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   2846篇
  免费   3篇
  国内免费   8篇
航空   1357篇
航天技术   1067篇
综合类   8篇
航天   425篇
  2021年   16篇
  2019年   18篇
  2018年   43篇
  2017年   39篇
  2016年   27篇
  2015年   13篇
  2014年   57篇
  2013年   75篇
  2012年   54篇
  2011年   123篇
  2010年   76篇
  2009年   135篇
  2008年   166篇
  2007年   78篇
  2006年   62篇
  2005年   72篇
  2004年   78篇
  2003年   106篇
  2002年   55篇
  2001年   106篇
  2000年   60篇
  1999年   95篇
  1998年   94篇
  1997年   68篇
  1996年   70篇
  1995年   91篇
  1994年   100篇
  1993年   39篇
  1992年   60篇
  1991年   26篇
  1990年   29篇
  1989年   59篇
  1988年   25篇
  1987年   39篇
  1986年   30篇
  1985年   90篇
  1984年   44篇
  1983年   49篇
  1982年   57篇
  1981年   81篇
  1980年   29篇
  1979年   27篇
  1978年   24篇
  1977年   19篇
  1976年   15篇
  1975年   19篇
  1974年   14篇
  1972年   17篇
  1969年   13篇
  1968年   13篇
排序方式: 共有2857条查询结果,搜索用时 15 毫秒
961.
In today's world of constrained budgets, one of the problems the military faces is the challenge of trying to maintain organic maintenance capabilities. Historically, the US Air Force has desired to achieve an autonomous capability to maintain the equipment it uses. This has been achieved, traditionally, by setting-up three levels of maintenance: organizational; intermediate (back-shop); and depot. The I-level back-shops often utilize militarized automatic test equipment (ATE) and test program sets (TPSs) to test today's complex aircraft line replaceable units (LRUs). And even though this is still a cost-effective maintenance philosophy, it has become costly to develop militarized ATE. The Department of Defense (DoD) has been very active in trying to reduce the total ownership cost of ATE in the government inventory. One approach is to utilize commercial, instead of Mil-Spec, ATE. However, utilizing commercial ATE at an USAF back-shop is not without its tradeoffs and challenges as it represents a significant deviation from the way the USAF maintenance squadrons are accustomed to "doing business." This paper documents the current success story of replacing the legacy C-17 I-level ATE with a commercial ATE.  相似文献   
962.
Air defense systems protect land and maritime resources from air attack. Depending on the regional characteristics and type of conflicting forces, air defense threats vary considerably. In regional conflicts, where forces with similar capabilities are involved and no air-superiority can be achieved, the role of air defense systems becomes critical. In combat terrains containing mountains (in mainland or in small islands), the man-operated or computer-controlled (using passive sensors) anti-aircraft artillery can be highly effective. The simulator presented in this work aims to exploit the capabilities provided by current commercial-off-the-shelf (COTS) communication and multimedia technologies for providing a training environment that improves the personnel capability for effective use of man-controlled anti-aircraft weapons.  相似文献   
963.
Based on the spectrophotometric data, by decomposition of the observed continuum, the power - law continuum characteristics of the central source are obtained. The behaviour of both electron temperature Te and density ne in the Narrow - Line Region (NLR) of NGC 7469 is discussed on the thermal and ionization equilibria calculations. Te in NLR of the Sy1 galaxies are higher than the Sy2 ones, and the possible explanation is the lack of the dense (ne 1010 cm−3) zone close to the central source in the Sy2 galaxies.  相似文献   
964.
X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field.  相似文献   
965.
Optical chemical sensors have been developed for monitoring several parameters relevant to plant growth systems. These sensors utilize porous polymer and porous glass as the sensing element, and optical fiber input/output lines connected to a custom optoelectronic interface. Present in the sensing element are immobilized colorimetric indicators, which react with the analyte to be sensed. This reaction results in a change in the optical properties of the sensor. These sensors are particularly suited to in-situ monitoring of nutrient solution parameters and atmospheric trace contaminants in life support and plant growth systems. Sensors for monitoring pH, ammonia, and ethylene will be discussed.  相似文献   
966.
For aeronautical weapon systems now entering development, avionics will equal or surpass airframe, propulsion, and weapons in establishing system effectiveness, both in terms of mission accomplishment and in terms of reliability, availability, supportability, and life cycle cost. Emerging technologies created the opportunity to satisfy demanding operational needs and to provide the means to upgrade systems against an evolving threat. This requires, however, basic changes in the skills, design methods, and constraints applied to avionics development. This paper highlights trends in avionics technology and systems and suggests some of the new ground rules which must be imposed to realize their full potential. Avionics considerations must be a central element of system development from the very outset, and the process of conceptual planning and program estimation must take the new realities of avionics into account.  相似文献   
967.
The allure of the solid state optical gyros is being challenged by the invention of a new spinning mass device, providing superior low noise drift performance, high reliability at a very low cost. The Minitact is a unique two axis gyroscope advancing the technology of spinning mass devices to set new low cost standards. Minitact provides all the low noise performance benefits of tuned rotor gyroscopes without the vagaries of fixture suspension systems. The Minitact design integrates together the advantages of a spherical hydrodynamic gas bearing and a three axis permanent magnet DC commutated, motor and torquer, into miniature two aids gyroscope with a dynamic range of greater than 10-7. The design is described and performance data presented illustrating the capability of this low cost next generation rate gyroscope. The Minitact is the result of Integrated Product Development (IPD) and Design to Unit Production Cost (DTUPC) design processes and is expected to become the tactical performance benchmark  相似文献   
968.
The dosimetric experiments Dose-M and Liulin as part of the more complex French-German-Bulgarian-Russian experiments for the investigation of the radiation environment for Mars-96 mission are described. The experiments will be realized with dosemeter-radiometer instruments, measuring absorbed dose in semiconductor detectors and the particle flux. Two detectors will be mounted on board the Mars-96 orbiter. Another detector will be on the guiderope of the Mars-96 Aerostate station. The scientific aims of Dose-M and Liulin experiments are: Analysis of the absorbed dose and the flux on the path and around Mars behind different shielding. Study of the shielding characteristics of the Martian atmosphere from galactic and solar cosmic rays including solar proton events. Together with the French gamma-spectrometer and the German neutron detectors the investigation of the radiation environment on the surface of Mars and in the atmosphere up to 4000 m altitude will be conducted.  相似文献   
969.
970.
In river systems confluences and bifurcations are typical features whose effects are largely dependent on the specific characteristics of the rivers involved. The study makes an attempt to analyse the planform configuration at the confluence zone of the Wainganga and Khobragadi rivers in the central part of India. The data used are the Landsat MSS CCT data of date 17.12.1972 for scene 154-046 and black and white aerial photographs of November, 1969. Supervised method of classification using maximum likelihood classification method is employed. The study confirms that the discharge and sediment transport in the individual rivers constitute the dominating factors in the changes of planform geometry at a confluence. The usefulness of the Landsat MSS data and digital analysis techniques. for generating the necessary data inputs for the study is also validated.  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