Simulation-based evaluation of a cold atom interferometry gradiometer concept for gravity field recovery

The prospects of future satellite gravimetry missions to sustain a continuous and improved observation of the gravitational field have stimulated studies of new concepts of space inertial sensors with potentially improved precision and stability. This is in particular the case for cold-atom interferometry (CAI) gradiometry which is the object of this paper. The performance of a specific CAI gradiometer design is studied here in terms of quality of the recovered gravity field through a closed-loop numerical simulation of the measurement and processing workflow. First we show that mapping the time-variable field on a monthly basis would require a noise level below $5\text{mE}/\sqrt{\text{Hz}}$. The mission scenarios are therefore focused on the static field, like GOCE. Second, the stringent requirement on the angular velocity of a one-arm gradiometer, which must not exceed ${10}^{-6}$?rad/s, leads to two possible modes of operation of the CAI gradiometer: the nadir and the quasi-inertial mode. In the nadir mode, which corresponds to the usual Earth-pointing satellite attitude, only the gradient $V_{\mathit{yy}}$, along the cross-track direction, is measured. In the quasi-inertial mode, the satellite attitude is approximately constant in the inertial reference frame and the 3 diagonal gradients $V_{\mathit{xx}}\text{,}{V}_{\mathit{yy}}$ and $V_{\mathit{zz}}$ are measured. Both modes are successively simulated for a 239?km altitude orbit and the error on the recovered gravity models eventually compared to GOCE solutions. We conclude that for the specific CAI gradiometer design assumed in this paper, only the quasi-inertial mode scenario would be able to significantly outperform GOCE results at the cost of technically challenging requirements on the orbit and attitude control.     

Effects of space weather on the ionosphere and LEO satellites’ orbital trajectory in equatorial,low and middle latitude

We study the effects of space weather on the ionosphere and low Earth orbit (LEO) satellites’ orbital trajectory in equatorial, low- and mid-latitude (EQL, LLT and MLT) regions during (and around) the notable storms of October/November, 2003. We briefly review space weather effects on the thermosphere and ionosphere to demonstrate that such effects are also latitude-dependent and well established. Following the review we simulate the trend in variation of satellite’s orbital radius (r), mean height (h) and orbit decay rate (ODR) during 15 October–14 November 2003 in EQL, LLT and MLT. Nominal atmospheric drag on LEO satellite is usually enhanced by space weather or solar-induced variations in thermospheric temperature and density profile. To separate nominal orbit decay from solar-induced accelerated orbit decay, we compute $r\text{,}h$ and ODR in three regimes viz. (i) excluding solar indices (or effect), where $r=r_0\text{,}h=h_0$ and $\mathit{ODR}={\mathit{ODR}}_0$ (ii) with mean value of solar indices for the interval, where $r=r_m\text{,}h=h_m$ and $\mathit{ODR}={\mathit{ODR}}_m$ and (iii) with actual daily values of solar indices for the interval ($r\text{,}h$ and ODR). For a typical LEO satellite at h?=?450?km, we show that the total decay in r during the period is about 4.20?km, 3.90?km and 3.20?km in EQL, LLT and MLT respectively; the respective nominal decay ($r_0$) is 0.40?km, 0.34?km and 0.22?km, while solar-induced orbital decay ($r_m$) is about 3.80?km, 3.55?km and 2.95?km. h also varied in like manner. The respective nominal ${\mathit{ODR}}_0$ is about 13.5?m/day, 11.2?m/day and 7.2?m/day, while solar-induced ${\mathit{ODR}}_m$ is about 124.3?m/day, 116.9?m/day and 97.3?m/day. We also show that severe geomagnetic storms can increase ODR by up to 117% (from daily mean value). However, the extent of space weather effects on LEO Satellite’s trajectory significantly depends on the ballistic co-efficient and orbit of the satellite, and phase of solar cycles, intensity and duration of driving (or influencing) solar event.     

An estimation of Envisat’s rotational state accounting for the precession of its rotational axis caused by gravity-gradient torque

The rotational state of Envisat is re-estimated using the specular glint times in optical observation data obtained from 2013 to 2015. The model is simplified to a uniaxial symmetric model with the first order variation of its angular momentum subject to a gravity-gradient torque causing precession around the normal of the orbital plane. The sense of Envisat’s rotation can be derived from observational data, and is found to be opposite to the sense of its orbital motion. The rotational period is estimated to be $(120.674\pm 0.068)·\exp \left((4.5095\pm 0.0096)\times {10}^{-4}·t\right)\text{s}$, where t is measured in days from the beginning of 2013. The standard deviation is 0.760?s, making this the best fit obtained for Envisat in the literature to date. The results demonstrate that the angle between the angular momentum vector and the negative normal of the orbital plane librates around a mean value of $8.53°\pm 0.42°$ with an amplitude from about $0.7°$ (in 2013) to $0.5°$ (in 2015), with the libration period equal to the precession period of the angular momentum, from about 4.8?days (in 2013) to 3.4?days (in 2015). The ratio of the minimum to maximum principal moments of inertia is estimated to be $0.0818\pm 0.0011$, and the initial longitude of the angular momentum in the orbital coordinate system is $40.5°\pm 9.3°$. The direction of the rotation axis derived from our results at September 23, 2013, UTC 20:57 is similar to the results obtained from satellite laser ranging data but about $20°$ closer to the negative normal of the orbital plane.     

海洋环境下三电极的电偶腐蚀仿真

为研究三电极的电偶腐蚀行为,测量了CF8611/AC531复合材料（CFRP）、7B04-T74铝合金（7B04）和镀锌30CrMnSiA钢（GSB）的极化曲线;开展了搭接件在模拟海洋环境下的全浸试验;设计了圆形三电极,推导了稳态腐蚀场和参数化扫描方程,建立了三电极和搭接件的电偶腐蚀模型。结果表明:稳态腐蚀场中的电势分布符合Laplace方程;电位最高的CFRP为阴极,最低的GSB为阳极,中间的7B04阴/阳极角色会随某一电极面积变化而转变,给出了转变的临界面积比,各电极表面电偶电流服从指数分布,相关系数近于1,拟合精度高;在搭接件中,搭接区电位和电流密度最高,并向两端对称递减,7B04和GSB均为阳极,电流密度分别提高约210倍和328倍,电偶腐蚀效应显著;搭接区7B04板全面腐蚀,厚度损失约1.011%;仿真所得点蚀敏感区宽度范围为3.9~7.6mm,实测所得宽度范围为4.667~8.872mm,二者范围、形状及变化规律吻合较好,表明模型有效、可靠。      

基于MIMO技术的卫星高速数据传输系统研究

多输入多输出（MIMO）系统作为提高频谱效率的一种有效方式,已经成为未来无线通信系统实现高速可靠数据传输的突破性技术之一,受到国际众多研究机构的普遍关注。文章通过研究MIMO信道模型、信道容量、卫星信道特征,构建了全新高速数传系统模型,解决了卫星大容量数据实时传输和频带限制之间的矛盾。     

复杂电磁环境下雷达目标距离像识别效果评估

分析了战场电磁环境复杂性及其对雷达目标高分辨距离像特征提取和目标识别影响,分别以距离像失真和识别率为度量标准,通过对比特征提取和目标识别效果,提出了一种复杂电磁环境下雷达目标距离像识别效能评估方法,并进行了仿真计算。     

月球表面太空风化作用及其效应

太空风化作用普遍发生在月球、小行星等一些无大气层的天体表面, 主要包括太阳风离子辐射和微陨石撞击等作用. 通过对真实月球样品太空风化作用的研究成果以及低能离子和激光照射等地面模拟实验结果的系统综述和分析, 重点分析了月壤颗粒中非晶质层和纳米铁的成因, 指出太空风化研究中存在的太阳风离子辐射和微陨石撞击贡献难以区分及模拟实验模拟效果尚不充分的问题.进而提出月球太空风化研究不能简单套用其他天体的模型, 并建议未来开展更多月球和陨石样品的精细化学分析.      

考虑围压效应的N15固体推进剂本构模型

利用自制围压试验系统,完成了不同围压下N15固体推进剂的力学性能测试,结果显示:围压对推进剂初始模量影响不大,但最大抗拉强度、断裂强度、最大伸长率和断裂伸长率均随着围压的增大而增大;结合推进剂细观结构及断面分析,初步揭示了围压对推进剂力学性能的影响机理,由于围压的存在,推进剂内部颗粒脱湿度降低,孔洞由于受到压迫,其形成扩展过程延缓,裂纹萌发扩展被抑制,从细观结构上证明了围压对推进剂具有强化效果;根据弹性-黏弹性对应原理建立了考虑围压效应的N15推进剂本构模型,结果表明所建立的本构模型准确度较好,能够准确描述围压对N15推进剂力学性能的影响。