铷频标物理系统的改进研究

物理系统是铷频标的核心部件,通过分析影响频率稳定度的因素,对物理系统内部结构进行了改进。改进后的物理系统采用分离滤光的三泡结构,增加了光学滤光技术,此外,物理系统还选用了磁控管微波腔。经测试,改进后的铷频标温度系数为9.7×10^-14/℃,频率稳定度约为1×10^-12/τ（1s≤τ≤10 000s）。     

压缩空气中悬浮油检测采样方法研究

设计了一套压缩空气中悬浮油检测的采样方法,并对该方法的采样时间、采样流量、采样吸收效率等进行了定量化试验和论证,确认了该方法的适用性。     

高超声速流场对自由流扰动波响应及边界层扰动波演化

采用高阶精度有限差分方法直接数值模拟了来流慢声波作用下的钝锥高超声速绕流瞬态流场,分析了自由流扰动波与流场的相互干扰,并应用Fourier频谱分析了边界层扰动波时域和空域演化.结果显示:弓形激波出现连续的“∽”变形,初始扰动被显著放大,边界层内外扰动模态存在明显差别.从空域(沿流线)演化来看,在球头附近低频扰动为主导模态,出了球头区,总扰动模态中的低频和高频成分比例迅速转变,高频模态成分显著地增大.至流场下游,大部分低频分量衰减,或者不再增长,边界层内仅存在特殊频率的不稳定波快速增长.从时域演化来看,比起其他模态,主导模态的发展对上游激励的依赖更大.无论时域还是空域演化,都存在模态竞争现象.      

基于自相关及相位差法的高精度频率估计算法

为提高高斯白噪声背景中正弦信号的频率估计精度,对基于自相关运算的频率估计算法的相关长度m进行了推导,得到了m的优化值与信噪比的关系式.当信噪比较高时,m的最优值为N/3(N为信号采样点数);信噪比较低时,m的最优值为N/2.通过对自相关法及分段FFT(Fast Fourier Transforms)相位差法特性的分析,提出了一种性能更优的频率估计综合算法. Monte Carlo仿真实验表明:新算法吸收了两种算法的优点,克服了其不足,在更大的信噪比范围内具有较高的频率估计精度,且计算量也较小.      

Modeling and Optimal Design of 3 Degrees of Freedom Helmholtz Resonator in Hydraulic System

Three degrees of freedom (3-DOF) Helmholtz resonator which consists of three cylindrical necks and cavities connected in series (neck-cavity-neck-cavity-neck-cavity) is suitable to reduce flow pulsation in hydraulic system. A novel lumped parameter model (LPM) of 3-DOF Helmholtz resonator in hydraulic system is developed which considers the viscous friction loss of hydraulic fluid in the necks. Applying the Newton’s second law of motion to the equivalent mechanical model of the resonator, closed-form expression of transmission loss and resonance frequency is presented. Based on the LPM, an optimal design method which employs rotate vector optimization method (RVOM) is proposed. The purpose of the optimal design is to search the resonator’s unknown parameters so that its resonance frequencies can coincide with the pump-induced flow pulsation harmonics respectively. The optimal design method is realized to design 3-DOF Helmholtz resonator for a certain type of aviation piston pump hydraulic system. The optimization result shows the feasibility of this method, and the simulation under optimum parameters reveals that the LPM can get the same precision as transfer matrix method (TMM).     

改进 K-均值算法在雷达辐射源信号预分选中的应用

雷达辐射源信号分选是电子对抗领域一个关键技术,随着电子技术的发展,电磁环境日趋复杂,信号分选的难度越来越大,在这样的条件下,我们应该寻求新的解决问题的方法.本文首先概述了雷达辐射源信号分选意义、系统组成和分选流程,然后介绍了改进 K-均值算法.为了有效实现信号分选,提出了基于改进K-均值算法的信号分选方法,该方法可对到达角、载频和脉宽参数进行分选.最后进行了仿真实验,结果表明该方法实现简单,分选效果较好     

气膜约束对轴系固有频率影响的试验

对10kg转子进行自由状态和有气膜约束两种状态下的试验模态分析,试验结果给出了转子平动、锥动和一阶弯曲时的固有频率及其振型等模态参数,分析了轴承供气压力变化对轴系固有频率的影响.进行了轴系升降速试验,结果表明:气膜约束下的模态分析结果比自由状态下的更接近于升降速试验的结果.试验结果为气体轴系固有频率的确定以及控制参数的选取提供了依据,也为下一步轴系振动控制奠定了基础.      

Ionospheric disturbances on 8 September, 2010: Was it connected with the incoming moderate Chongqing earthquake?

Earthquake prediction stimulates the searches for a correlation between seismic activity and ionospheric anomalies. Contrary to common focuses on strong earthquakes, we report the ionospheric disturbances, 2 days before a moderate Ms = 4.7 Chongqing earthquake (29.4°N, 105.5°E, depth = 7.0 km, occurred at 21:21 LT, 10 September, 2010) with the data of ground-based ionosondes and IGS receivers. The data covering the period under the quiet geomagnetic conditions and a geomagnetic storm was analyzed with upper and lower bounds. It is found that there were significant enhancements of foF2 and total electron content (TEC) on the afternoon of 8 September, 2010, with a limited area close to the epicentre, which was different from the feature of ionospheric perturbations triggered by the geomagnetic storm on 15 September. Taking into account the heliogeomagnetical condition, we conclude that the observed ionospheric enhancements were very likely associated with the forthcoming moderate Chongqing earthquake, which implies that the relationship between the amplitudes of ionospheric disturbances and earthquakes is very complicated.     

射频干扰对InSAR干涉处理的影响分析

窄带干扰是低频段InSAR系统面临的主要射频干扰之一,它的存在会对干涉相位产生严重影响,进而导致高程反演误差。本文推导了窄带干扰影响下的InSAR干涉相位的解析表达式,给出了干涉相位对窄带干扰各参数的敏感度方程,仿真分析了窄带干扰对P波段InSAR系统干涉处理的影响。结果表明,窄带干扰经过成像后其幅度调制函数近似为方位时间的sinc函数,在SAR图像中表现为沿距离向的干扰条带。干涉相位对窄带干扰信号的功率较敏感,对其频率较不敏感,干涉相位和高程反演误差随着干信比的增加而增加。     

一种航天器单脉冲差模角跟踪系统的标定及测试方法

介绍了航天器单脉冲差模角跟踪系统的组成和基本原理;针对捕获跟踪系统性能与射频通道相位特性的相关性,提出了在无线状态下通过用户目标在一定范围内按一定轨迹运动进行射频通道相位标定及跟踪性能测试的方法;在此基础上进一步提出了利用信号源模拟用户目标在角跟踪系统天线坐标系下不同位置时天线输出的射频信号,进行有线状态下射频通道性能的测试方法。标定及测试方法通过地面试验验证了可行性和有效性,可用于航天器单脉冲差模角跟踪系统在地面的通道相位标定、跟踪性能测试和射频通道性能测试,以实现角跟踪系统对用户目标高精度、稳定的跟踪。