ANALYSIS OF CRITERIA PREDICTING THE TENDENCY OF PILOT INDUCED OSCILLATION FOR LINEAR NON-LINEAR SYSTEM

Some problems encountered in applying Smith's technique to predict the PIO tendency for non-linear pilot-vehicle loop, are thoroughly analyzed. Subsequently, modified PIO predictable criteria are developed, in addition, to make also a certain improvement on Smith's PIO definition and PIO types. These modified criteria are applied to predict PIO tendency of various different configurations on the variable stability aircraft NT-33 in case of supposed non-linearity, and predicted results are compared with the flight tests and analytical results in the case of linear hypothesis given in Ref. (4)     

用导航星全球定位系统(GPS)第二阶段用户设备进行时间发布

导航星全球定位系统是用卫星建立的一种导航系统。它采用频谱扩展技术使用户能得到高精度的全球定位、速度和时间信息。这是用测量一组四颗卫星信号到达时间差的办法得到的。GPS 与其协调世界时(UTC)有确定关系,因此用户能够把 UTC 估算到亚微秒准确度。系统的关键性能是,GPS 用户设备(UE)对于 UE 参考钟和 GPS 时之间时差的估算能力。这种估算可通过精密控制硬件误差和计算误差来完成。因此,要求用户设备(UE)采用一种低相位噪声和可预报漂移特性的高质量石英晶振。在活动设施上实现精密守时是 GPS 用户设备装置的基本工作原理。该装置及其接收机利用导航星 GPS 空间部分供给的信息,向用户提供导航情报、GPS 时和对应的 UTC 数据。讨论了机械编排 GPS 用户设备装置守时功能程序的复杂性。讨论内容包括硬件和软件补偿技术。本文强调了该装置的晶体振荡器频标的应用和效果。     

美空军考虑改进B-52H使之适应21世纪作战要求

美国军方目前仍把远程轰炸机列为其关键武器装备范畴之内。为此,空军正在考虑进一步改进其现有的B-52H轰炸机,使之继续服役至2030年。改型的重点将放在改善其常规作战能力上,以使它们成为具有高效双任务能力的远程轰炸机,驰骋在下一世纪的战空。空军现拥有一个有94架B-52H轰炸机的机队,打算对其中的一多半飞机进行这种增强常规作战能力的改型。改型的主要项目包括:增加挂载AGM-84“鱼叉”导弹的设备;增加一个综合外挂武器管理系统;一个多用途炸弹舱连接装置和全球定位导航系统。其中10架改进型还打算布     

用于航天器姿态确定的卡尔曼滤波

一、引言这篇报告回顾了过去二十年中卡尔曼滤波在姿态确定中的应用及发展,这一回顾不是面面俱到,而只限于适合于配有三轴陀螺仪及姿态传感器的航天器姿态的算法。我们认为这一种航天器系统是最适合于使用卡尔曼滤波的。卡尔曼滤波器使用了系统随时间变化的动态模型以及这样一种传感器测量模型,即基于当前以及以前的测量值利用线性估计器而获得系统状态的最高精度估计的测量模型。因此,从     

用声表面波传感器作惯性制导和水声检测

本文研究声表面波传感器在惯性制导和水声检测方面的应用。在水声检测中,由于声表面波石英的动态负载而不是静态负载导致声表面波振荡器的频谱信息包含有边带。研究已经证明,传感器样机的灵敏度(用信噪比表示)至少与普通水听器的相当。声表面波作加速度计可提供高准确度、大动态范围(10~6)和不用模-数转换就可获得数字输出。采用比较简单的数字处理技术,加速度计就能够提供惯性数据,例如速度和位移。对声表面波加速度计的总误差率的研究证明,它们的误差率始终小于每小时一海里。声表面波振荡器传感器在高性能、中等价格的传感器应用方面是一种很有希望的竞争者。     

Molecular diversity of cyanobacteria inhabiting coniform structures and surrounding mat in a Yellowstone hot spring

Lithified coniform structures are common within cyanobacterial mats in Yellowstone National Park hot springs. It is unknown whether these structures and the mats from which they develop are inhabited by the same cyanobacterial populations. Denaturing gradient gel electrophoresis and sequencing and phylogenetic analysis of 16S rDNA was used to determine whether (1) three different morphological types of lithified coniform structures are inhabited by different cyanobacterial species, (2) these species are partitioned along a vertical gradient of these structures, and (3) lithified and non-lithified sections of mat are inhabited by different cyanobacterial species. Our results, based on multiple samplings, indicate that the cyanobacterial community compositions in the three lithified morphological types were identical and lacked any vertical differentiation. However, lithified and non-lithified portions of the same mat were inhabited by distinct and different populations of cyanobacteria. Cyanobacteria inhabiting lithified structures included at least one undefined Oscillatorialean taxon, which may represent the dominant cyanobacteria genus in lithified coniform stromatolites, Phormidium, three Synechococcus-like species, and two unknown cyanobacterial taxa. In contrast, the surrounding mats contained four closely related Synechococcus-like species. Our results indicate that the distribution of lithified coniform stromatolites may be dependent on the presence of one or more microorganisms, which are phylogenetically different from those inhabiting surrounding non-lithified mats.     

Trojans in habitable zones

With the aid of numerical experiments we examined the dynamical stability of fictitious terrestrial planets in 1:1 mean motion resonance with Jovian-like planets of extrasolar planetary systems. In our stability study of the so-called "Trojan" planets in the habitable zone, we used the restricted three-body problem with different mass ratios of the primary bodies. The application of the three-body problem showed that even massive Trojan planets can be stable in the 1:1 mean motion resonance. From the 117 extrasolar planetary systems only 11 systems were found with one giant planet in the habitable zone. Out of this sample set we chose four planetary systems--HD17051, HD27442, HD28185, and HD108874--for further investigation. To study the orbital behavior of the stable zone in the different systems, we used direct numerical computations (Lie Integration Method) that allowed us to determine the escape times and the maximum eccentricity of the fictitious "Trojan planets."     

Beyond the principle of plentitude: a review of terrestrial planet habitability

We review recent work that directly or indirectly addresses the habitability of terrestrial (rocky) planets like the Earth. Habitability has been traditionally defined in terms of an orbital semimajor axis within a range known as the habitable zone, but it is also well known that the habitability of Earth is due to many other astrophysical, geological, and geochemical factors. We focus this review on (1) recent refinements to habitable zone calculations; (2) the formation and orbital stability of terrestrial planets; (3) the tempo and mode of geologic activity (e.g., plate tectonics) on terrestrial planets; (4) the delivery of water to terrestrial planets in the habitable zone; and (5) the acquisition and loss of terrestrial planet carbon and nitrogen, elements that constitute important atmospheric gases responsible for habitable conditions on Earth's surface as well as being the building blocks of the biosphere itself. Finally, we consider recent work on evidence for the earliest habitable environments and the appearance of life itself on our planet. Such evidence provides us with an important, if nominal, calibration point for our search for other habitable worlds.