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   5430篇
  免费   11篇
  国内免费   18篇
航空   2844篇
航天技术   1922篇
综合类   23篇
航天   670篇
  2021年   30篇
  2019年   33篇
  2018年   57篇
  2017年   31篇
  2014年   82篇
  2013年   132篇
  2012年   110篇
  2011年   157篇
  2010年   116篇
  2009年   181篇
  2008年   247篇
  2007年   136篇
  2006年   140篇
  2005年   143篇
  2004年   114篇
  2003年   170篇
  2002年   101篇
  2001年   176篇
  2000年   109篇
  1999年   140篇
  1998年   164篇
  1997年   123篇
  1996年   169篇
  1995年   215篇
  1994年   182篇
  1993年   120篇
  1992年   127篇
  1991年   76篇
  1990年   62篇
  1989年   138篇
  1988年   62篇
  1987年   66篇
  1986年   60篇
  1985年   195篇
  1984年   150篇
  1983年   130篇
  1982年   133篇
  1981年   176篇
  1980年   58篇
  1979年   41篇
  1978年   50篇
  1977年   53篇
  1976年   38篇
  1975年   58篇
  1974年   38篇
  1973年   42篇
  1972年   51篇
  1971年   42篇
  1970年   44篇
  1969年   38篇
排序方式: 共有5459条查询结果,搜索用时 15 毫秒
511.
The building of great steam power plants with utmost consideration for public safety has created today's American miracle of abundant electricity. Behind this great achievement stand the mechanical engineers who have designed and built these power plants under their ethical responsibilities for public safety, in full compliance with the American Society of Mechanical Engineers (ASME) Boiler Code and under the strict disciplines imposed by a specialist insurance company willing to write 100 percent insurance coverage against boiler explosions. The historical origin of this great achievement dates back to the days of the Civil War and a small group of men with exceptional vision and a profound concern for public safety. They saw the opportunity to apply their talents and sense of responsibility to solving the problem of steam boiler explosions and the terrible losses of human lives. These early leaders have earned the esteem and honor of our present and future generations, but who remembers them? Today, in the scientific revolution of this century, there is a critical need for similar new leaders of exceptional integrity, responsibility, and initiative, both in engineering and in industry.  相似文献   
512.
The resolution and tracking of radar targets which are close to. gether is a classical problem which is receiving increased attention. Improving the angular resolution by sampling the received wave front along the aperture and then applying modern "high resolution" spectrum estimation techniques (e. g., maximum likelihood and maximum entropy processing) to the spatial sample data is discussed. Experimental results are presented for the application ion of these techniques to field data from a L- band line array.  相似文献   
513.
The LIMS experiment was launched on the Nimbus 7 satellite for the purpose of sounding the vertical structure of temperature and key upper atmosphere trace gases on a global scale. The technique of thermal infrared limb sounding was used to obtain measurements of O3, H2O, NO2, and HNO3. LIMS collected data almost continuously from late October to late May over the latitude range from 64°S to 84°N. Two of the gases, NO2 and HNO3, are important elements in the NOx chain of chemical reactions leading to ozone destruction. We will describe results for these gases in terms of zonal mean profiles and latitudinal distributions. The period selected for study is January–May 1979, when a major stratospheric warming occurred.  相似文献   
514.
515.
During the initial period of high voltage biasing of the SPEAR-3 sounding rocket payload, it was observed that electrical breakdown occurred in the gas surrounding the rocket. The breakdown occurred almost all the way to apogee of 289 km on the upleg, but did not recur on the downleg until the payload reached an altitude of 100 km. It is suggested that this behavior can be attributed to payload outgassing on the upleg leading to abnormally high gas pressure near the payload skin. Consideration of a modified Paschen discharge process with varying pressure along the discharge path was found to be consistent with the results.  相似文献   
516.
World-ocean distribution of the crossover altimetry data from Geosat, TOPEX/Poseidon (T/P) and the ERS 1 missions have provided strong independent evidence that NASA's/CSR's JGM 2 geopotential model (70 × 70 in spherical harmonics) yields accurate radial ephemerides for these satellites. In testing the sea height crossover differences found from altimetry and JGM 2 orbits for these satellites, we have used the sea height differences themselves (of ascending minus descending passes averaged at each location over many exact repeat cycles) and the Lumped Latitude Coefficients (LLC) derived from them. For Geosat we find the geopotential-induced LLC errors (exclusive of non-gravitational and initial state discrepancies) mostly below 6 cm, for TOPEX the corresponding errors are usually below 2 cm, and for ERS 1 (35-day cycle) they are generally below 5 cm. In addition, we have found that these observations agree well overall with predictions of accuracy derived from the JGM 2 variance-covariance matrix; the corresponding projected LLC errors for Geosat, T/P, and ERS 1 are usually between 1 and 4 cm, 1 – 2 cm, and 1 – 4 cm, respectively (they depend on the filtering of long-periodic perturbations and on the order of the LLC). This agreement is especially impressive for ERS 1 since no data of any kind from this mission was used in forming JGM 2.

The observed crossover differences for Geosat, T/P and ERS 1 are 8, 3, and 11 cm (rms), respectively. These observations also agree well with prediction of accuracy derived from the JGM 2 variance-covariance matrix; the corresponding projected crossover errors for Geosat and T/P are 8 cm and 2.3 cm, respectively. The precision of our mean difference observations is about 3 cm for Geosat (approx. 24,000 observations), 1.5 cm for T/P (approx. 6,000 observations) and 5 cm for ERS 1 (approx. 44,000 observations). Thus, these “global” independent data should provide a valuable new source for improving geopotential models. Our results show the need for further correction of the low order JGM 2 geopotential as well as certain resonant orders for all 3 satellites.  相似文献   

517.
This paper discusses the advantages of incorporating active sidesticks into a modern aircraft cockpit. Active sidestick controllers for manual pilot inputs in pitch and roll are examined for commercial transport aircraft. Options and requirements for sidesticks are reviewed. The recommendation of an active sidestick controller is developed providing both cross-cockpit coupling and autopilot backdrive capability. These characteristics provide pilot cues identical to traditional cable-linked column/yoke configurations  相似文献   
518.
A major technology barrier to the application of pulse compression for the meteorological functions required by a next generation ATC radar is range/time sidelobes which mask and corrupt observations of weak phenomena occurring near areas of strong extended meteorological scatterers. Techniques for suppressing range sidelobes are well known but without prior knowledge of the scattering medium's velocity distribution their performance degrades rapidly in the presence of Doppler. Recent investigations have presented a “doppler tolerant” range sidelobe suppression technique. The thrust of the work described herein is the extension of previous simulations to actual transmitted dispersed/coded waveforms using the S-band surveillance radar located at Rome Laboratory Surveillance Facility. The objectives of the experiment are: 1) to extend the verification of the simulation of the Doppler tolerant technique; and 2) to demonstrate that the radar transmitter, waveform generator, and receiver imperfections do not significantly degrade resolution, performance or reliability of meteorological spectral moment estimates  相似文献   
519.
A discussion of the current and emerging core technologies and philosophies that will enable Air Force personnel to quickly, accurately and intuitively diagnose faults in increasingly complex systems  相似文献   
520.
US Army helicopters that will allow single-crewmember operability in tactical environments require a high degree of automation. A tightly integrated flight control and navigation system has been studied. An architecture is discussed which meets anticipated flight control and navigation requirements, at an acceptable weight, using techniques of functional and physical integration. The architecture maximizes the sharing of sensor and computational resources in a flight critical environment and uses modular equipment packaging, dual fault-tolerant tetrad ring-laser-gyro inertial measurement units, and triplex self-checking processor pair-based flight critical processing channels  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