Hr 7308, a short period Cepheid with variable amplitude

HR 7308 is a F6 Ib-II star of 6^th magnitude. Semi-regular photometric variations were detected by Breger (1969), Percy et al. (1979) find that this star is a new small-amplitude Cepheid, and Burki and Mayor (1980) obtain a period of 1.49107 d. and detect a strongly variable amplitude.     

Recent work on Cepheid variables

Recent work on Cepheids is reviewed in the areas of (1) the large-amplitude mode behavior, (2) convection, and (3) Cepheid masses. Initial-value type nonlinear calculations have not yet yielded true double-mode behavior. Yet we have the beginnings of a promising theory of modal selection. Theoretical calculations also yield reasonably located red edges to Cepheid (and Cepheid-like) instability regions.Recent observational results have led to increased values of the pulsation mass, so that this mass is now in fair agreement with evolution theory. The Wesselink mass is also satisfactory. Thus now only bump and beat masses are possibly discrepant. Some possible ways which have been suggested to alleviate these discrepancies are reviewed. The proposal of helium enrichment in the outer stellar layers can apparently satisfactorily resolve the beat (and perhaps also the bump) mass anomaly. A recent suggestion that part of the pressure in the envelope is due to a tangled magnetic field (not unusually strong) resolves the above mass anomaly about as well as the helium-enrichment idea does.Recent results regarding duplicity and period changes in Cepheids are reviewed.     

Stretch: A Time-Transformation Technique

Stretch is a passive, linear, time-variant technique for performing temporal operations on many classes of signals. The technique employs three dispersive networks and a mixer. Signal slowdown, speedup, or time reversal can be attained by choice of network slopes. These temporal operations are performed within a signal "window," and the duration of the window is determined by the network time-bandwidth products. Both heuristic argumentation and rigorous analysis are presented, as are the results of a simple laboratory experiment.     

Checkout: A Changing Technology

On-board/in-flight checkout of future aerospace systems will necessitate a fundamental departure from today's design and checkout activities. The interrelationship of checkout, incorporated into prime vehicular functions, man as a subsystem, and standardization Of hardware/software must be considered as a functional and integral entity if the efforts of today's long-range planning are to become tomorrow's reality. This paper will describe future systems checkout, maintenance and support considerations, and actions and tentative measures necessary for implementation. The realization of the implementation of these concepts into functional elements will depend, accurately and economically, upon the degree of government perceptiveness and the extent of industrial support.     

Chaos and Determinism: A Paradigm

Policy makers depend upon ability to predict the outcome of their policies. Increasingly, in the field of international security ? in matters of war and peace ? prediction is based upon quantitative models. The point made here is that just because a model is mathematical, it does not follow that it can be used to make predictions, that non-linear models allow both predictive and chaotic regimes. After illustrating these matters with physical models, the paper turns to the modeling of an arms race between two antagonistic nations (or groups of nations). After discussing static and dynamic arms race models which are predictive, we turn to a simple non-linear arms race model which allows a transition from predictabililty to chaos. We then postulate that such a transition describes the transition from peace to war in the two party hostile competitive system being modeled by the nonlinear equations, that peace is to war in reality as determinism is to chaos in a mathematical model of that reality. Thus, escalation from peace to war through various levels of hostility may not be controllable, the ``slipping' from peace to war may be as inadvertent as the slipping from laminar flow to turbulence as an aircraft stalls and loses control. The lessons to be learned are to stay away from critical transition points if at all possible and ? more importantly ? to be very wary of claims of controllability in any escalation of a crisis.     

A320维修市场稳步增长

随着机队的快速扩大,A320的重检及C检需求也明显增加。空客与波音针锋相对的激烈竞争,使得A320与波音737成为世界各地最广泛使用的窄体中型双发客机。在过去的10年,空客开始鲸吞曾由波音737垄断的市场,并不断扩大市场份额,这意味着A320系列飞机的重检(HMV)     

仿生导航技术综述

仿生导航技术是一种模仿动物导航机理的新型导航技术,涉及认知科学、机器学习、计算机视觉和信息融合等多个学科。仿生导航因具有自主性好、自适应性强等特点,近年来成为了导航领域的研究热点之一。首先阐述了仿生导航的内涵,然后从仿生导航传感器技术和仿生导航方法两个方面简要介绍了国内外研究现状和发展趋势。其中,仿生导航传感器技术包括了仿生光罗盘、仿生磁罗盘、仿生复眼等内容;仿生导航方法主要涉及导航经验知识的表达与机器学习、仿生多源异质导航信息融合、面向任务的仿生路径规划与导航等。最后,对仿生导航技术进行了总结,并对未来发展进行了展望。     

A380:便于维修的"庞然大物"

站在A380的前面,会被它巨大的尺寸所震撼。但要对它进行维护的人员来说,其各种可达性均得到了尽可能的优化。从某种程度上说,这种飞机尺寸的大幅增加即便是对于有多年工作经验的工程师来说也会引起担忧,怕会增加维护的复杂性和难度。但空中客车公司认为,A380飞机在许多方面可能较其他任何飞机更易于维修。按照航空公司的要求,在A380的设计过程中充分考虑了维护人员的生理状况和安全性。例如,任何质量超过25kg的物品均会安排两人或者由专用设备来运送。在飞机的周围及内部充分体现了人机工效学,应用复杂的虚拟人体模型,在飞机的数字模型(DM…     

"发现"号"载险"而归新一代航天飞机呼之欲出

美国“发现”号的太空之旅之所以受到了世界备国的广泛关注,不仅因为“发现”号航天飞机在太空创造了多项第一,积累了很多重要的经验,也因为这是一次险象环生的飞行,发射一波三折,返航更是如履薄冰,从而加剧了对新一代航天飞机的呼声。     

Mercury: A post-Mariner 10 assessment

Our knowledge of Mercury has improved dramatically since the flight of Mariner 10. The planet is probably differentiated into a large iron-rich core (75% of the total radius) and a relatively thin (600 km) silicate mantle. Although the surface of Mercury superficially resembles the Moon, there are three main differences: (1) large areas of relatively old intercrater plains, (2) a widespread (probably global) distribution of lobate scarps, and (3) a similar albedo between young smooth plains and the older mercurian highlands. The origin of Mercury's plains units is still uncertain but a volcanic origin is favored for at least large tracts of the younger smooth plains. The older intercrater plains seem to span a range of ages, large tracts of which appear to have been implaced during the period of intense bombardment. The widespread distribution of lobate scarps probably resulted from a period of global contraction relatively late in Mercury's history. This period of contraction probably resulted primarily from cooling of the lithosphere and/or core following core formation. The crater diameter density distribution on the Moon, Mars and Mercury indicates that all the terrestrial planets experienced a period of intense bombardment early in their histories and that the objects responsible for this bombardment probably belonged to the same population(s).     

Energetics of reconnection: A reply

In this article we address several criticisms of Petschek-type reconnection models which have recently been raised by Heikkila. We discuss features of the time-dependent Petschek-type models in the context of the solar wind-magnetosphere interaction, and point out that such models can incorporate and reproduce observed features at the magnetopause, such as plasma jets and erosion of the current sheet. We argue that some of Heikkila's criticisms can be attributed to weaknesses in the analysis due to incomplete experimental information, rather than to flaws in the concept of reconnection per se; in this category we include the question of which instability leads to the localised breakup of the magnetopause current sheet. Other criticisms are based on an adherence to steady-state models, and cannot be sustained within the extended time-dependent theory. We discuss, for example, how the time-dependent model can provide a consistent picture of how energy from the incoming solar wind is transferred and converted as it enters the magnetosphere.     

Radar ECCM: A European Approach

The radar designer must optimize parameters for performance in electronic countermeasures (ECM) and avoid any constraints which could be exploited by ECM. ECM/electronic counter-counter measures (ECCM) strategy must take into account electromagnetic compatibility (EMC) including enemy EMC. Electronic scanning improves search performance, but also reduces ECCM performance in other ways. While electronic scanning avoids some constraints, it imposes others, particularly in the frequency domain. It is argued that simple radars of good performance may be as cost effective in ECM as more complex systems. Some examples of recent developments are given.     

成功的国际合作项目--CFM56发动机

法国斯奈克玛公司和美国GE公司联合研制的CFM56涡轮风扇民用发动机自1979年投入使用以来,至今已交付15300多台,占世界100座级以上商业飞机发动机市场的半壁江山.本文将研究其产生的背景,追踪联合的过程,分析运作的体制,显现经营的业绩,跟踪未来的计划和总结成功的经验.     

Nonlinear filtering: A hybrid approximation scheme

This paper is concerned with nonlinear filtering schemes for systems which allow non-Gaussian noise. Using the most probable trajectory (MPT) approach, a finite-dimensional recursive hybrid filtering scheme is derived. By appropriately selecting a switching process, a linear hybrid system can be obtained that approximates the original nonlinear system. Then the MPT approach is used to obtain the hybrid filtering schemes for the nonlinear systems. Numerical experiments are carried out and reported     

Phase-Locked Demodulator Threshold: A New Approach

In this paper the dependence of the threshold CNR of a simple phaselocked demodulator on the frequency deviation of an FM signal has been evaluated by the concept of quasi-stationary approximation. Experimental findings have been presented in support of the theoretical conclusion.