Far ultraviolet imaging from the IMAGE spacecraft. 3. Spectral imaging of Lyman-α and OI 135.6 nm

Two FUV Spectral imaging instruments, the Spectrographic Imager (SI) and the Geocorona Photometer (GEO) provide IMAGE with simultaneous global maps of the hydrogen (121.8 nm) and oxygen 135.6 nm components of the terrestrial aurora and with observations of the three dimensional distribution of neutral hydrogen in the magnetosphere (121.6 nm). The SI is a novel instrument type, in which spectral separation and imaging functions are independent of each other. In this instrument, two-dimensional images are produced on two detectors, and the images are spectrally filtered by a spectrograph part of the instrument. One of the two detectors images the Doppler-shifted Lyman- while rejecting the geocoronal `cold Ly-, and another detector images the OI 135.6 nm emission. The spectrograph is an all-reflective Wadsworth configuration in which a grill arrangement is used to block most of the cold, un-Doppler-shifted geocoronal emission at 121.567 nm. The SI calibration established that the upper limit of transmission at cold geocoronal Ly- is less than 2%. The measured light collecting efficiency was 0.01 and 0.008 cm² at 121.8 and at 135.6 nm, respectively. This is consistent with the size of the input aperture, the optical transmission, and the photocathode efficiency. The expected sensitivity is 1.8×10^–2 and 1.3×10^–2 counts per Rayleigh per pixel for each 5 s viewing exposure per satellite revolution (120 s). The measured spatial resolution is better than the 128×128 pixel matrix over the 15°×15° field of view in both wavelength channels. The SI detectors are photon counting devices using the cross delay line principle. In each detector a triple stack microchannel plate (MCP) amplifies the photo-electronic charge which is then deposited on a specially configured anode array. The position of the photon event is measured by digitizing the time delay between the pulses detected at each end of the anode structures. This scheme is intrinsically faster than systems that use charge division and it has a further advantage that it saturates more gradually at high count rates. The geocoronal Ly- is measured by a three-channel photometer system (GEO) which is a separate instrument. Each photometer has a built in MgF₂ lens to restrict the field of view to one degree and a ceramic electron multiplier with a KBr photocathode. One of the tubes is pointing radially outward perpendicular to the axis of satellite rotation. The optic of the other two subtend 60° with the rotation axis. These instruments take data continuously at 3 samples per second and rely on the combination of satellite rotation and orbital motion to scan the hydrogen cloud surrounding the earth. The detective efficiencies (effective quantum efficiency including windows) of the three tubes at Ly- are between 6 and 10%.     

民用飞机静气动弹性载荷计算方法研究

以某大型客机为例,根据气动外形利用有限元软件PANTRAN建立高阶气动面元,利用ZONAIR静气动弹性方法,分析了其弹性载荷,包含气动力系数、过载、迎角,并与刚性载荷进行了对比。此方法合理可靠,可用于民用飞机的弹性载荷设计分析。     

一种基于六自由度并联机构的动基座模拟器

介绍了一种基于六自由度并联机构的海浪模拟器。该模拟器系统采用 6UPS并联机器人作为机械台体 ,由伺服电机驱动 ,具有六个运动自由度 ,可以在一定程度上模拟舰艇水面水下运动情况 ,从而为各类舰艇导航系统和瞄准设备的研发提供支撑载体。实验和使用表明系统控制精度高、性能良好 ,安全稳定     

The Radio Plasma Imager investigation on the IMAGE spacecraft

Radio plasma imaging uses total reflection of electromagnetic waves from plasmas whose plasma frequencies equal the radio sounding frequency and whose electron density gradients are parallel to the wave normals. The Radio Plasma Imager (RPI) has two orthogonal 500-m long dipole antennas in the spin plane for near omni-directional transmission. The third antenna is a 20-m dipole along the spin axis. Echoes from the magnetopause, plasmasphere and cusp will be received with the three orthogonal antennas, allowing the determination of their angle-of-arrival. Thus it will be possible to create image fragments of the reflecting density structures. The instrument can execute a large variety of programmable measuring options at frequencies between 3 kHz and 3 MHz. Tuning of the transmit antennas provides optimum power transfer from the 10 W transmitter to the antennas. The instrument can operate in three active sounding modes: (1) remote sounding to probe magnetospheric boundaries, (2) local (relaxation) sounding to probe the local plasma frequency and scalar magnetic field, and (3) whistler stimulation sounding. In addition, there is a passive mode to record natural emissions, and to determine the local electron density, the scalar magnetic field, and temperature by using a thermal noise spectroscopy technique.     

橡胶O形圈密封结构的有限元分析

建立固体火箭发动机的橡胶O形圈的二维轴对称模型,采用非线性有限元方法计算了O形橡胶圈在库存和工作状态的变形及应力,分析了上下法兰张开间隙、初始压缩量、密封槽口及槽底倒角半径、密封槽宽、密封圈材料、O形圈截面尺寸及工作温度等典型参数对密封性能的影响:上下法兰张开间隙、密封圈的初始压缩率、密封圈材料等典型参数对最大接触压应力影响较大,而密封槽槽口和槽底处倒角对剪切应力影响明显.这些分析为密封结构的优化设计打下了基础;同时探讨O形圈应力分布规律,确定密封圈易受损和失效关键部位,初步确立了固体火箭发动机密封失效准则和失效模式.     

Performance Degradation Due to Specular Multipath Intersymbol Interference

Plots of performance degradation are used to compare the effects of fading and intersymbol interference in a two- component specular multipath digital communications channel. Similar plots are then used to compare two practical receivers designed to combat the interference. Degradation plots are shown to allow easy identification of each receiver's range of usefulness, as well as identification of variance bounds demanded of channel parameter estimates which the receivers require.     

On adaptive multiband signal detection with the SMI algorithm

In a nonstationary and/or nonhomogeneous interference environment, an adaptive system for target detection may suffer a severe performance degradation due to the lack of a sufficient amount of data from which the system can learn (estimate) the statistics of the environment. The detection performance of an adaptive system, which employs a frequency diversity (multiband) signaling waveform and a multiband sample matrix inversion algorithm (SMI), is analyzed. By comparison with the corresponding single-band system under the chosen system constraint, it is shown that the multiband system can significantly outperform the single band when the amount of data available from a single frequency band is severely limited by the environment     

The chemical profiles of polar ice on Earth and Mars: What we read from the first; what we could read from the second

An analogy is drawn between the current knowledge on terrestrial snow and ice-cap chemistry and the possible composition of snowfall and ice caps of Mars. Terrestrial snowfall reflects the composition of the Earth's atmosphere. Snow cover further interacts with the atmosphere and is the recipient of aerosol and particulate fall-out. The snow is transformed to firn and ice and the chemical signatures become locked into the perennial ice sheets. The chemical profiles of ice thus constitute environmental records of the Earth's past. By considering the present knowledge on the hydrologie cycle of Mars and the chemistry of the atmosphere, a simple analogous model for the chemical profile of the North polar ice cap is proposed. Three major constituents of the ice are discussed: water ice, dust, and occluded air bubbles. The seasonal fluctuations and interannual variability of these components are examined as possible chemical signatures for the dating of ice, elucidating hydrologie processes, and recording long-term climatic change. The model of the north polar cap in summer consists of water-ice fine-dust layers (30–200 m thick) sandwiched between annual dust layers of variable size distribution and thickness (< 1m– > 66 m). The water ice is subjected to metamorphism and grain growth. The interpretation of the physico-chemical profile could lead to increased knowledge on the recent climatic past (1,000–2,000 years), hydrologic reservoirs, and seasonal cycles in the atmospheric dynamics of the planet.     

实时微分控制的算法研究

实时微分控制是进行航天器轨道控制的主要方法。本文在费景高所提出的算法基础上 ,提出了一种有效的改进算法 ,扩大了算法的适用范围。文章对算法的稳定性进行了研究 ,并给出了此算法在弹道导弹显式制导中的应用实例。