全文获取类型
收费全文 | 9832篇 |
免费 | 658篇 |
国内免费 | 497篇 |
专业分类
航空 | 5028篇 |
航天技术 | 3068篇 |
综合类 | 446篇 |
航天 | 2445篇 |
出版年
2023年 | 47篇 |
2022年 | 120篇 |
2021年 | 214篇 |
2020年 | 133篇 |
2019年 | 149篇 |
2018年 | 316篇 |
2017年 | 244篇 |
2016年 | 219篇 |
2015年 | 197篇 |
2014年 | 347篇 |
2013年 | 384篇 |
2012年 | 393篇 |
2011年 | 529篇 |
2010年 | 480篇 |
2009年 | 571篇 |
2008年 | 552篇 |
2007年 | 396篇 |
2006年 | 301篇 |
2005年 | 321篇 |
2004年 | 265篇 |
2003年 | 305篇 |
2002年 | 291篇 |
2001年 | 317篇 |
2000年 | 229篇 |
1999年 | 261篇 |
1998年 | 255篇 |
1997年 | 185篇 |
1996年 | 231篇 |
1995年 | 265篇 |
1994年 | 241篇 |
1993年 | 157篇 |
1992年 | 187篇 |
1991年 | 80篇 |
1990年 | 74篇 |
1989年 | 156篇 |
1988年 | 77篇 |
1987年 | 64篇 |
1986年 | 72篇 |
1985年 | 193篇 |
1984年 | 149篇 |
1983年 | 109篇 |
1982年 | 120篇 |
1981年 | 217篇 |
1980年 | 51篇 |
1979年 | 47篇 |
1978年 | 51篇 |
1977年 | 42篇 |
1975年 | 50篇 |
1974年 | 40篇 |
1970年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
271.
Lee F.C. Carter R.A. Fang Z.D. 《IEEE transactions on aerospace and electronic systems》1983,(2):274-287
The stability and dynamic performances of a buck/boost regulator employing a current-injected control are examined. Small-signal models for the power state, the multiloop error processor, and the duty-cycle pulse modulator are developed. The error-processor model which incorporates the current-injected loop, the dc loop, and the compensation network permits evaluation of the effects of each individual control loop and their combined efforts toward shaping the performance characteristics of the closed-loop system. Comparisons are made between this modeling approach and earlier approaches. Some important yet subtle dissimilarities are discussed. This model predicts the constant-frequency 50-percent duty-cycle instability which is inherent to the current-injected control. 相似文献
272.
Rijsdijk F.B. van der Spek G.A. 《IEEE transactions on aerospace and electronic systems》1978,(2):226-236
For a planar-array antenna with a monopulse feed horn, this study describes a simple algorithm for the determination of the direction of target echoes. Antenna pattern measurements of the array indicate that the direction sines of a received wavefront can be independently obtained with one simple relation between a normalized difference channel output and a direction sine. This paper determines the accuracy of the algorithm. 相似文献
273.
S. J. Bauer L. H. Brace D. M. Hunten D. S. Intriligator W. C. Knudsen A. F. Nagy C. T. Russell F. L. Scarf J. H. Wolfe 《Space Science Reviews》1977,20(4):413-430
The current state of knowledge of the chemistry, dynamics and energetics of the upper atmosphere and ionosphere of Venus is reviewed together with the nature of the solar wind-Venus interaction. Because of the weak, though perhaps not negligible, intrinsic magnetic field of Venus, the mutual effects between these regions are probably strong and unique in the solar system. The ability of the Pioneer Venus Bus and Orbiter experiments to provide the required data to answer the questions outstanding is discussed in detail. 相似文献
274.
环境专业教学体系结构改革探索——以就业市场需求为导向 总被引:4,自引:0,他引:4
通过对西安航专2006与2007年环境监测与治理技术专业毕业生就业情况的对比分析,认为目前环境类专业的就业形势发生良性转变,需求范围增加,同时就业单位对毕业生的专业技术能力、技术管理能力等也提出了更高的要求。据此,我们从专业发展方向、课程体系结构以及实验实训三个方面进行建设与调整来满足就业单位的需求。 相似文献
275.
276.
The main goal of the paper is to demonstrate the presence of chaotic trajectories in the gaseous disk of a real spiral galaxy. As an example we have chosen NGC 3631. First, we show the stationarity of the 3-D velocity field restored from the observed line-of-sight velocity field of the gaseous disk. That allows to analyse behaviour of the trajectories of the fluid particles (gas clouds) in the disk, calculating the corresponding observed streamlines. We estimate the Lyapunov characteristic numbers using their independence of the metrics and show the existence of chaotic trajectories outside the vortices which are present in the velocity field, and in the vicinity of the saddle point. Related spectra of the stretching numbers for some trajectories are also calculated. 相似文献
277.
Shnidman D.A. Toumodge S.S. 《IEEE transactions on aerospace and electronic systems》2002,38(3):1023-1037
In radar systems, sidelobe blanking (SLB) is used to mitigate impulsive interference that enters the radar through sidelobes of the main antenna. SLB employs an auxiliary antenna channel with the output being compared with that of the main antenna channel and a decision is then made as to whether or not to blank the main channel output. SLB performance determination involves the evaluation of several probability functions. Based on the classical Maisel SLB architecture, this work extends previous performance results, in which detection was limited to the case of a single radar pulse with either Marcum or Swerling I target fluctuation. Probability expressions have been generalized to include both an arbitrary number of integrated pulses and target fluctuation models based on the gamma distribution. The Swerling fluctuation models are all special cases of the gamma distribution. Results are derived in terms of two generalized probability functions, one for detection and the other for blanking. With these generalized probability functions, the SLB design and performance results can be determined. Examples are presented and discussed. 相似文献
278.
从中国的国情入笔,论速了农业航空在发展国民经济中的重要作用;农业航空采用农业专用飞机是必然趋势;指出农业航空滑坡问题亟待解决。 相似文献
279.
J. Scudder F. Hunsacker G. Miller J. Lobell T. Zawistowski K. Ogilvie J. Keller D. Chornay F. Herrero R. Fitzenreiter D. Fairfield J. Needell D. Bodet J. Googins C. Kletzing R. Torbert J. Vandiver R. Bentley W. Fillius C. McIlwain E. Whipple A. Korth 《Space Science Reviews》1995,71(1-4):459-495
HYDRA is an experimental hot plasma investigation for the POLAR spacecraft of the GGS program. A consortium of institutions has designed a suite of particle analyzers that sample the velocity space of electron and ions between 2 keV/q – 35 keV/q in three dimensions, with a routine time resolution of 0.5 s. Routine coverage of velocity space will be accomplished with an angular homogeneity assumption of 16°, appropriate for subsonic plasmas, but with special 1.5° resolution for electrons with energies between 100 eV and 10 keV along and opposed to the local magnetic field. This instrument produces 4.9 kilobits s–1 to the telemetry, consumes on average 14 W and requires 18.7 kg for deployment including its internal shielding. The scientific objectives for the polar magnetosphere fall into four broad categories: (1) those to define the ambient kinetic regimes of ions and electrons; (2) those to elucidate the magnetohydrodynamic responses in these regimes; (3) those to assess the particle populations with high time resolution; and (4) those to determine the global topology of the magnetic field. In thefirst group are issues of identifying the origins of particles at high magnetic latitudes, their energization, the altitude dependence of the forces, including parallel electric fields they have traversed. In thesecond group are the physics of the fluid flows, regimes of current, and plasma depletion zones during quiescent and disturbed magnetic conditions. In thethird group is the exploration of the processes that accompany the rapid time variations known to occur in the auroral zone, cusp and entry layers as they affect the flow of mass, momentum and energy in the auroral region. In thefourth class of objectives are studies in conjunction with the SWE measurements of the Strahl in the solar wind that exploit the small gyroradius of thermal electrons to detect those magnetic field lines that penetrate the auroral region that are directly open to interplanetary space where, for example, the Polar Rain is observed. 相似文献
280.