全文获取类型
收费全文 | 6021篇 |
免费 | 41篇 |
国内免费 | 18篇 |
专业分类
航空 | 2741篇 |
航天技术 | 2142篇 |
综合类 | 24篇 |
航天 | 1173篇 |
出版年
2021年 | 62篇 |
2019年 | 41篇 |
2018年 | 147篇 |
2017年 | 111篇 |
2016年 | 101篇 |
2015年 | 49篇 |
2014年 | 171篇 |
2013年 | 181篇 |
2012年 | 184篇 |
2011年 | 280篇 |
2010年 | 190篇 |
2009年 | 315篇 |
2008年 | 375篇 |
2007年 | 184篇 |
2006年 | 144篇 |
2005年 | 181篇 |
2004年 | 162篇 |
2003年 | 203篇 |
2002年 | 132篇 |
2001年 | 195篇 |
2000年 | 96篇 |
1999年 | 125篇 |
1998年 | 155篇 |
1997年 | 121篇 |
1996年 | 115篇 |
1995年 | 157篇 |
1994年 | 145篇 |
1993年 | 94篇 |
1992年 | 124篇 |
1991年 | 63篇 |
1990年 | 50篇 |
1989年 | 114篇 |
1988年 | 51篇 |
1987年 | 52篇 |
1986年 | 45篇 |
1985年 | 142篇 |
1984年 | 126篇 |
1983年 | 104篇 |
1982年 | 115篇 |
1981年 | 155篇 |
1980年 | 65篇 |
1979年 | 45篇 |
1978年 | 47篇 |
1977年 | 33篇 |
1976年 | 42篇 |
1975年 | 36篇 |
1974年 | 30篇 |
1973年 | 29篇 |
1972年 | 28篇 |
1971年 | 28篇 |
排序方式: 共有6080条查询结果,搜索用时 0 毫秒
291.
J. L. Kohl S. Fineschi R. Esser A. Ciaravella S. R. Cranmer L. D. Gardner R. Suleiman G. Noci A. Modigliani 《Space Science Reviews》1999,87(1-2):233-236
Ultraviolet emission line profiles have been measured on 15-29 September 1997 for H I 1216 Å, O VI 1032, 1037 Å and Mg X 625 Å in a polar coronal hole, at heliographic heights ? (in solar radii) between 1.34 and 2.0. Observations of H I 1216 Å and the O VI doublet from January 1997 for ? = 1.5 to 3.0 are provided for comparison. Mg X 625 Å is observed to have a narrow component at ? = 1.34 which accounts for only a small fraction of the observed spectral radiance, and a broad component that exists at all observed heights. The widths of O VI broad components are only slightly larger than those for H I at ? = 1.34, but are significantly larger at ? = 1.5 and much larger for ? > 1.75. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least ? = 2.0, but never reach the values of O VI. 相似文献
292.
The remote sensing of comets in the ultraviolet bandpass has been a valuable tool for studying the structure, composition, variability, and physical processes at work in cometary comae. By extension, these studies of comae have revealed key insights into the composition of cometary nuclei. Here we briefly review the ultraviolet studies of comets, and then take a look toward the future of such work as anticipated by the advent of several key new instruments. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
293.
High resolution radar clutter statistics 总被引:6,自引:0,他引:6
Anastassopoulos Lampropoulos G.A. Drosopoulos A. Rey N. 《IEEE transactions on aerospace and electronic systems》1999,35(1):43-60
The generalized compound probability density function (GC-pdf) is presented for modeling high resolution radar clutter. In particular, the model is used to describe deviation of the speckle component from the Rayleigh to Weibull or other pdfs with longer tails. The GC-pdf is formed using the generalized gamma (GΓ) pdf to describe both the speckle and the modulation component of the radar clutter. The proposed model is analyzed and thermal noise is incorporated into it. The validation of the GC-pdf with real data is carried out employing the statistical moments as well as goodness-of-fit tests. A large variety of experimental data is used for this purpose. The GC-pdf outperforms the K-pdf in modeling high resolution radar clutter and reveals its structural characteristics 相似文献
294.
Ovshinsky S.R. Dhar S.K. Fetcenko M.A. Corrigan D.A. Reichman B. Young K. Fierro C. Venkatesan S. Gifford P. Koch J. 《Aerospace and Electronic Systems Magazine, IEEE》1999,14(5):17-23
While Ovonic NiMH batteries are already in high volume commercial production for portable applications, advances in materials technology have enabled performance improvements in specific energy (100 Wh/kg), specific power (600-1000 W/kg), high temperature operation, charge retention, and voltage stability. Concurrent with technology advances, Ovonic NiMH batteries have established performance and commercial milestones in electric vehicles, hybrid electric vehicles, as well as scooter, motorcycle and bicycle applications. As important as these advances, significant manufacturing cost reductions have also occurred which allow continued growth of NiMH technology. In this paper, advances in performance, applications and cost reduction are discussed with particular emphasis on the improved proprietary metal hydride and nickel hydroxide materials that make such advances possible 相似文献
295.
Jao J.K. Lee C.F. Ayasli S. Haywiser E.J. 《Aerospace and Electronic Systems Magazine, IEEE》1999,14(6):5-9
In July and October 1995, a large-scale airborne SAR experiment was conducted in the Yuma Proving Ground, Yuma, Arizona, to investigate ground penetration radar phenomenology and buried target detection. This paper describes the Yuma experiment and measurement results for many tactical, utility, and environmental targets deployed in the test 相似文献
296.
Pulsone N.B. Raghavan R.S. 《IEEE transactions on aerospace and electronic systems》1999,35(3):903-916
Coherent signal detection in non-Gaussian interference is presently of interest in adaptive array applications. Conventional array detection algorithms inherently model the interference with a multivariate Gaussian random vector. However, non-Gaussian interference models are also under investigation for applications where the Gaussian assumption may not be appropriate. We analyze the performance of an adaptive array receiver for signal detection in interference modeled with a non-Gaussian distribution referred to as a spherically invariant random vector (SIRV). We first motivate this interference model with results from radar clutter measurements collected in the Mountain Top Program. Then we develop analytical expressions for the probability of false alarm and the probability of detection for the adaptive array receiver. Our analysis shows that the receiver has constant false alarm rate (CFAR) performance with respect to all the interference parameters. Some illustrative examples are included that compare the detection performance of this CFAR receiver with a receiver that has prior knowledge of the interference parameters 相似文献
297.
A two-way, noncoherent spacecraft navigation technique has been developed to provide velocity accuracy comparable to that available from a deep space coherent transponder (0.1 mm/s). The technique is compatible with the normal tracking procedures of the Deep Space Network (DSN) ground stations and does not require the use of a highly stable oscillator on-board the spacecraft. The measurement technique, potential error sources, and test results obtained with prototype hardware are described 相似文献
298.
The major radio aids to air navigation are listed. Underlined are those whose signal format is standardized by the International Civil Aviation Organization (ICAO) and they have now all been the subject of the IEEE Aerospace and Electronic Systems Society Pioneer Award, as follows: airborne direction finder/nondirectional beacon (ADF/NDB), Busignies and Moseley, 1959; VHF omnidirectional range (VOR), Stuart, 1962; instrument landing system (ILS), Kramar 1964, Alford 1965; air traffic control radar beacon system (ATCRBS), Williams, Bowden, and Harris, 1973; distance measuring equipment (DME), Dodington, 1980. A brief history of the development of the distance measuring equipment, which also formed the basis for an IEEE National Aerospace Electronics Conference luncheon address, is given. 相似文献
299.
The heating of solar coronal loops by the resonant absorption or phase-mixing of incident wave energy is investigated in the framework of 3D nonlinear magnetohydrodynamics (MHD) by means of numerical simulations. 相似文献
300.
T. S. Bastian 《Space Science Reviews》1994,68(1-4):261-274
Radio and X-ray observations of stellar flares provide the most direct probes of energy relaase particle acceleration, and energy transport on stars other than the Sun. In this review, the observational basis for our understanding of the flare phenomenon on other stars is briefly described and outstanding interpretive and theoretical issues are discussed. I shall confine my attention to objects which are solar-like, to the extent that they possess deep convective envelopes and display activity which is presumed to be magnetic in origin. These include pre-main sequence objects, classical flare stars, and close binaries. Future directions are briefly discussed. 相似文献