首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8626篇
  免费   1044篇
  国内免费   414篇
航空   4611篇
航天技术   2906篇
综合类   495篇
航天   2072篇
  2024年   53篇
  2023年   79篇
  2022年   128篇
  2021年   215篇
  2020年   150篇
  2019年   161篇
  2018年   242篇
  2017年   222篇
  2016年   204篇
  2015年   185篇
  2014年   300篇
  2013年   332篇
  2012年   339篇
  2011年   433篇
  2010年   402篇
  2009年   510篇
  2008年   503篇
  2007年   346篇
  2006年   294篇
  2005年   313篇
  2004年   263篇
  2003年   305篇
  2002年   269篇
  2001年   282篇
  2000年   228篇
  1999年   237篇
  1998年   223篇
  1997年   202篇
  1996年   214篇
  1995年   264篇
  1994年   231篇
  1993年   128篇
  1992年   187篇
  1991年   84篇
  1990年   77篇
  1989年   144篇
  1988年   59篇
  1987年   49篇
  1986年   68篇
  1985年   181篇
  1984年   138篇
  1983年   108篇
  1982年   135篇
  1981年   165篇
  1980年   47篇
  1979年   32篇
  1978年   40篇
  1977年   37篇
  1976年   31篇
  1974年   35篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
The original basis for the Lorentz transformations, and thus special relativity, was the assumption that the observed velocity of interaction of light with matter represents a unique velocity of the electromagnetic wave. This arbitrary decision is not borne out by Maxwell's theories or by any test that might prove that EM energy actually travels in a continuum of velocities. The second postulate as stated by Einstein does not deserve the status of a postulate, as it is at once overly restrictive and ultimately phenomenological-the nature of c is based on experimental measurement rather than on analysis of first principles. The radiation continuum model's (RCM) modified second postulate, however, says nothing about the actual propagation of EM energy, but only of the relative speed with which it must interact with matter to be detected. Utilizing this modified light principle we obtain an intuitive Galilean form invariance for Maxwell's equations. RCM places no upper limit on attainable velocities, and allows for the possibility of communications between humans or particles at speeds far in excess of c. This precludes many of the compatibility problems between the highly successful quantum mechanics and relativity theory  相似文献   
912.
Parameterization of dynamical and thermal effects of stationary orographic gravity waves (OGWs) generated by the Earth’s surface topography is incorporated into a numerical model of general circulation of the middle and upper atmosphere. Responses of atmospheric general circulation and characteristics of planetary waves at altitudes from the troposphere up to the thermosphere to the effects of OGWs propagating from the earth surface are studied. Changes in atmospheric circulation and amplitudes of planetary waves due to variations of OGW generation and propagation in different seasons are considered. It is shown that during solstices the main OGW dynamical and heat effects occur in the middle atmosphere of winter hemispheres, where changes in planetary wave amplitudes due to OGWs may reach up to 50%. During equinoxes OGW effects are distributed more homogeneously between northern and southern hemispheres.  相似文献   
913.
    
The use of wireless power transmission in Space Solar Power (SSP) activities creates significant policy issues regarding the beam right-of-way. There will not be a single beam, there may well be hundreds of beams for economical systems. Are some or all of these power beams to be afforded priorities of space for unobstructed power delivery, or must the beaming systems be designed to be capable of detecting any and all potential beam interceptions and appropriately responding? The repeated interruptions for guaranteed safety of transit for freely moving air and space traffic are of great consequence. The safety issues are critical, but the implications for equipment transient protection, energy storage system costs and the quality of power delivery service are also significant for wireless power transmission economics. A scenario of precursor wireless power transmission developments leading up to and including SSP applications will be used to frame and to discuss the beamed power technology implications and policy issues.  相似文献   
914.
    
The results of modeling the dynamics of the Space Radiotelescope are presented. The results of ground-based vibration-dynamic tests are used to validate the calculation models and estimate the damping levels. The dynamic distortions of the reflecting surface caused by the operation of the pointing mechanism of the high-gain antenna are estimated.  相似文献   
915.
    
In the past studies, different soil moisture estimation models were developed for bare soil areas by using remotely-sensed data. However, there are few models that can be used to estimate soil moisture in vegetated areas. Water Cloud Model (WCM) model is a widely used soil moisture estimation model has been developed for vegetated areas. In this study, the WCM model was extended to take soil roughness parameter into consideration. The modeling and its accuracy assessment were done by using multi-polarization Airborne Synthetic Aperture Radar (AIRSAR) images and ground data collected during field Soil Moisture Experiments.  相似文献   
916.
林震  王长辉  刘宇 《推进技术》2012,33(6):897-901
为探索催化点火气氢气氧推力器的点火规律和相关性能,设计并搭建了1N催化点火气氢气氧推力器试验系统,进行了冷热试试验。试验成功实现了不同工况下的多次催化点火。结果表明:供给氧氢混合比控制在100时,推力器预燃室产生的点火温度为700℃,能够满足燃烧室点火要求;催化剂面积体积比对催化氢氧点火具有决定性影响,采用面积体积比大的催化载体可以实现常温(8℃)下的催化点火;另外,试验中氢氧燃烧产生的高温和试验结束后推力器内残留的液态水也对催化组件提出了一定的耐温性和防水性要求。  相似文献   
917.
环境与灾害监测预报小卫星星座--环境减灾-1A、1B卫星经历了方案阶段、初样阶段、正样阶段的研制工作,研制中解决了星座设计与优化、全新载荷的研制难题,克服了主动段时间长和任务与接口要求变化大等问题,积累了丰富的经验和教训,可以为其它型号研制提供借鉴.  相似文献   
918.
A thorough observability analysis of the Mars entry navigation using radiometric measurements from ground based beacons is performed. This analysis involves the evaluation of the Fisher information matrix which is derived from the maximum likelihood estimation. A series of navigation cases with multiple beacons are investigated, and both range and range-rate measurements are considered. The determinant of Fisher information matrix is used to quantify the observability of navigation system, while the trace of Fisher information matrix is used to determine the lower-bound of estimation errors. For one and two beacon cases, the navigation system is unobservable. However, the eigenvectors of Fisher information matrix give the observable and unobservable component. When three or more beacon measurements are employed, the states of entry vehicle become observable. Some valuable analytic conclusions on the relationship between the geometric configuration of beacons and observability are obtained consequently. Finally, simulation results from two navigation examples indicate that our effort is useful for understanding and assessing the observability of the Mars entry navigation using radiometric measurements.  相似文献   
919.
直升机六自由度仿真研究   总被引:4,自引:0,他引:4  
朱宇  秦超敏  高浩 《飞行力学》2004,22(1):45-47,51
建立了直升机剐体运动的六自由度仿真模型。用试飞数据进行参数识别,建立了气动导数数据库并以此计算气动力;将仿真结果与试飞结果对比,并进行了直升机机动飞行仿真计算。结果表明:六自由度仿真模型能够得到较好的仿真结果。  相似文献   
920.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号