全文获取类型
收费全文 | 21144篇 |
免费 | 696篇 |
国内免费 | 621篇 |
专业分类
航空 | 11827篇 |
航天技术 | 6173篇 |
综合类 | 661篇 |
航天 | 3800篇 |
出版年
2021年 | 303篇 |
2019年 | 224篇 |
2018年 | 333篇 |
2017年 | 236篇 |
2016年 | 259篇 |
2015年 | 219篇 |
2014年 | 598篇 |
2013年 | 664篇 |
2012年 | 592篇 |
2011年 | 803篇 |
2010年 | 673篇 |
2009年 | 1012篇 |
2008年 | 987篇 |
2007年 | 577篇 |
2006年 | 568篇 |
2005年 | 538篇 |
2004年 | 535篇 |
2003年 | 623篇 |
2002年 | 600篇 |
2001年 | 661篇 |
2000年 | 464篇 |
1999年 | 558篇 |
1998年 | 504篇 |
1997年 | 405篇 |
1996年 | 455篇 |
1995年 | 520篇 |
1994年 | 497篇 |
1993年 | 392篇 |
1992年 | 377篇 |
1991年 | 281篇 |
1990年 | 263篇 |
1989年 | 419篇 |
1988年 | 225篇 |
1987年 | 248篇 |
1986年 | 248篇 |
1985年 | 648篇 |
1984年 | 525篇 |
1983年 | 409篇 |
1982年 | 489篇 |
1981年 | 615篇 |
1980年 | 245篇 |
1979年 | 184篇 |
1978年 | 189篇 |
1976年 | 156篇 |
1975年 | 185篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1969年 | 147篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Rogers S.K. Ruck D.W. Kabrisky M. Tarr G.L. 《Aerospace and Electronic Systems Magazine, IEEE》1990,5(9):17-19
A brief summary of research done at the Air Force Institute of Technology (AFIT) in the area of neural networks is provided. It has been shown that backpropagation, used for feedforward artificial neural networks, is just a degenerate version of an extended Kalman filter, and that networks can do about as well as the optimum statistical classification technique. A method of finding the importance of features for use by a neural network classifier has been determined. Techniques for using neural networks for image segmentation have been developed. In optical pattern recognition, techniques that allow the processing of real FLIR (forward-looking infrared) images with existing binary spatial light modulators have been devised. An optical direction of arrival detector applicable to laser illumination direction determination has been designed and tested; the design is similar to a fly's eye. Coated mirrors for the optical confocal Fabry-Perot interferometer have been designed, specified, fabricated, and installed. Significant progress has been made in the use of neural networks for processing multiple-feature sets for speech recognition 相似文献
22.
S. K. Solanki M. Fligge 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2002,29(12):L169-1940
The contribution to total solar irradiance variations by the magnetic field at the solar surface is estimated. Detailed models of the irradiance changes on the basis of magnetograms show that magnetic features at the solar surface account for over 90% of the irradiance variations on a solar rotation time scale and at least 70% on a solar cycle time scale. If the correction to the VIRGO record proposed by Fröhlich & Finsterle (2001) is accepted, then magnetic features at the solar surface are responsible for over 90% of the solar cycle irradiance variations as well. 相似文献
23.
Ahmad F. Frazer G.J. Kassam S.A. Amin M.G. 《IEEE transactions on aerospace and electronic systems》2004,40(1):206-220
A coarray-based near-field, wideband synthetic aperture beamformer using stepped-frequency signal synthesis and post-data acquisition processing is presented. While coarray techniques offer significant reduction in the number of array elements for a given angular resolution, the hybrid subarray-stepped frequency realization of wideband systems simplifies implementations and offers flexibility in beamforming. Proof of concept is provided using real data collected in an anechoic chamber for several pulse shapes and array weightings. 相似文献
24.
Fiber optic fence sensor developments 总被引:1,自引:0,他引:1
Many detection technologies have been employed for perimeter detection, to sensitize a barrier. These outdoor perimeter fence detection sensors must reliably detect intruders attempting to cut or climb the barrier, while ignoring the effects of environmental noise, including nearby activity. In recent conference proceedings, the new IntelliFIBER/spl trade/ fiber optic based product was introduced and compared with previous technologies. IntelliFIBER is designed to provide the advantages of a nonconductive "dielectric" cable sensor, e.g., resistance to electromagnetic interference and the ability to provide longer cable zones. It utilizes the proven processor of the Intelli-FLEX/spl trade/ triboelectric cable fence sensor, which provides relay or bidirectional networked serial communications and is compatible with its calibration module. IntelliFIBER also capitalizes on the Intelli-FLEX's adaptive detection algorithms that were developed through extensive field-testing, to provide exceptional immunity to environmental alarms while still detecting the skilled intruder. 相似文献
25.
The results of the experimental studies of damping characteristics of a composite torsion bar in the helicopter main rotor
hingeless hub are presented. A variant of their improvement is considered. 相似文献
26.
We present an algorithm for calculating the shells of revolution with the branching meridian using the triangular finite elements, the rigidity matrices of which are formed based on the vector method of displacement interpolation [1]. The correct kinematic and static conditions of shell conjugation on the line of their coupling have been developed. The shell structure consisting of a cylinder and adjacent shells is calculated under various conditions of support. 相似文献
27.
Lenz R. Schuler K. Younis M. Wiesbeck W. 《Aerospace and Electronic Systems Magazine, IEEE》2006,21(5):30-33
In April 2006, the TerraSAR-X satellite was launched. This paper describes the development of a novel and highly integrated, digitally-controlled active SAR system calibrator (DARC). It consists of both an active transponder path for absolute radiometric calibration and a calibrated receiver chain for antenna pattern evaluation of the satellite antenna. A total of 16 active transponder and receiver systems and 17 receiver-only systems will be fabricated for a calibration campaign in 2006. 相似文献
28.
The James Webb Space Telescope (JWST) project at the NASA, Goddard Space Flight Center (GSFC) is responsible for the development, launch, flight, and science operations for the telescope. The project is in phase B with its launch scheduled for no earlier than June 2013. The project is a partnership among NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The JWST mission team is fully in place, including major ESA and CSA subcontractors. This provides an overview of the planned JWST science, current architecture focusing on the instrumentation, and mission status, including technology developments, and risks. 相似文献
29.
Ulander L.M.H. Hellsten H. Stenstrom G. 《IEEE transactions on aerospace and electronic systems》2003,39(3):760-776
Exact synthetic aperture radar (SAR) inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, back-projection integration in time domain can also be used. This technique has the benefit of handling a general aperture geometry. In the past, however, back-projection has seldom been used due to heavy computational burden. We show that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization. By representing images in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms. The algorithm is applied to data from the airborne ultra-wideband CARABAS SAR and shown to give a reduction in processing time of two to three orders of magnitude. 相似文献
30.
以小车式起落架为例,利用ADAMS/Airc- raft建立了起落架滑跑试验仿真模型.给出了起落架滑跑运动方程,并进行了滑跑试验仿真分析。仿真结果与理论要求对比具有较好的一致性。 相似文献