The definition of life: a brief history of an elusive scientific endeavor

In spite of the spectacular developments in our understanding of the molecular basis that underlies biological phenomena, we still lack a generally agreed-upon definition of life, but this is not for want of trying. Life is an empirical concept; and, as suggested by the many unsuccessful efforts to define it, this task is likely to remain, at best, a work in progress. Although phenomenological characterizations of life are feasible, a precise definition of life remains an elusive intellectual endeavor. This is not surprising: as Nietszche once wrote, there are concepts that can be defined, whereas others only have a history. The purpose of this essay is to discuss some of the manifold (and often unsatisfactory) definitions of life that have been attempted from different intellectual and scientific perspectives and reflect, at least in part, the key role that historical frameworks play. Although some efforts have been more fruitful, the lack of an all-embracing, generally agreed-upon definition of life sometimes gives the impression that what is meant by life's origin is defined in somewhat imprecise terms and that several entirely different questions are often confused. The many attempts made to reduce the nature of living systems to a single living compound imply that life can be so well defined that the exact point at which it started can be established with the sudden appearance of the first replicating molecule. On the other hand, if the emergence of life is seen as the stepwise (but not necessarily slow) evolutionary transition between the non-living and the living, then it may be meaningless to draw a strict line between them.     

海上舰船目标的机载高分辨率ISAR成像

本文提出了机载高分辨率二维逆合成孔径雷达(ISAR)对舰船目标的侧视成像问题。通过采用舰船运动的简单模型，避免了采用高级的联合-时间频率(JTF)变换。研究了包括运动估计与校正、最佳处理时间与持续时间估计以及目标形状提取的鲁棒处理方案。强调了这样一个事实，即该处理的鲁棒性造就了单个ISAR成像分析而不必由一组雷达图像进行数据融合。     

用鲁棒算法使宽带雷达信号与目标实现最佳匹配的目标识别新方法

介绍了一种采用耦合振荡器阵列进行宽带雷达信号传输的新方法。这种方法表明振荡器阵列的总体输出是一种能够与特定方位目标匹配的灵活信号。此外，还介绍了一种新的基于相关矩阵本征向量的鲁棒算法，用此算法可以使耦合振荡器阵列产生的信号与目标在有限方位范围内实现匹配。因为雷达不能够精确估计真实目标的方位，所以具有这种性能是很重要的。在这种用于雷达目标识别的新方法中，在估计了未知目标的近似方位后，依次合成并向目标发射了大量与不同潜在目标相匹配的各种波形。这样，后向散射信号的最大峰值功率就会标明出目标的身份。计算机模拟表明这项技术对附加噪声和方位角不定性尤其有效。     

MRO市场隐约出现产能危机

<正>随着全球机队规模不断壮大,一些经典款的飞机和发动机机队增长迅猛,尤其是在新一代飞机缓慢投入运营的过程中,经典机型的维修产能压力较大。即使运营商和MRO企业想到了通过快速兴建维修设施来化解这一危机,似乎仍然无法改变当前的MRO市场正在由买方市场变为卖方市场的事实。     

钛合金扩散连接界面完整性超声定量评估

利用高分辨率超声扫描显微成像检测技术对钛合金扩散连接界面的微缺陷分布进行研究 ,从而得到有关扩散界面微观完整性的定量信息 ,给出了钛合金扩散连接界面的超声扫描成像检测的典型试验结果     

高压气动逻辑

引言“气动逻辑”能以多种形式和用普通原理来实现。“气动逻辑”是通过使用压缩空气而不是电作为动力,为工厂和设备提供类似继电器或固体电子逻辑的顺序闭锁和控制功能。“逻辑”要涉及到器件预先规定的操作和将闭锁线路表示为“逻辑图”的绘图习惯和符号。1.历史和发展逻辑线路曾用传统的运动充气组件(通常是与空气压缩设备相连的短管阀)组成,但这样的组件有很多的缺点,如:     

Astronaut-induced disturbances in microgravity

This Note describes the dynamic load sensors (DLS) spaceflight experiment that measured middeck astronaut-induced disturbances during the 14-day STS-62 Space Shuttle mission in March 1994. The DLS experiment was flown in conjunction with the reflight of the Middeck 0-Gravity Dynamics Experiment (MODE). The objective of MODE was to investigate effects of the microgravity environment on large space structures. Where Skylab experiments focused on measuring the forces exerted during vigorous soaring activities, the DLS experiment quantified the reaction forces and moments exerted by the crew going about their normal on-orbit activities. The objective of this Note is to present DLS force data and frequency analysis that characterize astronaut-induced loads during spaceflight.     

用于ESM系统的CFAR处理器

作者研究了一种用于ESM接收机自适应门限系统的CFAR处理器的设计。对于这种用途，ESM系统的重要技术指标是接收不同宽度和速率的脉冲以及噪声干扰性。确定背景噪声统计的主要难题是干扰信号问题。在背景噪声功率的允差范围内，提出一种MEx-CFAR处理器，它具有良好的抗干扰脉冲的能力。当噪声功率变化大时，建议采用AMEx-CFAR处理器的性能。采用反馈环路结构。AMEx处理器能快速适应新的背景条件。本文用蒙特卡罗模拟研究了所提出的CFAR处理器在各种情况下的性能，讨论了用以确定处理器设计参数的方法。     

新技术将如何改变MRO领域

<正>未来可能应用于MRO领域的新技术有很多,有的是受上游产业影响而发展的,有的是新技术的直接应用,如复合材料修理、增材制造、数字孪生、预测性维修、增强现实、自动化、机器人等等,但这些技术真正应用于MRO领域后会是何种程度,应用中还存在哪些障碍,亦或是其他新的商业模式还会催生出哪些新的技术,这些都应该提前思考。