A nonlinear equivalent circuit method for analysis of passive intermodulation of mesh reflectors

Passive intermodulation(PIM) has gradually become a serious electromagnetic interference due to the development of high-power and high-sensitivity RF/microwave communication systems, especially large deployable mesh reflector antennas. This paper proposes a field-circuit coupling method to analyze the PIM level of mesh reflectors. With the existence of many metal–metal(MM) contacts in mesh reflectors, the contact nonlinearity becomes the main reason for PIM generation. To analyze these potential PIM sources, an equivalent circuit model including nonlinear components is constructed to model a single MM contact so that the transient current through the MM contact point induced by incident electromagnetic waves can be calculated. Taking the electric current as a new electromagnetic wave source, the far-field scattering can be obtained by the use of electromagnetic numerical methods or the communication link method. Finally, a comparison between simulation and experimental results is illustrated to verify the validity of the proposed method.     

基于外辐射源直达波的对消及仿真研究

利用外辐射源（如调频广播、电视、GPS等的发射信号）的无源雷达探测技术一直是国内外研究的热点。在目标检测上,直达波信号的干扰不但对通道的动态范围提出了极高的要求,同时对微弱目标回波的检测构成了限制。文中利用二元阵的方法对直达波的干扰进行抑制,并对其对消性能进行了仿真分析和研究。     

Passive cooling of superconducting magnetic systems in space

The equilibrium temperature of a system in space can be lowered by a suitable choice of its geometry and its attitude. This remark is important for devices based on medium temperature and high temperature superconducting materials, and offers the possibility of their fully passive cooling without or with a marginal recourse to active systems. General parameterizations are given and simple schemes discussed. The adopted geometrical configuration and the attitude can enhance the role of passive cooling of the large superconducting magnetic systems required for protecting from ionizing radiation manned habitats in deep space. A specific example based on MgB₂ cable for protecting large volume habitats (500 and 1000 m³) is treated. The systems can be run in deep space at equilibrium temperatures around 20 K mainly by passive cooling, provided that their geometry and attitude would be suitably chosen.     

某型空气发生器二级导向器组合定位销脱落故障排除

某型空气发生器进行定检工作时,发现二级导向器组合定位销孔穿透裂纹、定位销脱落故障,本文对故障原因进行分析,并探讨了故障排除方法以及故障件修理注意事项。     

粗糙度对超高负荷低压涡轮边界层影响

基于IET-LPTA叶型研究了被动控制中的粗糙度对分离转捩的影响,以寻求一种优化的粗糙度布置方式来减小叶型损失。研究以CFX数值模拟为主,对数值结果进行试验验证。主要考察了不同粗糙高度及不同粗糙度布置位置对叶型损失的影响,提出了变粗糙高度的优化布置方案,并使用间歇因子、位移厚度、动量厚度、形状因子、湍流度等特征参数分析了粗糙度控制分离的机理。研究发现粗糙度对减小叶型损失,推迟开式分离泡的出现有显著作用,同时变粗糙高度结合前缘至转捩点的布置方式能够更好地抑制分离,减小叶型损失。     

改进的推广KALMAN滤波器在导弹被动制导中的应用

本文研究将改进的推广Ｋａｌｍａｎ滤波器应用于导弹被动制导问题。为了提高系统的可观性，我们采用一种双重指标来设计最优导引律。仿真结果表明，如何系统状态的均值较高，波动较小，那么ＭＥＫＦ的估计精度明显优于推广Ｋａｌｍａｎ滤波器。     

基于IMM理论的大型舰船被动定位算法研究

针对相控阵导引头跟踪航母等大型舰船目标的要求,为了给雷达成像进行运动补偿、给目标识别提供更多的先验信息,必须对目标运动参数进行准确的估计,但是常规被动定位算法通常采用单一目标运动模型,难以适应大型军舰运动规律的多样化,导致模型不匹配,使得误差较大,难以对目标参数进行准确估计,为解决上述问题,提出了一种基于交互式多模型(IMM)理论的大型舰船被动定位方法,使用不同的模型来匹配舰船不同的运动状态。仿真结果表明,提出的方法与常规的被动定位算法相比,定位精度明显提高,且易于工程实现。     

Design of passive fault-tolerant flight controller against actuator failures

The problem of designing passive fault-tolerant flight controller is addressed when the normal and faulty cases are prescribed. First of all, the considered fault and fault-free cases are formed by polytopes. As considering that the safety of a post-fault system is directly related to the maximum values of physical variables in the system, peak-to-peak gain is selected to represent the relationships among the amplitudes of actuator outputs, system outputs, and reference commands. Based on the parameter dependent Lyapunov and slack methods, the passive fault-tolerant flight controllers in the absence/presence of system uncertainty for actuator failure cases are designed, respectively. Case studies of an airplane under actuator failures are carried out to validate the effectiveness of the proposed approach     

Passive optical detection of submillimeter and millimeter size space debris in low Earth orbit

Understanding of the space debris environment and accuracy of its observation-validated models are essential for optimal design and safe operation of satellite systems. Existing ground-based optical telescopes and radars are not capable of observing debris smaller than several millimeters in size. A new experimental and instrumental approach – the space-based Local Orbital Debris Environment (LODE) detector – aims at in situ measuring of debris with sizes from 0.2–10 mm near the satellite orbit. The LODE concept relies on a passive optical photon-counting time-tagging imaging system detecting solar photons (in the visible spectral range) reflected by debris crossing the sensor field of view. In contrast, prior feasibility studies of space-based optical sensors considered frame detectors in the focal plane. The article describes the new experimental concept, discusses top-level system parameters and design tradeoffs, outlines an approach to identifying and extracting rare debris detection events from the background, and presents an example of performance characteristics of a LODE sensor with a 6-cm diameter aperture. The article concludes with a discussion of possible sensor applications on satellites.     

Bernoulli particle flter with observer altitude for maritime radiation source tracking in the presence of measurement uncertainty

For maritime radiation source target tracking in particular electronic counter measures（ECM）environment,there exists two main problems which can deteriorate the tracking performance of traditional approaches.The frst problem is the poor observability of the radiation source.The second one is the measurement uncertainty which includes the uncertainty of the target appearing/disappearing and the detection uncertainty（false and missed detections）.A novel approach is proposed in this paper for tracking maritime radiation source in the presence of measurement uncertainty.To solve the poor observability of maritime radiation source target,using the radiation source motion restriction,the observer altitude information is incorporated into the bearings-only tracking（BOT）method to obtain the unique target localization.Then the two uncertainties in the ECM environment are modeled by the random fnite set（RFS）theory and the Bernoulli fltering method with the observer altitude is adopted to solve the tracking problem of maritime radiation source in such context.Simulation experiments verify the validity of the proposed approach for tracking maritime radiation source,and also demonstrate the superiority of the method compared with the traditional integrated probabilistic data association（IPDA）method.The tracking performance under different conditions,particularly those involving different duration of radiation source opening and switching-off,indicates that the method to solve our problem is robust and effective.