一种新型多普勒噪声抑制技术对BepiColombo任务无线电科学实验的性能提升

在欧洲空间局和日本宇宙开发机构联合开展的BepiColombo水星任务中,将开展水星轨道器无线电科学实验,包括估计水星的引力场及其旋转状态,并对广义相对论进行验证。目前地面系统和星上设备的主流配置可以在无线电科学实验中建立X/X、X/Ka和Ka/Ka多个频段的链路,测速精度可达3 um/s（1 000 s积分）,测距精度为20 cm。提出了基于时延机械噪声对消技术提高无线电科学实验性能的方案。时延机械噪声对消技术需要处理在两个测站不同时刻的测量数据,一个测站实施双向多普勒测距,对另一个单收测站的要求较为严格,该测站需要具有较好的对流层条件。这种方法能够显著降低Ka频段双向链路的主要测量噪声,包括由对流层和天线机械系统震动引起的噪声。我们给出了端到端的仿真性能,并估计了在使用时延机械噪声对消技术前提下的水星引力场和旋转状态。考虑使用NASA位于美国本土戈尔德斯敦的DSS-25天线或欧空局位于阿根廷马拉圭的DSA-3天线作为双程测量站,并考虑使用位于智力的APEX天文观测天线作为单收站。分析结果表明在最好的噪声条件下,使用DSA-3天线作为双程测量站时,时延机械噪声对消技术可将待估计的全局和局部参数的估计精度提升一倍。对于无线电科学实验的目标,这一可能的性能提升对行星地质物理学很有意义,它将有益于研究水星内部的结构。     

Coronal Observations of CMEs

CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future.     

收敛形进气道数值研究的验证

介绍一种新型的、具有最小喉道面积的三维高超声速进气道 (称之为收敛形进气道 )的数值和实验研究结果。表明使用这种形式的进气道 ,在整个飞行速度范围内可以降低阻力和高超声速发动机表面的热防护要求 ,通过降低外压缩表面的倾斜度和减少进气道及燃烧室壁的面积就可以做到这一点。在采用低维次流动的气体动力设计方法的基础上设计成这种形式的进气道。计算是在无粘气体模型构架内用有限体积法进行的。同时用边界层方程计算出计及粘性的气流特性和进气道特性。数值算法是通过收敛形进气道的有限宽楔形外压缩表面的计算和实验数据来验证的。进行实验研究的马赫数M=2～ 1 0 7,基于模型进气道高度的雷诺数Re=( 1～ 5) × 1 0 6。数值计算与实验结果一致性很好。这些结果也和通常的二维进气道的数据作了比较。     

Ulysses observations of a coronal origin particle event at 32° south heliographic latitutde

A remarkable streaming beam-like particle event of 60 keV-5 MeV ions and of 38–315 keV electrons has been reported previously. This event has been associated with the passage of a Coronal Mass Ejection (CME) over the Ulysses spacecraft on June 9–13, 1993. At this time, the spacecraft was located at 4.6 AU from the sun and at an heliolatitude of 32° south. It was proposed (Armstrong et al., 1994) that the particle injection source could have been of coronal origin. In this study, we analyse the solar activity during this period. We identify a region of solar radio noise storms in the corona and in particular, a flare on June 7 that presents all the required characteristics to produce the hot plasma beam observed in the interplanetary medium.     

Rocket Exhaust Scattering Effects on Incident Microwave Propagation Characteristics

This paper considers the effect of the rocket exhaust turbulence and scattering within the surrounding medium upon the propagation characteristics of incident electromagnetic waves. The exhaust is represented by a cylindrical plasma beam, diffusing through the surrounding medium. The equations of propagation of EM waves are derived for both TE and TM modes. By using a quasi-linear perturbation technique the exhaust is further separated into an inner homogeneous cylindrical plasma beam, and an outer conical inhomogeneous turbulent region. The isotropic change in the temperature of the outer region and its effects on the fluctuations in the density of electrons, collision frequency, and plasma index of refraction are analyzed in detail. It is found that the exhaust turbulence and scattering effects produce linear fluctuations in the E and H fields computed from the exhaust inner region effect. The equations of this paper can be used in the prediction of the radar cross sections and the attenuation of microwaves by rocket exhaust plumes.     

Comments on `Detector relative efficiencies in the presence ofLaplace noise' by M.I. Dadi and J.R. Marks II

In the above-titled paper (see ibid., vol.AES-23, p.568-82, July 1987) M.I. Dadi and J.R. Marks II studied the relative efficiencies of the Neyman-Pearson optimal detector with respect to the linear and sign detectors, for the detection of a constant signal in additive Laplace noise. By applying the central limit theorem, they derived expressions for three types of asymptotic relative efficiencies (AREs). However, as noted in the above paper, the Gaussian approximation to the sign detector fails to yield the correct asymptotic efficiency. The commenter derives the correct ARE of the optimal detector with respect to the sign detector for the Laplace noise     

Device for mass measurement under zero-gravity conditions.

The problem considered in this paper is the investigation of the properties of a mass-meter, i.e. the device for determining the mass of cosmonaut's body under zero-gravity conditions. The estimates of accuracy of mass measurement by this device are given, and the results of measuring the masses of cosmonauts' bodies on the Salyut 5 and 6 orbital stations are presented.     

Computational Research of the Main Rotor Hover and Vertical Descent States Based on the Nonlinear Blade Vortex Model

The research is conducted of aerodynamic performance of the Mi-8 helicopter main rotor in the hover state and the vertical descent state, including vortex ring states (VRSs). Total and distributed aerodynamic performance, wake patterns, and main rotor flow patterns are calculated. The findings and results were compared with those provided by other researchers.     

Adaptive control and stabilization of elastic spacecraft

This work treats the question of large angle rotational maneuver and stabilization of an elastic spacecraft (spacecraft-beam-tip body configuration). It is assumed that the parameters of the system are completely unknown. An adaptive control law is derived for the rotational maneuver of the spacecraft. Using the adaptive controller, asymptotically decoupled control of the pitch angle of the space vehicle is accomplished, however this maneuver causes elastic deformation of the beam connecting the orbiter and tip body. For the stabilization of the zero dynamics (flexible dynamics), a stabilizer is designed using elastic mode velocity feedback. In the closed-loop system including the adaptive controller and the stabilizer, reference pitch angle trajectory tracking and vibration suppression are accomplished. Simulation results are presented to show the maneuver capability of the control system