Studies of polar mesosphere summer echoes with the EISCAT VHF and UHF radars: Information contained in the spectral shape

The nature of PMSE in the VHF and UHF frequency range is considered taking into account the shape of corresponding Doppler spectra. Assuming a turbulence-based model of PMSE it is argued that for cases where a VHF radar detects strong PMSE, the UHF radar could either detect enhanced coherent scattering caused by the same physical process as in the VHF (i.e., turbulence with large charged ice particles), there could be incoherent scattering modified by the charged ice particles, or there could be a mixture of both. In order to distinguish these cases a simple but robust method is introduced to characterize the shape of the Doppler spectra derived from observations at both frequencies. Spectral shapes are quantified with one simple fitting parameter of a generalized fit to the autocorrelation function (=Fourier transform of the Doppler spectrum). This parameter takes a value of 1 for a Lorentzian spectrum indicative of pure incoherent scatter from the D-region, a value of 2 for coherent scatter owing to turbulence, and a value of less than 1 for incoherent scatter modified by the presence of charged aerosol particles. This method is applicable to observations at altitudes between ∼70 and ∼90 km. Simultaneous observations with the EISCAT VHF and UHF radar are presented in which all three cases mentioned above are identified. For the case of incoherent scatter modified by the presence of charged aerosol particles we quantify the radius of the involved ice particles to exceed ∼5 nm. Most importantly, however, for the case where the UHF-signal exceeded the incoherent scatter signal significantly, the spectrum revealed a clear Gaussian shape indicative of a coherent scattering process with identical spectral width as for the VHF-observations. This finding gives strong support that both echoes are created by the same turbulence-based mechanism and not by different mechanisms as speculated by several previous authors.     

基于仿真的正交试验在对接锁系运动同步性研究中的应用

对接锁系运动同步性是空间对接机构完成刚性连接和分离的关键技术,通过采用 正交试验的方法合理设计仿真试验,高效找出影响对接锁系运动同步性的主要因素,明确了 提高锁系运动同步性的改进方向。     

KM6空间环境模拟设备运动模拟器内腔温度控制设计方案

文章在运动模拟器初始通风冷却温度控制方案的基础上, 提出了风扇换热器组件冷却方案, 并采用CFD模拟两种方案下模拟器内部流场和温度场分布。此外, 为了验证风扇换热器组件冷却方案的可行性, 采用遗传算法进行了三流程板翅式换热器的优化设计。计算表明风扇换热器组件冷却方案可行有效, 明显改善运动模拟器内部气体流动和换热。     

双臂自由飞行空间机器人捕捉目标实验研究

本文主要介绍了在地面上模拟的空间微重力环境下双臂自由飞行空间机器人捕捉目标的实验。首先介绍了双臂自由飞行空间机器人地面实验平台，该实验平台由机器人模型、视觉系统、无线通讯系统、网络系统、操作台、运动规划系统、气浮系统七个子模块构成。其次简单介绍了双臂自由飞行空间机器人捕捉目标策略，最后介绍了基于该实验平台的几种实验。     

伞舱系统在火星环境中的运动特性分析

建立了火星大气环境模型和伞舱系统多体动力学模型,通过仿真分析了火星环境下伞舱系统中进入舱的运动特性,并对其在火星环境和地球环境的运动稳定性进行仿真比较分析,得出的相关结论可为探测器火星登陆技术提供参考。     

Multi-beam sounding ionograms in the polar cap region: Absorption induced by proton precipitations

The simulation of the multi-beam ionograms in the polar cap region, assessing absorption effect is performed. It is reasonable to distinguish among four different mechanisms responsible for absorption: regular absorption due to solar UV illumination, absorption associated with energetic particles precipitation, absorption connected with X-rays flare and additional absorption in Auroral oval area. In this paper the absorption attributed to proton precipitations is envisaged. The computational model of the high-latitude ionosphere with irregularities oriented to application for the high frequency wave propagation problem was elaborated (Zaalov et al., 2005). A number of the quasi-vertical ionograms in the polar cap region were simulated on the basis of this model. A well-known algorithm (Sauer and Wilkinson, 2008) is applied for the absorption effects calculation. The simulated high-latitude ionograms with the absorption effect and the measured ionograms exhibit quite a good resemblance. This paper illustrates the importance of the understanding and taking into account the absorption effect in the presence of the various structural features in the polar ionosphere (in particular, patches of enhanced electron density) in interpreting ionosonde data.     

Motion-planning and pose-tracking based rendezvous and docking with a tumbling target

Rendezvous and docking (RVD) with a tumbling target is challenging. In this paper, a novel control scheme based on motion planning and pose (position and attitude) tracking is proposed to solve the pose control of a chaser docking with a tumbling target in the phase of close range rendezvous. Firstly, the current desired motion of the chaser is planned according to the motion of the target. In planning the desired motion, the “approach path constraint” is considered to avoid collisions between the chaser and the target, and the “field-of-view constraint” is considered to make sure the vision sensors on the chaser to obtain tight relative pose knowledge of the target with respect to the chaser. Then, the difference between the chaser’s motion and the desired motion is gradually reduced by a pose tracking controller. This controller is based on the non-singular terminal sliding mode (NTSM) method to make the tracking error converge to zero in finite time. Since the chaser nearly moves along the desired motion and the motion is reasonable, (1) it could safely arrive at the docking port of the target with a suitable relative attitude, (2) it will be always suitably oriented to observe the target well, and (3) the magnitude of the needed control inputs are less than that in existing literatures. The numerical results demonstrate the above three advantages of the proposed method.     

Polar caps and auroral zones under idealized axisymmetric magnetic fields

The geomagnetic field, modified by the solar wind, determines the shape, area and location of polar caps and auroral zones, among other magnetosphere and upper atmosphere characteristics. Since the field varies greatly with time it is of interest to analyze polar caps and auroral zones variations linked to magnetic field variations of intensity and pattern. Polar caps and auroral zones locations and areas for various single harmonic axial field configurations are obtained analytically assuming a simple magnetopause model. As the axial degree n increases, the polar caps and auroral zones total number, given by n + 1 and 2n respectively, also increase. However, their total areas decrease from a larger value in the case of an axial dipole to a minimum for an axial octupole (n = 3), and then increase for increasing degrees. The increasing rate is much higher in the auroral zones case to the point that it exceeds the dipolar value at n = 5 while in the polar caps case this occurs at n = 8. The absolute latitudes of the auroral zones and polar caps that reside around the geographical poles increase with axial degree. Our results represent an end-member case of the evolution of auroral zones and polar caps during polarity reversals if the transition involves axial dipole energy cascade to higher axial degrees. Evidence for such an energy transfer is found in the historical record of the geomagnetic secular variation.     

基于移动最小二乘法的机载SAR成像运动误差补偿方法

机载SAR平台在飞行过程中产生的运动误差会导致成像质量下降,利用测量数据提取运动误差是机载SAR运动补偿的重要手段之一。通过对机载SAR运动误差理论的推导,提出一种基于移动最小二乘法的机载SAR成像运动补偿方法,方法借助于惯性导航系统记录的东北天三维速度信息,对其进行处理,进而获得雷达平台的三维空间信息,再与后向投影算法结合,完成SAR成像运动补偿。方法是对最小二乘法进行改进,使其不需要对数据进行分段估计和平滑,还可以保证估计结果的正确性。实验结果验证方法对运动误差估计的准确性和有效性。