New results on planetary lightning

We present the latest observations from spacecraft and ground-based instruments in search for lightning activity in the atmospheres of planets in the solar system, and put them in context of previous research. Since the comprehensive book on planetary atmospheric electricity compiled by Leblanc et al. (2008), advances in remote sensing technology and telescopic optics enable detection of additional and new electromagnetic and optical emissions, respectively. Orbiting spacecraft such as Mars Express, Venus Express and Cassini yield new results, and we highlight the giant storm on Saturn of 2010/2011 that was probably the single most powerful thunderstorm ever observed in the solar system. We also describe theoretical models, laboratory spark experiments simulating conditions in planetary mixtures and map open issues.     

Effects of eccentricity of the reference orbit on multi-tethered satellite formations

The paper discusses the relevance of eccentric reference orbits on the dynamics of a tethered formation, when a massive cable model is included in the analysis of a multi-tethered satellite formation. The formations examined in this study are hub-and-spoke (HAS) and closed-hub-and-spoke (CHAS) configurations for in-plane and Earth-facing spin planes. Stability of the formations is studied by means of numerical simulation, together with the evaluation of the effects of eccentricity on tether elongation, agents relative position, and formation orientation and shape.     

Robust stationkeeping and reconfiguration of underactuated spacecraft formations

Feasibility of achieving reliable control for spacecraft formation keeping and reconfiguration without the need for thrust in either the radial or along-track direction is explored in this paper. Analysis of the linearized dynamics without along-track input indicates the presence of an uncontrollable eigenvalue at the origin. A nonlinear controller is designed to indirectly stabilize the uncontrollable modes to a stable manifold around the equilibrium. Conditions for robustness against unmatched uncertainties and disturbances are derived to establish the regions of asymptotic stabilization. The benefits of the proposed control method are also validated via numerical simulations to show that precise formation maintenance can be achieved by dealing with the issues of system nonlinearities, variations in initial conditions, and external disturbances, concurrently.     

Effect of Electromagnetic Stirring on the Quality of K417 Superalloy Ingots

The effect of electromagnetic stirring on the inner quality of K417 superalloy ingots is studied with EPMA and optical microscope.The results show that while an electromagnetic stirring with 50Hz frequency and 80A current is imposed,the equiax crystals of K417 superalloy ingots can be effectively refined and increased,and the central shrinkage porosity and the dendritic segregation of K417 superalloy ingots are greatly reduced,so the inner quality of K417 superalloy ingots is obviously improved.     

基于MPPSO的线性分布细金属柱体成像研究

基于线性分布细金属柱体的散射模型,用多极子展开求解电磁散射问题。以测量的散射场和计算散射场偏差为目标函数,将待优化变量设置为描述细金属柱体中心位置的向径和幅角,通过多相粒子群算法（MPP-SO）优化待优化变量,通过使目标函数达到最小值实现自由空间中线性分布的细金属柱体族的电磁成像。算例表明：MPPSO有较好的收敛性能和成像精度,以及较强的抗随机噪声干扰能力。     

Enhanced electromagnetic-interference shielding effectiveness and mechanical strength of Co-Ni coated aramid-carbon blended fabric

An efficient method was proposed to prepare high-performance conductive Aramid-Carbon Blend Fabrics (ACBF) with cobalt–nickel (Co-Ni) alloy coatings, which is conducive to industrial production. The grid-like substrate composed of aramid and carbon fibers was innovatively used in flexible Electromagnetic Interference (EMI) shielding materials. The natural network structure is advantageous to the uniform deposition of metal particles to the establishment of conductive pathways subsequently in order to improve conductivity. The induction of a synergistic effect from Electromagnetic (EM) wave-reflection and EM wave-absorption through the whole carbon-Co-Ni-ternary system notably enhanced the EMI Shielding Effectiveness (SE) value to an average of 42.57 dB in the range of 30–6000 MHz. On the other hand, together with the inherent toughness of the alloy coatings, the tensile strength of the aramid fibers used for bulletproof made a significant contribution to the desired mechanical properties. The light weight of the resultant composite made it applicable to aerospace vehicles simultaneously. X-ray Photoelectron Spectroscopy (XPS) was conducted to investigate the variations of elements and groups on the sample surface in pre-treating process. The elemental components and surface morphologies of fabric samples during different stages of the process were investigated by Scanning Electron Microscope (SEM) and Energy Dispersive spectrometer (EDX) measurements. X-Ray Diffraction (XRD) results indicated that the obtained Co-Ni alloy coating had a combined Hexagonal Closed-Packed (HCP) and Faced-Centered Cubic (FCC) crystalline phase. The relatively high corrosion resistance demonstrated in different acid and alkaline conditions was instrumental in more complex environments as well.     

Influence of sensor and actuator errors on impulsive satellite formation control methods

This paper investigates how sensor and actuator errors are impacting formation control accuracy and propellant consumption of a two-satellite formation in a low Earth orbit. Realistic relative navigation errors are implemented, based on the results from the PRISMA mission, as well as realistic actuator uncertainty and actuator constraints. Two impulsive control methods are investigated. The first method is based on a controller that is implemented onboard PRISMA and the second method uses linear programming to arrive at a model predictive controller. The control methods are tested in a simulation environment and are subjected to orbital perturbations and realistic sensor errors and actuator errors. Both control methods are able to maintain the desired relative geometry of a projected circular orbit in the presence of the errors. The PRISMA control method demonstrates lower propellant consumption, while the model predictive controller shows better control accuracy. The results show that, based on the used scenario, sensor errors dominate both the formation control accuracy and propellant consumption. The versatility of the model predictive controller is demonstrated in a challenging formation control scenario including formation maintenance and formation reconfiguration tasks.     

分离模块异构航天器集群松散队形设计

针对近地轨道分离模块航天器集群,考虑航天器之间面质比差异,提出一种能在较长时间内维持两星之间相对距离的松散队形设计方法。该方法以相对偏心率向量和相对轨道倾角向量描述松散队形,以两颗卫星之间的轨道长半轴之差和相对相位角之差为优化变量,并且通过设置虚拟空间圆解决避碰问题,保证在考虑气动力摄动的情况下,异构航天器松散编队能够有较长时间满足距离要求。仿真研究表明,针对轨道高度为500 km的参考轨道,当两星面质比分别为0.003 m 2/kg和0.0032 m 2/kg时,两颗卫星之间的距离能够在50天内处于200 m到20 km之间。     

控制受限的编队航天器鲁棒自适应控制

研究了考虑控制受限的编队航天器鲁棒自适应轨道跟踪控制问题。针对航天器编队飞行系统中控制受限、外部扰动和模型不确定性的情况,利用反步控制方法和指令滤波设计提出了一种鲁棒自适应控制策略。指令滤波器用于补偿控制受限对于控制器的影响,同时设计了自适应律对未知参数进行估计,并且利用Lyapunov稳定性理论分析了闭环系统的渐近稳定性。和滑模控制等传统鲁棒控制不同,所设计的鲁棒自适应控制器是连续的,更便于航天器编队飞行系统的实现。仿真结果表明所设计的控制器既能实现高精度的编队飞行跟踪控制,又能保证控制受限的要求。     

卫星群反作用飞轮的故障诊断方法研究

卫星群飞行技术是航天领域的新兴技术,多个低成本小卫星可以完成较为艰巨的太空任务,但与此同时也会带来更多的故障问题。面对复杂的空间环境,针对卫星群可能发生的故障问题,提出了一种分布式卫星群系统故障诊断方案,通过基于Elman神经网络的故障诊断方法完成对卫星群姿态控制系统的故障诊断。试验结果表明,该方案能够快速检测出卫星群中反作用飞轮发生的故障,并且让邻近卫星通过神经网络也能检测到该故障的发生,表现出很好的准确性和实时性,在实际应用中有效可行。