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   8371篇
  免费   17篇
  国内免费   25篇
航空   3744篇
航天技术   2855篇
综合类   31篇
航天   1783篇
  2021年   92篇
  2019年   52篇
  2018年   252篇
  2017年   165篇
  2016年   151篇
  2015年   62篇
  2014年   220篇
  2013年   278篇
  2012年   258篇
  2011年   386篇
  2010年   286篇
  2009年   411篇
  2008年   435篇
  2007年   278篇
  2006年   189篇
  2005年   233篇
  2004年   220篇
  2003年   258篇
  2002年   184篇
  2001年   273篇
  2000年   142篇
  1999年   181篇
  1998年   220篇
  1997年   136篇
  1996年   188篇
  1995年   243篇
  1994年   223篇
  1993年   129篇
  1992年   170篇
  1991年   54篇
  1990年   61篇
  1989年   159篇
  1988年   65篇
  1987年   66篇
  1986年   74篇
  1985年   221篇
  1984年   177篇
  1983年   135篇
  1982年   141篇
  1981年   247篇
  1980年   65篇
  1979年   55篇
  1978年   58篇
  1977年   51篇
  1975年   58篇
  1974年   46篇
  1973年   37篇
  1972年   46篇
  1971年   44篇
  1970年   42篇
排序方式: 共有8413条查询结果,搜索用时 281 毫秒
801.
Monopulse radars have played an important role in air and missile defense systems since the development of the monopulse technique in the late 1940s. This paper outlines the application of monopulse radars in Russian defense systems, starting with the Moscow ABM system and continuing in instrumentation and air defense radars now widely deployed in Russia and elsewhere  相似文献   
802.
The complex system of linear features on Phobos—the inner moon of Mars—found by the Viking Orbiters in 1976, can be classified morphologically into three types according to their appearance as well as their geometrical distribution on the surface. One kind of grooves appears to form arcs of small circles normal to the Phobos-Mars direction. We propose that these grooves are the surface manifestation of layering within Phobos[4–6] which could have been formed when Phobos was a part of a much larger and geologically active parent planet.Here we suggest some measurements to observe whether Phobos has a layered structure or not. The suggestion contains remote sensing measurements (particularly determination of the magnetic field by magnetometer and by electron reflection method) by a spacecraft orbiting around Mars as well as surface measurements (active seismic reflection experiment) by a landing module on Phobos.  相似文献   
803.
The paper elaborates on “ lessons learned” from two recent ESA workshops, one focussing on the role of Innovation in the competitiveness of the space sector and the second on technology and engineering aspects conducive to better, faster and cheaper space programmes. The paper focuses primarily on four major aspects, namely:
1. a) the adaptations of industrial and public organisations to the global market needs;
2. b) the understanding of the bottleneck factors limiting competitiveness;
3. c) the trends toward new system architectures and new engineering and production methods;
4. d) the understanding of the role of new technology in the future applications.

Under the pressure of market forces and the influence of many global and regional players, applications of space systems and technology are becoming more and more competitive. It is well recognised that without major effort for innovation in industrial practices, organisations, R&D, marketing and financial approaches the European space sector will stagnate and loose its competence as well as its competitiveness. It is also recognised that a programme run according to the “better, faster, cheaper” philosophy relies on much closer integration of system design, development and verification, and draws heavily on a robust and comprehensive programme of technology development, which must run in parallel and off-line with respect to flight programmes.

A company's innovation capabilities will determine its future competitive advantage (in time, cost, performance or value) and overall growth potential. Innovation must be a process that can be counted on to provide repetitive, sustainable, long-term performance improvements. As such, it needs not depend on great breakthroughs in technology and concepts (which are accidental and rare). Rather, it could be based on bold evolution through the establishment of know-how, application of best practices, process effectiveness and high standards, performance measurement, and attention to customers and professional marketing. Having a technological lead allows industry to gain a competitive advantage in performance, cost and opportunities. Instrumental to better competitiveness is an R&D effort based on the adaptation of high technology products, capable of capturing new users, increasing production, decreasing the cost and delivery time and integrating high level of intelligence, information and autonomy. New systems will have to take in to account from the start what types of technologies are being developed or are already available in other areas outside space, and design their system accordingly. The future challenge for “faster, better, cheaper” appears to concern primarily “cost-effective”, performant autonomous spacecraft, “cost-effective”, reliable launching means and intelligent data fusion technologies and robust software serving mass- market real time services, distributed via EHF bands and Internet.

In conclusion, it can be noticed that in the past few years new approaches have considerably enlarged the ways in which space missions can be implemented. They are supported by true innovations in mission concepts, system architecture, development and technologies, in particular for the development of initiatives based on multi-mission mini-satellites platforms for communication and Earth observation missions. There are also definite limits to cost cutting (such as lowering heads counts and increasing efficiency), and therefore the strategic perspective must be shifted from the present emphasis on cost-driven enhancement to revenue-driven improvements for growth. And since the product life-cycle is continuously shortening, competitiveness is linked very strongly with the capability to generate new technology products which enhance cost/benefit performance.  相似文献   

804.
The problem of soil and vegetation recognition with the use of satellite-derived digitized images and sub-satellite spectral brightness measurements at test sites data is discussed. A technique for retrieval of soil humus content is suggested.  相似文献   
805.
The needs of coastal engineering are different from open ocean monitoring or fisheries potential monitoring. A high resolution of 20 meters is essential for identifying several processes that are taking place and influences the coast. A case of application of Landsat MSS to evaluate littoral process highlights the needs for improved spatial resolution. Nevertheless Landsat MSS data for 1981 and 1982 showed that the extent of accretion and erosion that has taken place during this period. Some problems to be addressed for marine remote sensing are mainly regarding the need for insitu data. These aspects are mentioned.  相似文献   
806.
Cir X-1 was extremely faint when we observed it with EXOSAT. The light curve clearly shows the source in two states; a faint variable state and a very faint but more constant state. The spectrum is very complicated but clearly shows the existence of an iron line.  相似文献   
807.
808.
The results of numerical and experimental study of physico-mechanical properties of composite materials are proposed and variations in rigidity characteristics of the hub working part of the hingeless type within the entire range of helicopter operational temperatures are evaluated.  相似文献   
809.
810.
ARTEMIS Mission Design   总被引:2,自引:0,他引:2  
The ARTEMIS mission takes two of the five THEMIS spacecraft beyond their prime mission objectives and reuses them to study the Moon and the lunar space environment. Although the spacecraft and fuel resources were tailored to space observations from Earth orbit, sufficient fuel margins, spacecraft capability, and operational flexibility were present that with a circuitous, ballistic, constrained-thrust trajectory, new scientific information could be gleaned from the instruments near the Moon and in lunar orbit. We discuss the challenges of ARTEMIS trajectory design and describe its current implementation to address both heliophysics and planetary science objectives. In particular, we explain the challenges imposed by the constraints of the orbiting hardware and describe the trajectory solutions found in prolonged ballistic flight paths that include multiple lunar approaches, lunar flybys, low-energy trajectory segments, lunar Lissajous orbits, and low-lunar-periapse orbits. We conclude with a discussion of the risks that we took to enable the development and implementation of ARTEMIS.  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