Forest biomass monitoring with GNSS-R: Theoretical simulations

GNSS-Reflectometry (GNSS-R) is a remote sensing technique which performs bistatic measurements of the earth surface scattering. This paper presents some theoretical simulations of the specular scattering coefficient of a forested area, with the aim of demonstrating the potentiality of GNSS-R in monitoring forest biomass. The study is performed by means of an electromagnetic model developed in the past years and tested over several vegetation covered sites in its active and passive version. Here, after showing a comparison between model results and measurements over a forest site in the monostatic configuration, and after summarizing other previous validations, the extension to the specular configuration, typical of GNSS-R systems, will be presented. Namely, simulations are carried out at circular polarization and a sensitivity analysis of the received power in the specular configuration to some soil and forest parameters is shown.     

High-precision RCS measurement of aircraft’s weak scattering source

The radar cross section(RCS) of weak scattering source on the surface of an aircraft is usually less than 40 d Bsm.How to accurately measure the RCS characteristics of weak scattering source is a technical challenge for the aircraft's RCS measurement.This paper proposes separating and extracting the two-dimensional(2D) reflectivity distribution of the weak scattering source with the microwave imaging algorithm and spectral transform so as to enhance its measurement precision.Firstly,we performed the 2D microwave imaging of the target and then used the 2D gating function to separate and extract the reflectivity distribution of the weak scattering source.Secondly,we carried out the spectral transform of the reflectivity distribution and eventually obtained the RCS of the weak scattering source through calibration.The prototype experimental results and their analysis show that the measurement method is effective.The experiments on an aircraft's low-scattering conformal antenna verify that the measurement method can eliminate the clutter on the surface of aircraft.The precision of measuring a 40 d Bsm target is 3–5 d B better than the existing RCS measurement methods.The measurement method can more accurately obtain the weak scattering source's RCS characteristics.     

基于神经网络的翼盒结构响应重构方法应用

翼盒结构复杂，航行中承载条件恶劣，利用有限测点信息重构其它位置响应对于实时健康监测具有很强的现实意义。通过误差反向传播神经网络训练得到响应之间的非线性关系，建立基于神经网络的响应重构方法，开展有限元分析对其进行数值仿真验证，并将该方法应用于实测随机激励环境下翼盒典型承力结构的响应重构及损伤定位与判断分析。结果表明：采用该方法重构出的预测响应功率谱密度的均方根相对误差不超过1.90 dB，主要频点误差小于10%；判断出翼盒关键测点e 的损伤或故障发生在所截取片段数据3 s 后，其故障特征频率为240 Hz 左右，该方法应用于响应重构预示及健康监测分析具有可行性。     

螺旋形纳米碳纤维的电磁波吸收特性分析

根据手征材料的本构方程,讨论了螺旋形手征纳米气相生长碳纤维(VGCF)手征参数ξ与电磁参数ε,μ的关系,并分析螺旋形VGCF几何结构产生的手征特性,与电磁波相互作用时产生电磁交叉极化的耦合吸收电磁波的机理,研究了改善手征材料ξ和性能的方法。结果表明,取螺旋形VGCF半径与螺距比为0.23左右,增加手征体含量可改善手征介质的手征参数,手征体的含量适当、螺旋形VGCF的尺寸与电磁波的波长接近能提高材料的吸波能力。     

Gamma-hadron separation at high energies in view of recent cross section measurements

Recent measurements of the cross section for hadron photoproduction at HERA are reviewed and their importance for gamma ray astronomy is outlined. Various methods of distinguishing gamma and hadron primaries among cosmic rays in the air shower range are then discussed in view of our (insufficient) knowledge of the relevant hadron hadron cross sections. Theoretical model calculations indicate that the fraction of proton primaries mimicking gamma rays may be as high as one in a thousand.     

非圆截面弹体附加表面积工程估算方法研究

引入附加质量、附加表面积的概念 ,给出基于细长体理论的非圆截面弹体附加表面积的工程估算方法 ,并提出了计算带倒角圆的正方形和不同顶角菱形附加表面积的通用公式 ,通过计算值与理论值的比较 ,误差在 2 %以内。并通过利用附加表面积计算的弹体法向力系数与试验值进行比较 ,法向力系数计算误差在 10 %以内 ,说明用本方法计算的附加表面积可以在工程初步估算时应用。     

Reduction of satellite electromagnetic scattering by carbon nanostructured multilayers

We propose a low observable satellite covered by layered materials. The dielectric properties of the composite materials are analyzed using the coaxial air-line method. Our solution could result in cost-effective applications for satellite stealthness. The particle swarm optimization algorithm is used to design and optimize layered absorbers. The obtained reflection is below −20 dB in many frequency subranges, also under an oblique incident condition, within the band 2–18 GHz. The optimized layered absorbers are used as a cloak for cube satellites. In particular, we investigated a very simple geometry by using a finite element commercial software in the band from 2 to 18. Finally, the transmission through an aperture on the satellite surface is analyzed. It is highlighted that the optimized layered absorbers attenuate the electric field within the satellite, improving its immunity against electromagnetic interferences.     

国内对横电磁传输室的研究

主要论述如下几个问题:(1)我国学者对横电磁传输室理论的研究情况及成果;(2)对高阶模的模式判别、截止频率、谐振频率、最高可用频率问题的讨论;(3)关于我国学者提出的设计上的新思想和新方法;(4)对未来的展望及建议。对某些国内学术界尚有分歧的问题作了研讨,提出了横电磁传输室电压驻波比第一谐振峰频率与横向尺寸关系的经验曲线,讨论了与研制GTEM有关的问题。     

5A90铝锂合金型材折斜角成形性能的试验研究

型材一次折斜角成形是航空零件制造工艺的重要方法之一。本文通过新研制的5A90铝锂合金型材折斜角成形试验的研究，得到了极限开斜角和闭斜角值，为新机的设计及生产工艺提供了重要的指导依据。