等离子体对GPS信号载噪比的影响

飞行器再入过程产生等离子体,等离子体中电子密度的变化对电磁波信号造成幅度衰减和相位偏移,严重时导致通信中断。理论分析了等离子体对GPS信号载噪比的影响;利用实验室环境下的等离子体模拟装置,设计了GPS二键相控(BPSK)调制信号透过等离子体的地面模拟方案,进行了等离子体对GPS接收机性能影响的实验研究,获得了不同电子密度下的衰减和导航信号载噪比的数据。实验结果与理论分析一致,不仅证明了等离子体下衰减与导航信号载噪比的变化是一致的结论,而且验证了实验方法的有效性。     

压力对Fe_（16）N_2结构和磁性的影响

用基于密度泛函理论的第一性原理方法,系统地研究了压力对Fe16N2体系结构和磁性的影响。结果表明仅仅离N位最远的Fe Ⅲ拥有一个约为2.84μB的大磁矩,而FeⅠ和Fe Ⅱ原子没有大磁矩产生。此外,还发现从0到47 GPa,总磁矩和单胞体积随着压力的增加单调减小,然而,在48 GPa的压力作用下,伴随着单胞体积的剧减,体系总磁矩消失。     

织构对TC1棒材动态冲击力学行为的影响

挤压成形是TC1棒材成形的主要手段。在挤压过程中,钛合金往往会形成织构,从而导致棒材的性能变化。本研究通过对比分析,研究了织构对动态冲击力学行为的影响。结果表明,在动态冲击过程中TC1合金出现了大量孪晶,且孪晶的数量随着冲击速度的提高而增加;0．4～0．7MPa气压气压范围内冲击时,具有强织构的冲击试样动态屈服强度变化不明显,而具有弱织构的冲击试样动态屈服强度由于位错强化有所增加,表现出了应变率正效应;当气压增高到0．9MPa,由于更多孪晶的开动,具有强、弱织构的样品都出现负的应变速率效应。     

S频段小数分频锁相环频率合成器实现与应用

传统的整数分频锁相环频率合成技术无法在单个环路实现高频率、低分辨率和低相噪的目标,小数分频锁相环在提高鉴相频率的同时减小分频计数值,从而降低相位噪声。针对USB统一测控系统的需要,本文提出基于单片小数分频锁相环的微波频率合成方法。实验结果表明,小数分频锁相环频率合成器具备良好的信号输出特性,可以为测控系统提供低成本频率合成方案。     

Effects of simulated weightlessness on the intestinal mucosal barrier of rats

This study employed a rat tail-suspension model to investigate the effects of simulated weightlessness on the intestinal mucosal barrier. Twenty-four Wistar rats were randomly divided into control (CON), 14-day tail-suspension (SUS-14d), and 21-day tail-suspension (SUS-21d) groups (n = 8 per group). Expression of occludin and zonula occludins-1 (ZO-1), proteins of the tight junction (TJ), in the intestinal mucosa was measured by immunohistochemical analysis, Western blotting, and mRNA fluorescent quantitation PCR. Plasma concentrations of diamine oxidase (DAO) and d-lactate were determined using an enzymatic spectrophotometric assay. Expression of occludin and ZO-1 was reduced in the SUS-14d and SUS-21d groups as compared to the CON group, with lowest expression observed in the SUS-21d group (P < 0.01). Examination by transmission electron microscopy (TEM) of the jejunal epithelium revealed increased intercellular space, decreased TJ and desmosome densities, and destruction of microvilli in the SUS-14d and SUS-21d groups. Plasma DAO and d-lactate concentrations in the SUS-21d group were higher than those in SUS-14d group and significantly higher than those in the CON group (P < 0.01). In all three groups, the expression of occludin and ZO-1 was found to correlate negatively with DAO (P < 0.01) and d-lactate (P < 0.01) concentrations. It is concluded that simulated weightless results in down-regulation of expression of TJ proteins in the rat intestinal mucosa. Simulated weightlessness is proposed to increase intestinal permeability through damage to the TJ.     

Debris Discs Around Stars: The 2004 ISO Legacy

Debris discs around stars were first discovered by the Infrared Astronomical Satellite (IRAS) in 1983. For the first time material orbiting another star than the Sun, but distinct from a circumstellar envelope, was observed through its far infrared emission. This major discovery motivated astronomers to investigate those discs by further analyzing the IRAS data, using ground-based telescopes for the hunting of exoplanets, developing several projects using the Infrared Space Observatory (ISO), and now exploiting the ISO Data Archive (IDA). This review presents the main ISO results, statistical as well as individual, on debris discs in orbit around pre-main-sequence and main-sequence stars. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

Thermoelectric Properties of Mg2Si1-xSnx Synthesized by Bulk Mechanical Alloying

Bulk mechanical alloying （BMA） has been successfully applied to solid-state synthesis of p-type and n-type thermoelectric materials Mg2Si1-xSnx （x = 0, 0.2, 0.4, 0.6, 0.8, 1） from element-powders at the room temperature in a relatively short time. The electrical conductivity, the Seebeck coefficient and the thermal conductivity of the Mg2Si1-xSnx are quite sensitive to the x-content. With the x-content rising, the electrical conductivity increases. When x = 0.6, it reaches the lowest and Mg2Si shows an n-type of semi-conducting However, when x = 0.2 and T 〉525 K, the Seebeck coefficient of the samples will change the opposite way. While x≥0.4, the samples present a p-type of semi-conducting. The figure of merit, Z of Mg2Si1-xSnx will be obtained in the range from 300 K to 700 K. When x = 0.6, Z proves to be higher than that of other samples at 300 K≤ T≤650 K.     

CBERS-1卫星红外多光谱扫描仪主光学装置设计

文章介绍了CBERS -1卫星IRMSS主光学装置的设计 ,叙述了光学系统和结构选择、装配调试、像质的稳定性、可靠性问题。该装置在焦面Φ8mm范围内弥散园优于Φ0.010mm ,调制传递函数MTF优于0.91。     

两种信道编码记录方式在光盘应用中的对比

依据数字编码理论对几种光盘编码调制的结构参数性能进行了比较，着重分析了光盘的两种信道编码记录方式：用于磁光（ＭａｇｎｅｔｏＯｐｔｉｃａｌｄｉｓｋ－ＭＯ）光盘的游程长度受限（ＲｕｎＬｅｎｇｔｈＬｉｍｉｍｔｅｄ）脉宽调制（ＰｕｌｓｅＷｉｄｔｈＭｏｄｕｌａｔｉｏｎ）［ＲＬＬ（１，７）－ＰＷＭ］记录方式，及用于相变（ＰｈａｓｅＣｈａｎｇｅ－ＰＣ）光盘的游程长度受限（ＲｕｎＬｅｎｇｔｈＬｉｍｉｍｔｅｄ）脉位调制（ＰｕｌｓｅＰｏｓｉｔｉｏｎＭｏｄｕｌａｔｉｏｎ）［ＲＬＬ（２，７）－ＰＰＭ］记录方式，并结合记录介质的特点，阐明了两者并存的原因．     

基于DTW的HJ-1B卫星地表温度产品质量检验

HJ-1B卫星热红外数据应用广泛,但其地表温度反演产品的质量检验工作尚不完善。以黑河流域为研究区,利用普适性单通道算法得到HJ 1B地表温度,基于7个地面站点（下垫面为荒漠、沙漠、植被、农作物、雪地和湿地）同步观测资料和MODIS地表温度产品（MOD11A1）,引入动态时间规整方法（DTW）对站点处HJ 1B地表温度进行验证。试验结果表明：HJ 1B地表温度反演产品与地面观测值的偏差值在沙漠和荒漠站点为1K以内,均方根误差在05K左右;对于植被和农作物站点的偏差在2K以内,均方根误差为1~2K;基于DTW的验证对时序不匹配的数据评价结果与现有指标表现一致。HJ-1B地表温度反演产品与地面站点的相对偏差均低于其与MODIS地表温度反演产品的相对偏差。