitter on the ground, such a Digisonde in a low-flying satellite (100–400 km) can receive signals from ducted radio waves launched into the ionosphere near natural or artificial ionization depletion areas for the study of large distance radiowave propagation.     

Modeling of artificial plasma “bubble” in ionosphere

Results of a combination of radio-crossing and in situ measurements of plasma density in an artificial plasma “bubble” in the ionosphere are presented. Shaped — charge barium injection was made at short distance (≤50 m) to plasma diagnostics on the rocket. After injection the rocket passed through expanding plasma shell. Plasma density depletion inside was more than one order and plasma enhancements on the boundary about 3–5 times that of background. When the rocket passed the shell and went away by 2.1 km an abrupt drop of telemetry signal level (≤ 65 dB) was registered though plasma density was not more than 3×10³sm⁻³. An estimation of high frequency signal refraction on the plasma shell is in good accordance with refraction data of geostationary satellite signals on equatorial bubbles.     

Investigation of magnetospheric processes with the use of a source of strong magnetic field in the ionosphere

More than 20 years ago V.P. Shabansky suggested that the magnetic system installed aboard the satellite, could be used as a physical instrument for studying the processes which occur in the near Earth space. The corresponding space scales of an artificial “magnetosphere”—“magnisphere”—are 10 m in the experiment with relatively small magnets in the ionosphere and 100 m in the solar wind. The corresponding similarity criteria are estimated. The possible scheme of the experiment with a superconducting magnet (magnetic moment 10⁵ A · m²) installed aboard the satellite is considered. The experimental complex includes a number of systems for measuring the fluxes of charged particles in a wide energy range, DC electric and magnetic fields, the electromagnetic fields in different frequency bands (from X-rays to radio). The scientific objectives are discussed in detail.     

Magnetopause poleward of the cusp: Comparison of plasma and magnetic signature of the boundary for southward and northward directed interplanetary magnetic field

A coherent data set of high-latitude dayside magnetopause encounters by old (Heos 2, Hawkeye, Prognoz 7, 8) and new (Polar, Interball Tail, Cluster) spacecraft is needed to build a realistic model of the magnetopause (MP) including an indentation in the cusp. In building such a coherent data set a caution is necessary as the dayside magnetopause at high-latitudes may be less clearly defined than in the case of observations at low latitudes. It is due to expected presence of bundles of newly-reconnected magnetic field lines forming an extended boundary layer on the magnetosheath (MS) side of the magnetopause in the cusp region. Moreover, numerical magnetohydrodynamic (MHD) models of the solar wind-magnetosphere interaction predict that under northward interplanetary magnetic field (IMF) an additional thin current sheet should form inside the magnetopause at high latitudes on the dayside (e.g., Wu, 1983; Palmroth et al., 2001). Such a thin currect sheet is absent in empirical magnetosphere models. This internal current sheet, if a real one, may be mistaken for the magnetopause if magnetic field data are only taken into account and/or plasma data are unavailable. The Interball-Tail orbit allows for a full transition of magnetopause boundary layers at high-latitudes. We compare plasma and magnetic field signatures of the magnetopause poleward of the cusp for southward and northward IMF. The distance between the magnetic signature of the magnetopause (the current layer) and a cold and laminarly antisunward flowing MS plasma (so called free-flow MS) was found to be 0.5 to 1 R_E, at least. These observations were made under nominal solar wind of v350 km/s and p_dyn=1 to 4 nPa. We also observed several transient magnetic field reversals in the cusp related to pulses of solar wind dynamic pressure and/or the IMF discontinuity arrival. These transient reversals occurred at the same distance to the model MP as well defined full MP crossing, so most probably they represent just short encounters with the magnetopause current layer. Our analysis suggests that an indentation of the magnetopause with a subtle structure dependent on the local magnetic shear would explain and allow to predict the magnetic configuration in the high-altitude cusp.     

基于多通道天线高分辨率星载SAR 实现方法研究

对基于多通道天线实现高分辨率星载SAR的方法进行了深入研究,建立了三通道天线SAR的回波信号模型,提出了一种回波处理方式,将单发三收模式下的回波数据转变为传统的单发单收模式下的回波数据,可用常规条带式成像方法进行成像处理.对采用多通道天线实现高分辨率星载SAR涉及到的脉冲重复频率PRF偏离问题进行了深入分析和研究,推导出PRF偏离造成的虚假目标的理论位置和虚假目标与真实目标的峰值功率比值,计算机仿真试验证明了上述内容的有效性.      

直反信号协同的GNSS-R BSAR距离多普勒成像算法

针对目前基于全球导航卫星系统反射信号的双基地合成孔径雷达（GNSS-R BSAR)在一站固定模式下的大斜视,斜距历程复杂,回波信号方位空变导致回波信号难以处理的问题,提出改进的距离多普勒成像新算法。所提算法采用GNSS信号作为辐射源,根据一站固定模式下GNSS-R BSAR合成孔径时间长的特点,引入高阶等效斜视距离模型,得到导航卫星与目标斜距相对时间变化的精确描述。先通过直射信号与回波信号时域对消进行距离徙动校正,实现全场景目标距离徙动的精确校正;再通过方位向分块混合相关处理来克服回波信号方位向的移变性质,实现全场景高效精确成像。所提算法的成像效率优于传统后向投影时域（BP）算法,成像精度与BP算法相当,且可根据需要通过调整方位分块的宽度来提升聚焦效果。最后,用GPS-L5信号进行仿真和实验,仿真和实验结果验证了所提算法的可行性和高效性。     

基于双谱分析的高原灌木回波信号分类

目标的回波信号是无线电引信获得目标信息的最重要方式,为了太赫兹频段的引信前端未来能够投入高原战场,适应高原不同的地貌环境,利用双谱对高原在灌木地形下不同高度的太赫兹波回波特性进行了分析。为减少分类时间,对双谱数据进行积分,得到实采信号双谱切片的特征,进而利用最邻近算法对此进行分类。利用经验模态分解（EMD）提取原始数据内在模态函数的特征,再次分类并与前一组分类结果进行对比。通过一系列数据的分类,结果表明:利用一维的积分双谱信息可以有效提取出距离地面分别为2 m、3 m、4 m、5 m时的特征并进行分类,经验模态分解也可以有效提高分类的成功率,成功率最高可达90%以上。      

火星空间大尺度电流体系分布特征

在MHD模型基础上, 对火星空间环境的电流分布进行了模拟. 结果表明, 火星空间存在着弓激波电流、磁堆积区电流、电离层电流和磁尾电流. 弓激波电流在激波曲面上均由北向南自成体系, 电流密度在弓激波顶区域较大. 在向阳面磁堆积区边界电流与电离层电流彼此耦合形成完整的回路, 在背阳面磁堆积区边界电流与磁尾中心电流片耦合形成完整的回路.      

电离层对星载SAR影响的多相位屏仿真方法

利用多相位屏技术对电离层色散、相位闪烁、幅度闪烁等影响的P波段星载SAR(Synthetic Aperture Radar)回波进行了仿真.将电离层建模为只改变信号相位的多个薄屏,信号在屏之间的衍射效应实现电离层的幅度闪烁和相位闪烁.衍射结果由斜入射条件下球面波的抛物方程在自由空间的解析解得到.穿过多相位屏的信号复振幅中包含了电离层色散、相位闪烁、幅度闪烁等影响.回波信号通过发射信号的频谱与受电离层影响的复振幅相乘得到.仿真表明,该方法可以真实反映电离层色散、相位闪烁、幅度闪烁等效应,为研究电离层影响下的星载SAR成像提供真实可靠的数据.     

针对地线扰动的一种地线电流自动补偿方法

本文分析了地线干扰的主要原因与途径，指出了抑制地线干扰的几种措施，同时为了解决地线电流通过地线阻抗形成差模电压对高精度测量性能造成影响的问题，提出了一种基于地线电流自动补偿技术的解决方案，利用本技术方案，可以自动地将流经地线的直流电流和杂波噪声电流补偿为零，即使地线存在直流电阻和交流阻抗，在地线的两端也不产生压降，提高了系统信号准确度。     

针对地线扰动的一种地线电流自动补偿方法

本文分析了地线干扰的主要原因与途径,指出了抑制地线干扰的几种措施,同时为了解决地线电流通过地线阻抗形成差模电压对高精度测量性能造成影响的问题,提出了一种基于地线电流自动补偿技术的解决方案,利用本技术方案,可以自动地将流经地线的直流电流和杂波噪声电流补偿为零,即使地线存在直流电阻和交流阻抗,在地线的两端也不产生压降,提高了系统信号准确度。     

CRIT II electric, magnetic, and density observations in an ionizing neutral jet

Measurements inside a high velocity neutral barium beam show a factor of six increase in plasma density in a moving ionizing front. This region was co-located with intense electric fields (δE ≈ 300 mV/m²) perpendicular to the local geomagnetic field and field aligned currents all fluctuating at frequencies well under the lower hybrid frequency for barium but above the oxygen cycloton frequency. It was determined that these structures were moving with the barium stream near the neutral barium velocity. Large quasi-dc electric and magnetic field fluctuations were also detected. The heart of the ionizing front, a cross beam current of nearly 10 mA/m², was estimated from the magnetic field variation. This is three orders of magnitude higher than typical auroral zone currents associated with auroral arcs. This current sheet was co-located with fluxes of soft electrons which saturated the particle detector. An Alfvén wave with a finite electric field component parallel to the geomagnetic field was observed to propagate along B_o where it was detected by an instrumented sub-payload.     

Current collection through the transport of electrons across magnetic field lines

Collection of electrons by a long conducting cylinder in a flowing plasma is studied by means of numerical simulations. The plasma flow simulates the relative motion between a spacecraft and plasma. The sheath structures and the levels of electron current collections for the cases with and without an ambient magnetic field ( ) are studied. It is found that for the flow perpendicular to the magnetic field, the current is considerably enhanced depending on the relative drift velocity. In the case of a non-zero magnetic field perpendicular to the cylinder axis, the potential structure is a two-dimensional double layer with dimensions L L_|, where L and L_| are the dimensions perpendicular and parallel to , respectively. L is found to be the current limiting radius given by the Parker-Murphy model. For the flow along , the electron current is found to be smaller than that for the flow perpendicular to . This is explained in terms of the potential structures.     

Energy deposition in the upper atmosphere in the EXCEDE III experiment

The EXCEDE III sounding rocket flight of April 27, 1990 used a 18 Ampere 2.5 keV electron beam to produce an artificial aurora in the region 90 to 115 km. A “daughter” sensor payload remotely monitored the low-energy X-ray spectrum while scanning photometers measured the spatial profile of prompt emissions of N₂⁺ (1N) and N₂ (2P) transitions (3914Å and 3805Å, respectively). Two Ebert-Fastie spectrometers measured the spectral region from 1800 to 8000Å. On the “mother” accelerator payload, the return current electron differential energy spectra were monitored by an electrostatic analyzer (up to 10 keV) and by a retarding potential analyzer (0 eV to 100 eV). We present an overview of the results from this experiment.     

一种基于PCI总线的可编程雷达信号模拟器实现

针对目前雷达回波信号模拟器功能单一、信号回放速度低的问题,提出了采用计算机PCI总线式结构、高速FIFO和高速D/A的实现方案,使模拟器的数据更新速度达到8 MHz,信号回放速率达到73 MHz;采用FPGA技术,使系统参数程控可选,且可方便地改动其组成结构以适应不同类型的雷达.通过对合成孔径雷达(SAR,Synthetic Aperture Radar)回波信号的仿真输出,并经现场运行表明,该方案可行,样机运行可靠,满足对高分辨率图像的仿真要求.