全文获取类型
收费全文 | 6133篇 |
免费 | 20篇 |
国内免费 | 17篇 |
专业分类
航空 | 2906篇 |
航天技术 | 2189篇 |
综合类 | 23篇 |
航天 | 1052篇 |
出版年
2021年 | 58篇 |
2019年 | 32篇 |
2018年 | 123篇 |
2017年 | 83篇 |
2016年 | 86篇 |
2015年 | 45篇 |
2014年 | 166篇 |
2013年 | 167篇 |
2012年 | 176篇 |
2011年 | 254篇 |
2010年 | 170篇 |
2009年 | 277篇 |
2008年 | 346篇 |
2007年 | 164篇 |
2006年 | 139篇 |
2005年 | 171篇 |
2004年 | 156篇 |
2003年 | 209篇 |
2002年 | 117篇 |
2001年 | 191篇 |
2000年 | 102篇 |
1999年 | 136篇 |
1998年 | 171篇 |
1997年 | 119篇 |
1996年 | 142篇 |
1995年 | 187篇 |
1994年 | 169篇 |
1993年 | 105篇 |
1992年 | 147篇 |
1991年 | 75篇 |
1990年 | 54篇 |
1989年 | 130篇 |
1988年 | 59篇 |
1987年 | 61篇 |
1986年 | 56篇 |
1985年 | 163篇 |
1984年 | 128篇 |
1983年 | 116篇 |
1982年 | 135篇 |
1981年 | 177篇 |
1980年 | 59篇 |
1979年 | 61篇 |
1978年 | 55篇 |
1977年 | 45篇 |
1976年 | 47篇 |
1975年 | 41篇 |
1974年 | 41篇 |
1972年 | 35篇 |
1970年 | 36篇 |
1969年 | 34篇 |
排序方式: 共有6170条查询结果,搜索用时 125 毫秒
191.
Mismatched Filtering of Sonar Signals 总被引:1,自引:0,他引:1
A replica correlator (matched filter) is an optimum processor for a receiver employing a pulse of continuous wave (CW) signal in a white Gaussian noise background. In an active sonar, however, when the target of interest has low Doppler shift and is embedded in a high reverberation background, this is not so. High sidelobes of the correlator frequency response pass a significant portion of the signal contained in the mainlobe of the reverberation spectrum. In order to reduce the sidelobes of the correlator output spectrum and at the same time keep the increase in its 3 dB bandwidth to a small amount, we propose lengthening of the replica of the transmitted signal and weighting it by a Kaiser window. It is demonstrated that by extending the weighted replica by 50 percent compared with the transmitted signal, it is possible to reduce the sidelobe levels to at least 40 dB below the mainlobe peak, with the concomitant increase of the 3 dB band-width by less than 5 percent. The degradation in signal-to-noise ratio (SNR) performance for such a ?mismatched? filter receiver with respect to the matched filter is less than 1.5 dB. 相似文献
192.
Jovanovic M.M. Lee F.C. Chen D.Y. 《IEEE transactions on aerospace and electronic systems》1986,(2):138-145
Reverse-bias safe operating area (RBSOA) of high-power Darlington transistors is characterized using a 120 A/1000 V nondestructive reverse-bias second breakdown tester designed and fabricated at Virginia Polytechnic Institute and State University. Elaborate RBSOA characteristics are generated with different forward/reverse base drives and collector current levels. The effects of elevated case temperature and second-base drive on RBSOA of four-terminal Darlington devices are also discussed. 相似文献
193.
J. P. Cox J. C. Wheeler C. J. Hansen D. S. King A. N. Cox S. W. Hodson 《Space Science Reviews》1980,27(3-4):529-535
The radial pulsations of very luminous, low-mass models (L/M 104, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (strange) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars. 相似文献
194.
Magnetic Position and Orientation Tracking System 总被引:3,自引:0,他引:3
Raab F.H. Blood E.B. Steiner T.O. Jones H.R. 《IEEE transactions on aerospace and electronic systems》1979,(5):709-718
Three-axis generation and sensing of quasi-static magneticdipole fields provide information sufficient to determine both the position and orientation of the sensor relative to the source. Linear rotation transformations based upon the previous measurements are applied to both the source excitation and sensor output vectors, yielding quantities that are linearly propotional to small changes in the position and orientation. Changes are separated using linear combinations of sensor output vectors, transformed to the desired coordinate frame, and used to update the previous measurements. Practical considerations for a head-tracking application are discussed. 相似文献
195.
A track-while scan (TWS) algorithm is developed for targets in a clutter environment. The problem has been studied using only the position measurements [1, 5-8], but the simulation results have not been satisfactory. Modern processing techniques (FFT processor) ) in air traffic control and surveillance radar receivers provide both position and radial velocity. The radial velocity measurement may be conveniently used in the target-track correlation process, which will reduce the association ambiguity in the clutter environment. t. In the clear environment the algorithm using the position and radial velocity measurements has been treated in [3, 4]. A TWS algorithm, using both position and radial velocity measurements for targets in a clutter environment, is presented here. The algorithm obtained is nonlinear and adaptive. In order to evaluate the improvement due to radial velocity measurement a simulation has been performed on a digital computer. The algorithm was run with and without radial velocity measurements to compare its performances. An improvement was noted especially when the target path included an accelerated portion. 相似文献
196.
197.
An Adaptive Extended Kalman Filter for Target Image Tracking 总被引:1,自引:0,他引:1
198.
A Detection Algorithm for Optical Targets in Clutter 总被引:2,自引:0,他引:2
There is active interest in the development of algorithms for detecting weak stationary optical and IR targets in a heavy opticalclutter background. Often only poor detectability of low signal-to-noise ratio (SNR) targets is achieved when the direct correlation method is used. In many cases, this is partly obviated by using detection with correlated reference scenes [1, 2].This paper uses the experimentally justified assumption that most optical clutter can be modeled as a whitened Gaussian randomprocess with a rapidly space-varying mean and a more slowlyvarying covariance [2]. With this assumption, a new constant falsealarm rate (CFAR) detector is developed as an application of the classical generalized maximum likelihood ratio test of Neyman and Pearson. The final CFAR test is a dimensionless ratio. This test exhibits the desirable property that its probability of a false alarm(PFA) is independent of the covariance matrix of the actual noiseencountered. When the underlying noise processes are complex intime, similar considerations can yield a sidelobe canceler CFARdetection criterion for radar and communications. Performance analyses based on the probability of detection (PD)versus signal-to-noise ratio for several given fixed false alarm probabilities are presented. Finally these performance curves are validated by computer simulations of the detection process which use real image data with artificially implanted signals. 相似文献
199.
H. Borg L. -A. Holmgren B. Hultovist F. Cambou H. Rème A. Bahnsen G. Kremser 《Space Science Reviews》1978,22(5):511-535
Some preliminary new results are presented of the keV plasma experiment on GEOS-1. Electrons and ions have been observed to stream along the magnetic field lines in the dayside magnetosphere from the ionospheric side of the satellite towards the equatorial plane during magnetic storms, with streaming velocities corresponding to a kinetic energy of the order of a keV. The opposite streaming velocity has also been seen, but primarily in the ions only and with a smaller flux ratio for the two opposite directions along the field lines. The transition between the two opposite streaming directions, as seen by the satellite, has been found to occur even in a fraction of a second.In magnetic storm conditions azimuthal asymmetries in ion fluxes corresponding to electric field intensities of several tens of millivolts per meter have been observed. 相似文献
200.
本文研究DO28试验机飞行试验数据一致性检验及其误差校正。建立了由非线性运动学方程及内容广泛的校正环节构成的一致性检验模型,解决了大地风场重构、测量信号时间延迟估计、ψ转换及测量控制信号的校正。为DO28参数估计提供了可靠、准确的飞行试验数据。 相似文献