全文获取类型
收费全文 | 20473篇 |
免费 | 326篇 |
国内免费 | 489篇 |
专业分类
航空 | 11187篇 |
航天技术 | 6035篇 |
综合类 | 662篇 |
航天 | 3404篇 |
出版年
2021年 | 187篇 |
2018年 | 253篇 |
2016年 | 212篇 |
2014年 | 519篇 |
2013年 | 616篇 |
2012年 | 531篇 |
2011年 | 745篇 |
2010年 | 573篇 |
2009年 | 886篇 |
2008年 | 955篇 |
2007年 | 545篇 |
2006年 | 527篇 |
2005年 | 507篇 |
2004年 | 514篇 |
2003年 | 620篇 |
2002年 | 542篇 |
2001年 | 670篇 |
2000年 | 446篇 |
1999年 | 587篇 |
1998年 | 574篇 |
1997年 | 419篇 |
1996年 | 481篇 |
1995年 | 521篇 |
1994年 | 519篇 |
1993年 | 439篇 |
1992年 | 391篇 |
1991年 | 307篇 |
1990年 | 280篇 |
1989年 | 437篇 |
1988年 | 218篇 |
1987年 | 273篇 |
1986年 | 233篇 |
1985年 | 646篇 |
1984年 | 522篇 |
1983年 | 417篇 |
1982年 | 490篇 |
1981年 | 614篇 |
1980年 | 245篇 |
1979年 | 188篇 |
1978年 | 189篇 |
1977年 | 146篇 |
1976年 | 156篇 |
1975年 | 187篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
本文提出一种近似随机选取法,计算一维非定常的高温平衡流动。采用稀疏激波替代等熵稀疏波,选用拟合经验方程计算高温热力学性质,并利用切割迭代方法加快收敛,从而计算大为简化,并有很好精度。 相似文献
992.
993.
Nonbinary m -sequences (maximal length sequences) for spread-spectrum communication systems that have a two-level autocorrelation are presented. The autocorrelation function of an m -sequence over the Galois field of q elements GF(q ), where q =p k, for p a prime and k an integer greater than 1, is developed and shown to be bilevel when the elements of GF(q ) are expressed as elements of a vector space over the p th roots of unity 相似文献
994.
在计算机系统的接地系统中,一些问题至今尚无定论,本文结合几个计算机系统安装调试的经验,及现有的ATC雷达模拟系统实体,就机房屏蔽地线、防雷击地线、交流地线、系统地线给予介绍和简单的论述,并给出一些经验值,还说明了直流地悬空的危害和地桩的埋设方法。另外,简要介绍了机房的防火设施,以便有关单位参考。 相似文献
995.
W. Hermsen 《Space Science Reviews》1989,49(1-2):17-39
The correlation between diffuse galactic gamma rays and gas tracers is studied using the final COS-B database and H i and CO surveys covering the entire galactic plane. A good quantitative fit to the gamma rays is obtained, with a small galacto-centric gradient in the gamma-ray emissivity per hydrogen atom. The average ratio of H2 column density to integrated CO temperature is determined, the best estimate being (2.3 ± 0.3) × 102 molecules cm–2 (K km s–1)–1. Strictly taken, this value is an upper limit. The corresponding mass of molecular hydrogen in the inner galaxy, derived using both 1st and 4th quadrants, is 1.0 × 109
M
.The softer gamma-ray spectrum towards the inner galaxy found in previous work can be attributed to a steeper emissivity gradient at low energies and/or to a softer gamma-ray spectrum of the emission distributed like molecular gas. A steeper emissivity gradient at low energies could be related to cosmic-ray spectral variations in the Galaxy, to different distributions of cosmic-ray electrons and nuclei, or to a contribution from discrete sources. A softer spectrum for the emission associated with molecular clouds may be physically related to the clouds themselves (i.e., cosmic-ray spectral variations) or to an associated discrete source distribution.New results on the temporal and spectral characteristics of the high-energy (50 MeV to 5 GeV) gammaray emission from the Vela pulsar are presented. The whole pulsed flux is found to exhibit long-term variability. Five discrete emission regions within the pulsar lightcurve have been identified, with the spectral characteristics and long-term behaviour being different. These characteristics differ significantly from those reported earlier for the Crab pulsar. However, geometrical pulsar models have been proposed (e.g., Morini, 1983; Smith, 1986) which could explain many of these features. 相似文献
996.
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
997.
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
998.
999.
1000.