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   1198篇
  免费   251篇
  国内免费   176篇
航空   746篇
航天技术   316篇
综合类   162篇
航天   401篇
  2024年   4篇
  2023年   10篇
  2022年   26篇
  2021年   41篇
  2020年   39篇
  2019年   34篇
  2018年   36篇
  2017年   28篇
  2016年   39篇
  2015年   48篇
  2014年   66篇
  2013年   61篇
  2012年   77篇
  2011年   85篇
  2010年   95篇
  2009年   99篇
  2008年   73篇
  2007年   57篇
  2006年   71篇
  2005年   54篇
  2004年   34篇
  2003年   64篇
  2002年   74篇
  2001年   58篇
  2000年   43篇
  1999年   44篇
  1998年   37篇
  1997年   42篇
  1996年   26篇
  1995年   25篇
  1994年   24篇
  1993年   16篇
  1992年   15篇
  1991年   15篇
  1990年   8篇
  1989年   20篇
  1988年   7篇
  1987年   5篇
  1986年   1篇
  1985年   3篇
  1984年   3篇
  1983年   1篇
  1982年   7篇
  1981年   2篇
  1980年   1篇
  1978年   3篇
  1977年   1篇
  1969年   1篇
  1968年   2篇
排序方式: 共有1625条查询结果,搜索用时 15 毫秒
991.
直译与意译是中外翻译界长期争论的一个问题。由于人们看待这个问题的角度不同,这场争论至今尚无定论。回顾了中外翻译史不同时期内,翻译学家根据自己对翻译的不同理解而展开的论战,指出走融合不走极端,已成为现代翻译的趋势。  相似文献   
992.
针对机场跑道异物(FOD) 识别检测,分析了机场跑道环境特性,建立了针对机场FOD图像识别数据库, 提出了一种基于Faster-RCNN 的机场跑道异物识别定位系统,通过无人机系统对机场跑道进行图像采集,运用 Faster-RCNN 算法框架完成异物种类识别,并结合无人机地理位置信息进行位置换算求解,得到机场FOD 的 类别信息和位置信息。经验证,该系统可高效完成机场FOD 检测识别及定位任务。  相似文献   
993.
A new adaptive nonlinear guidance law is proposed here. The fourth order state equation for integrated guidance and control loop is formulated taking into consideration the target uncertainties and control loop dynamics. The state equation is further changed into the normal form by nonlinear coordinate transformation. Using the normal form of state equation, an adaptive nonlinear guidance law is proposed to compensate for the uncertainties in both target acceleration and control loop dynamics. The proposed law adopts the sliding mode control approach with adaptation for unknown bound of uncertainties. The present approach can effectively solve the existing guidance problem against target maneuver and the limited performance of control loop. We have provided the stability analyses and performed simulations comparing favorably our approach to the state of the art.  相似文献   
994.
Nowadays, GNSS-Reflectometry (GNSS-R) can be a new promising remote sensing tool in the ocean, snow/ice and land surfaces, e.g., vegetation biomass monitoring. Although GNSS-R provides a potentially special L-band multi-angular and multi-polarization measurement, the theoretical vegetation scattering properties and mechanisms for GNSS-R are not understood clearly. In this paper, the GNSS-R vegetation polarization scattering properties are studied and modeled at different incidence angles (specular direction). The bistatic scattering model Bi-mimics is employed, which is the first-order radiative transfer equation. As a kind of forest stand, the Aspen’s crown layer is composed of entire leaves, and its parameters in Mimics handbook are used as model input. The specular circular polarizations (co-polarization RR and cross-polarization LR) are simulated. For cross-polarization, the received polarization is assumed as a linear (horizontal and vertical) polarizations and ±45° linear polarizations. Therefore, the HR VR, +45R and −45R polarizations are simulated here. Contributions from different scattering components at RR, LR and VR polarization are also presented. For co-polarization, it is large in the whole specular angles (10–80°). The scattering trends of the other cross polarization (HR, LR, +45R and −45R) are a little similar when compared to the RR and RV. Therefore, the RHCP and V polarizations are more favorable to collect the reflected signals. The trunk heights and crown depths do not affect the scattering trends of RR, RV and RL, while the trunk height has some effect on the scattering amplitude of different polarizations. The azimuth angle has more effects on RR, RL and RV scattering, especially in lower than 50°. The observation angles and polarization combinations are extremely important for GNSS-R remote sensing.  相似文献   
995.
服从任意分布的部件可用度快速计算方法   总被引:2,自引:0,他引:2  
工程中通常认为电子产品故障密度和修复服从指数分布,但是现代的航空和航天系统使用部件,并不总是指数分布,指数分布部件构成的系统也并不一定服从指数分布。文中提出了故障密度和修复密度任意分布的部件可用度快速计算方法,并且以故障密度和修复密度服从常用分布情形,即指数分布、正态分布、对数正态分布和威布尔分布情形,分析了任意分布部件可用度的变化规律,所提出的方法,便于工程实际应用。  相似文献   
996.
现代液体火箭发动机的推力室钎焊身部、发生器钎焊身部及游机推力室钎焊身部均是在高温、高压环境下工作,采用波纹板和铣槽式结构,而这种结构的钎焊和扩散焊的焊接质量直接关系到产品的承压强度及冷却效果。因此,对钎焊和扩散焊的质量检测尤为重要。着重讨论了对波纹板结构和铣槽结构的钎焊、扩散焊质量采用不同无损检测方法进行检测时,方法的可行性和存在的问题,并给出不同方法对产品的可检测性结论。  相似文献   
997.
纵观世界各国和地区的经济发展,产业梯度转移是一个普遍存在的客观经济规律,这种梯度推进规律为欠发达国家或地区发挥比较和后发优势、实现跨越式发展提供了机遇.从现实性考察,江西近年在招商引资中也开创出崭新的局面.但尽快制订相应对策,促进产业转移承接基地建设仍然是实现江西在中部率先崛起的一条有效路径.  相似文献   
998.
This paper analyzes the step load response of a current-mode-controlled pulsewidth modulation (PWM) converter and also presents design guidelines for obtaining a good step load response. Analytical expressions for the step load response are derived in terms of the power stage and feedback compensation parameters. Control design to minimize the overshoot and settling time of the output voltage is presented. Analysis results are verified by large-signal simulations.  相似文献   
999.
采用被动式电磁阻尼器控制磁悬浮轴承转子系统振动.首先,设计了一个被动式电磁阻尼器,可以通过电流变化控制阻尼力.然后,基于通用有限元软件NASTRAN,对磁悬浮轴承转子系统分析了不同阻尼下的不平衡响应仿真.最后通过系统高速旋转实验,研究了不同阻尼器控制参数下该系统的不平衡响应.仿真与试验结果表明,增加被动式电磁阻尼器能够改善系统的动态性能并降低转子的不平衡振动,使转子成功越过一阶弯曲临界转速15 600 r/min,稳定运行在21 000 r/min.  相似文献   
1000.
Sustaining weapons system hardware and software represents a significant and ever-increasing portion of total system cost. Hardware components are becoming obsolete much sooner while weapons system lifetimes are increasing, We must identify more cost-effective solutions to engineering and reengineering these subsystems. Verifying and validating weapons systems are two of the most costly parts of either engineering process. Traditionally, hardware validation and verification is done by simulation and testing, In the past few years, math- and logic-based formal methods tools have begun to scale up to and be applied successfully to real-world problems. Incorporating formal verification methods into engineering and reengineering processes will cost-effectively and significantly improve the level of trust and the quality of our weapons systems. Formal methods are especially well suited for redesigning current weapon systems which have become unsupportable due to component obsolescence because they help minimize the astronomical costs of rigorously reverifying the reengineered components. We believe that formal methods are an important tool for effective engineering of future weapon systems  相似文献   
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