全文获取类型
收费全文 | 5009篇 |
免费 | 472篇 |
国内免费 | 234篇 |
专业分类
航空 | 2765篇 |
航天技术 | 1576篇 |
综合类 | 254篇 |
航天 | 1120篇 |
出版年
2023年 | 38篇 |
2022年 | 115篇 |
2021年 | 115篇 |
2020年 | 97篇 |
2019年 | 94篇 |
2018年 | 129篇 |
2017年 | 148篇 |
2016年 | 102篇 |
2015年 | 130篇 |
2014年 | 188篇 |
2013年 | 186篇 |
2012年 | 180篇 |
2011年 | 275篇 |
2010年 | 212篇 |
2009年 | 218篇 |
2008年 | 280篇 |
2007年 | 176篇 |
2006年 | 158篇 |
2005年 | 165篇 |
2004年 | 136篇 |
2003年 | 179篇 |
2002年 | 145篇 |
2001年 | 141篇 |
2000年 | 116篇 |
1999年 | 132篇 |
1998年 | 128篇 |
1997年 | 115篇 |
1996年 | 103篇 |
1995年 | 113篇 |
1994年 | 122篇 |
1993年 | 74篇 |
1992年 | 84篇 |
1991年 | 52篇 |
1990年 | 48篇 |
1989年 | 79篇 |
1988年 | 50篇 |
1987年 | 35篇 |
1986年 | 35篇 |
1985年 | 114篇 |
1984年 | 81篇 |
1983年 | 79篇 |
1982年 | 59篇 |
1981年 | 104篇 |
1980年 | 25篇 |
1979年 | 27篇 |
1978年 | 33篇 |
1977年 | 25篇 |
1975年 | 39篇 |
1972年 | 33篇 |
1971年 | 25篇 |
排序方式: 共有5715条查询结果,搜索用时 31 毫秒
271.
272.
273.
Optimal speckle reduction in polarimetric SAR imagery 总被引:9,自引:0,他引:9
Speckle is a major cause of degradation in synthetic aperture radar (SAR) imagery. With the availability of fully polarimetric SAR data, it is possible to use the three complex elements (HH , HV , VV ) of the polarimetric scattering matrix to reduce speckle. The optimal method for combining the elements of the scattering matrix to minimize image speckle is derived, and the solution is shown to be a polarimetric whitening filter (PWF). A simulation of spatially correlated, K -distributed, fully polarimetric clutter is then used to compare the PWF with other, suboptimal speckle-reduction methods. Target detection performance of the PWF, span, and single-channel |HH |2 detectors is compared with that of the optimal polarimetric detector (OPD). A novel, constant-false-alarm-rate (CFAR) detector (the adaptive PWF) is as a simple alternative to the OPD for detecting targets in clutter. This algorithm estimates the polarization covariance of the clutter, uses the covariance to construct the minimum-speckle image, and then tests for the presence of a target. An exact theoretical analysis of the adaptive PWF is presented; the algorithm is shown to have detection performance comparable with that of the OPD 相似文献
274.
275.
276.
S. Hayakawa J. Kuijpers J. Kleczek Boris Valnicek L. Kresak H. R. De Ruiter 《Space Science Reviews》1991,57(1-2):189-192
277.
The proposed KAAD (knowledge-based automated air defense) system demonstrates a man-machine environment for airspace defense systems. When the unknown aircraft is hostile, a threat rating and response methods are generated by the system. It serves as a double-check decision-making system for a war control center. In addition to this application, the KAAD system can also be a useful tool as a training program for the war controller. The capabilities of the system are limited due to the shortage of knowledge resources. It requires communications among war controllers and air fighter pilots to organize a practical knowledge base. It is shown that the KAAD system can be combined with an automated ATC (air traffic control) system to become a practical system for air defense applications 相似文献
278.
279.
B. Holback S. -E. Jansson L. Åhlén G. Lundgren L. Lyngdal S. Powell A. Meyer 《Space Science Reviews》1994,70(3-4):577-592
The Wave Experiment, F4, on the Swedish/German satelliteFreja, is designed to measure the electric wave fields up to 4 MHz, the magnetic wave fields up to 16 kHz and the plasma density and its relative variations up to 2 kHz. Six wave signals and four density probe signals can be measured simultaneously. The wave forms of all signals are transmitted to ground without any analysis onboard. The limited TM allocation does not allow continuous sampling of the wave signals, so normally the measurements are made in snapshots of various lengths dependent on sampling frequency, etc. Continuous sampling can be made for shorter time periods by using a 6 Mbyte memory as a buffer. 相似文献
280.
伪距解除相关法在GPS/SINS紧组合系统中的应用研究 总被引:3,自引:0,他引:3
介绍了伪距解除相关法的原理 ,设计了一种加权平均跟踪误差估计器 ,用以估计 GPS接收机码环跟踪误差。研究结果表明 ,与未对伪距测量值进行修正时相比 ,应用伪距解除相关技术可以减小组合系统的导航误差 ,提高系统的导航精度 相似文献