全文获取类型
收费全文 | 7023篇 |
免费 | 25篇 |
国内免费 | 24篇 |
专业分类
航空 | 3516篇 |
航天技术 | 2337篇 |
综合类 | 185篇 |
航天 | 1034篇 |
出版年
2021年 | 51篇 |
2019年 | 42篇 |
2018年 | 104篇 |
2017年 | 70篇 |
2016年 | 67篇 |
2014年 | 123篇 |
2013年 | 159篇 |
2012年 | 168篇 |
2011年 | 241篇 |
2010年 | 159篇 |
2009年 | 280篇 |
2008年 | 280篇 |
2007年 | 175篇 |
2006年 | 138篇 |
2005年 | 153篇 |
2004年 | 166篇 |
2003年 | 213篇 |
2002年 | 233篇 |
2001年 | 277篇 |
2000年 | 140篇 |
1999年 | 165篇 |
1998年 | 201篇 |
1997年 | 139篇 |
1996年 | 194篇 |
1995年 | 236篇 |
1994年 | 225篇 |
1993年 | 127篇 |
1992年 | 169篇 |
1991年 | 83篇 |
1990年 | 86篇 |
1989年 | 175篇 |
1988年 | 75篇 |
1987年 | 78篇 |
1986年 | 77篇 |
1985年 | 211篇 |
1984年 | 189篇 |
1983年 | 148篇 |
1982年 | 164篇 |
1981年 | 217篇 |
1980年 | 67篇 |
1979年 | 60篇 |
1978年 | 79篇 |
1977年 | 54篇 |
1976年 | 50篇 |
1975年 | 70篇 |
1974年 | 55篇 |
1972年 | 62篇 |
1971年 | 57篇 |
1970年 | 43篇 |
1969年 | 51篇 |
排序方式: 共有7072条查询结果,搜索用时 348 毫秒
361.
McGoey-Smith A.D. Vant M.R. 《IEEE transactions on aerospace and electronic systems》1992,28(3):666-674
When the basic step transform algorithm is used to compress synthetic-aperture radar (SAR) signals in azimuth, the linear FM rate and sampling rate must satisfy certain tight constraints. In practice, these constraints cannot be satisfied and errors are introduced into the compressed SAR image. A modification is described of the basic step transform which incorporates interpolation and resampling into the algorithm. These changes allow the removal of the constraints and make the step transform more useful for the compression of real data. An autofocusing capability is also included, without introducing much additional complexity 相似文献
362.
363.
介绍了俄罗斯铝锂合金铸锭熔炼和铸造方法的发展和改进,评述了铝锂合金熔铸特点,熔体氧化,防护添加剂,熔剂和氩气防护,氧、碳、氮和氢等非金属夹杂,铸造裂纹(冷裂和热裂)敏感性,结晶状态,不均匀组织等方面的研究结果。 相似文献
364.
High-frequency-link power conversion and distribution based on a resonant inverter has been recently proposed. The design of several topologies is reviewed and a simple approximate design procedure is developed for the phase-controlled parallel-loaded resonant inverter. This design procedure seeks to ensure the expected benefits of resonant conversion and is verified by data from a laboratory 2.5 kVA, 20-kHz converter. A simple phasor analysis is introduced as a useful approximation for design purposes. Experimental results show that under transient conditions such as load short-circuit, a reversal of the expected commutation sequence is possible. This should be accounted for in the design of the power circuit, or prevented by the design of the controller 相似文献
365.
Frequency estimation techniques for high dynamic trajectories 总被引:7,自引:0,他引:7
Vilnrotter V.A. Hinedi S. Kumar R. 《IEEE transactions on aerospace and electronic systems》1989,25(4):559-577
A comparison is presented of four different estimation techniques applied to the problem of continuously estimating the rapidly varying parameters of a sinusoidal signal, observed in the presence of additive noise. Frequency estimates are emphasized, although phase and/or frequency rate are also estimated by some of the algorithms. These parameters are related to the velocity, position, and acceleration of the maneuvering receiver or transmitter. Estimated performance at low carrier-to-noise ratios and high dynamics is investigated for the purpose of determining the useful operating range of an approximate maximum likelihood estimator, an extended Kalman filter, a cross-product automatic frequency loop and a phase-locked loop. Numerical simulations are used to evaluate performance while tracking a common trajectory exhibiting high dynamics 相似文献
366.
367.
S. Hayakawa J. Kuijpers J. Kleczek Boris Valnicek L. Kresak H. R. De Ruiter 《Space Science Reviews》1991,57(1-2):189-192
368.
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
369.
有限体积法的分析与改进 总被引:10,自引:0,他引:10
本文对以Jameson为代表提出的常用的一种有限体积法进行了较深入的分析,指出了它的适用范围,网格对计算精度的影响并提出了改进措施。算例表明本文提出的分析与改进措施是正确、有效的。 相似文献
370.
A white light coronagraph was launched into orbit aboard the space shuttle OV 103 (Discovery) on 7 April 1993. This device was one of two instruments included in the SPARTAN 201-1 payload, a completely autonomous sub-satellite deployed from the shuttle for a period of about 47 hours. The WLC system is an externally occulted coronagraph system which incorporates a rotating half-wave plate polarimeter, and the image data is used to infer the brightness, the polarized brightness and the degree of polarization of the white light emission from the solar corona. These data are in turn used to infer estimates of the K- and F-coronal brightnesses and density distributions. We shall present preliminary results of the electron density estimate in the coronal streamer and hole region and describe the methods employed.affiliated to USRA 相似文献