全文获取类型
收费全文 | 7754篇 |
免费 | 18篇 |
国内免费 | 33篇 |
专业分类
航空 | 3397篇 |
航天技术 | 2784篇 |
综合类 | 32篇 |
航天 | 1592篇 |
出版年
2021年 | 77篇 |
2019年 | 47篇 |
2018年 | 203篇 |
2017年 | 147篇 |
2016年 | 144篇 |
2015年 | 55篇 |
2014年 | 200篇 |
2013年 | 247篇 |
2012年 | 228篇 |
2011年 | 327篇 |
2010年 | 235篇 |
2009年 | 395篇 |
2008年 | 429篇 |
2007年 | 247篇 |
2006年 | 183篇 |
2005年 | 232篇 |
2004年 | 226篇 |
2003年 | 262篇 |
2002年 | 176篇 |
2001年 | 260篇 |
2000年 | 149篇 |
1999年 | 176篇 |
1998年 | 195篇 |
1997年 | 156篇 |
1996年 | 181篇 |
1995年 | 242篇 |
1994年 | 216篇 |
1993年 | 108篇 |
1992年 | 170篇 |
1991年 | 58篇 |
1990年 | 65篇 |
1989年 | 154篇 |
1988年 | 50篇 |
1987年 | 52篇 |
1986年 | 68篇 |
1985年 | 216篇 |
1984年 | 172篇 |
1983年 | 135篇 |
1982年 | 157篇 |
1981年 | 214篇 |
1980年 | 62篇 |
1979年 | 44篇 |
1978年 | 47篇 |
1977年 | 45篇 |
1976年 | 39篇 |
1975年 | 37篇 |
1974年 | 42篇 |
1972年 | 33篇 |
1971年 | 30篇 |
1970年 | 32篇 |
排序方式: 共有7805条查询结果,搜索用时 15 毫秒
131.
Yu. F. Gortyshov V. M. Gureev R. Sh. Misbakhov I. F. Gumerov A. P. Shaikin 《Russian Aeronautics (Iz VUZ)》2009,52(4):488-490
We present the results of experimental studies of the fuel hydrogen additive influence on the characteristics of a gas-piston
engine converted for operation by natural gas under changes of an ignition advance angle (IAA). The results of investigations
were used to determine the influence of the hydrogen additive on the effective engine efficiency and fuel consumption under
IAA changes. 相似文献
132.
133.
Drumheller D.M. Titlebaum E.L. 《IEEE transactions on aerospace and electronic systems》1991,27(1):2-10
The problem of determining the cross-correlation properties of signals based on algebraically constructed Costas arrays is addressed by examining the discrete cross-correlation of the algebraically constructed Costas arrays for a given construction and dimension. Finding two arrays that minimally correlate implies that the signals based on these arrays also minimally correlate. The properties of finite fields are reviewed, and the major algebraic constructions for Costas arrays are presented, i.e. the Welch construction and the Golomb construction. The discrete cross-correlation properties of the Costas arrays are derived for arrays of the same dimension derived from the same construction. The use of Costas arrays in the signal design problem is discussed, and examples are given to show the cross-correlation of the signals based on the algebraically constructed arrays 相似文献
134.
Calculation of optimum gain for minimum distortion due to A/D (analog-to-digital) conversion requires the estimation of the input signal strength. To use a common AGC (automatic gain control) for both the I/Q (in-phase and quadrature) signals, it is efficient to estimate the input signal strength using the quantized A/D output from both channels. Assuming a Gaussian input, the relationship between σ of the input of the A/D converter and E (|x |+|y |) and E (max(|x |,|y |)+1/2 min (|x |,|y |)) for t quantized I/Q output x and y is derived. Numerical results obtained using the derived expression and the statistical data obtained through simulation show excellent agreement. It is concluded that, because of its simplicity, the cubic equation obtained by fitting the numerical results should be useful 相似文献
135.
Beamspace ML bearing estimation incorporating low-angle geometry 总被引:1,自引:0,他引:1
A problem in low-angle radar tracking, namely, bearing estimation in the presence of a strong specular multipath component that arrives within the beamwidth of the direct path signal, is studied. Three-dimensional beamspace domain maximum likelihood (3D-BDML) is a computationally simple ML bearing estimation algorithm applicable in this scenario which operates in a 3-D beamspace. A variation of 3D-BDML incorporating the multipath geometry as a priori information is presented. In symmetric 3D-BDML the pointing angle of the center beam is equal to the bisector angle between the direct path ray and the image ray, which may be estimated a priori given only the radar height and the target range. The effect of the inclusion of a priori information on the performance of 3D-BDML is analyzed in terms of the dependence on the relative phase difference between the direct and specular path signals, the sensitivity to error in the bisector angle estimate, and the results of operation when no specular multipath component is present in the data. In addition, computationally simple schemes for coherently incorporating multifrequency data into 3D-BDML are investigated 相似文献
136.
Varghese M. Fuchs A. Mukundan R. 《IEEE transactions on aerospace and electronic systems》1991,27(5):784-796
The authors attempt to integrate two areas in robotics that are maturing rapidly: the control of kinematically redundant rigid robots and feedback linearization. Kinematically redundant robots have several advantages over nonredundant robot arms. The extra degrees of freedom are used to reach around or between obstacles. Unlike nonredundant rigid link robots that do not exhibit zero dynamics, the self motions of kinematically redundant robots can be used for torque optimization or avoiding obstacles. It is shown that induced self motions can be chaotic. The main contribution is to demonstrate chaos in cases of feedback linearization when the decoupling matrix is nonsquare and there are less outputs than inputs 相似文献
137.
Cantrell B.H. Gordon W.B. Trunk G.V. 《IEEE transactions on aerospace and electronic systems》1981,(2):213-221
The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface. 相似文献
138.
The performance of electronically scanned radar systems is evaluated for clear and barrage jamming environments. Radar figures of merit in jamming are derived for search and track modes and are directly related to antenna pattern relative sidelobe levels. A random sidelobe model is analyzed with various numbers of independent jammers. Probability distributions of detection probability are derived to determine detection performance versus average jamming level and number of jammers. 相似文献
139.
140.
The effect of a logarithmic receiver on the detection performance of a radar has been determined by Green by numerical computations for up to 100 pulses integrated. It was further suggested that the loss in decibels as compared to a square-law detector would appear to be proportional to the logarithm of the number of pulses integrated. In this correspondence we show that this ?conjecture? is false, and that the additional loss when N becomes very large approaches a constant value of 2.2 dB. 相似文献