全文获取类型
收费全文 | 18281篇 |
免费 | 34篇 |
国内免费 | 126篇 |
专业分类
航空 | 9880篇 |
航天技术 | 5470篇 |
综合类 | 251篇 |
航天 | 2840篇 |
出版年
2021年 | 159篇 |
2018年 | 206篇 |
2016年 | 150篇 |
2014年 | 443篇 |
2013年 | 525篇 |
2012年 | 432篇 |
2011年 | 581篇 |
2010年 | 401篇 |
2009年 | 747篇 |
2008年 | 797篇 |
2007年 | 370篇 |
2006年 | 437篇 |
2005年 | 403篇 |
2004年 | 433篇 |
2003年 | 537篇 |
2002年 | 484篇 |
2001年 | 567篇 |
2000年 | 369篇 |
1999年 | 452篇 |
1998年 | 431篇 |
1997年 | 328篇 |
1996年 | 395篇 |
1995年 | 474篇 |
1994年 | 457篇 |
1993年 | 359篇 |
1992年 | 333篇 |
1991年 | 251篇 |
1990年 | 242篇 |
1989年 | 413篇 |
1988年 | 206篇 |
1987年 | 241篇 |
1986年 | 237篇 |
1985年 | 653篇 |
1984年 | 526篇 |
1983年 | 413篇 |
1982年 | 490篇 |
1981年 | 612篇 |
1980年 | 247篇 |
1979年 | 187篇 |
1978年 | 189篇 |
1977年 | 146篇 |
1976年 | 155篇 |
1975年 | 194篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 149篇 |
1970年 | 144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 0 毫秒
801.
H. M. Fischer J. D. Mihalov L. J. Lanzerotti G. Wibberenz K. Rinnert F. O. Gliem J. Bach 《Space Science Reviews》1992,60(1-4):79-90
The Energetic Particles Investigation (EPI) instrument operates during the pre-entry phase of the Galileo Probe. The major science objective is to study the energetic particle population in the innermost regions of the Jovian magnetosphere — within 4 radii of the cloud tops — and into the upper atmosphere. To achieve these objectives the EPI instrument will make omnidirectional measurements of four different particle species — electrons, protons, alpha-particles, and heavy ions (Z > 2). Intensity profiles with a spatial resolution of about 0.02 Jupiter radii will be recorded. Three different energy range channels are allocated to both electrons and protons to provide a rough estimate of the spectral index of the energy spectra. In addition to the omnidirectional measurements, sectored data will be obtained for certain energy range electrons, protons, and alpha-particles to determine directional anisotropies and particle pitch angle distributions. The detector assembly is a two-element telescope using totally depleted, circular silicon surfacebarrier detectors surrounded by a cylindrical tungsten shielding with a wall thickness of 4.86 g cm-2. The telescope axis is oriented normal to the spherical surface of the Probe's rear heat shield which is needed for heat protection of the scientific payload during the Probe's entry into the Jovian atmosphere. The material thickness of the heat shield determines the lower energy threshold of the particle species investigated during the Probe's pre-entry phase. The EPI instrument is combined with the Lightning and Radio Emission Detector (LRD) such that the EPI sensor is connected to the LRD/EPI electronic box. In this way, both instruments together only have one interface of the Probe's power, command, and data unit. 相似文献
802.
CFAR data fusion center with inhomogeneous receivers 总被引:1,自引:0,他引:1
Elias-Fuste A.R. Broquetas-Ibars A. Antequera J.P. Yuste J.C.M. 《IEEE transactions on aerospace and electronic systems》1992,28(1):276-285
Detection systems with distributed sensors and data fusion are increasingly used by surveillance systems. A system formed by N inhomogeneous constant false alarm rate (CFAR) detectors (cell-averaging (CA) and ordered statistic (OS) CFAR detectors) is studied. A recursive formulation of an algorithm that permits a fixed level of false alarms in the data fusion center is presented, to set the optimum individual threshold levels in the CFAR receivers and the optimum `K out of N ' decision rule in order to maximize the total probability of detection. The algorithm also considers receivers of different quality or with different communication channel qualities connecting them with the fusion center. This procedure has been applied to several hypothetical networks with distributed CA-CFAR and OS-CFAR receivers and for Rayleigh targets and interference, and it was seen that in general the fusion decision OR rule is not always the best 相似文献
803.
Improved strapdown coning algorithms 总被引:2,自引:0,他引:2
Three improved algorithms for strapdown attitude computation utilizing accumulated gyro increments from the previous and current update are developed and evaluated analytically under classical coning motion. The error criterion of Miller is derived directly from the rotation vector concept. The accuracy of updating rotation vector estimation can be improved at least two orders of magnitude as compared with those of conventional algorithms. The proposed algorithm is equivalent to increasing the number of gyro samples used in the conventional method and it requires less computer loading 相似文献
804.
McGoey-Smith A.D. Vant M.R. 《IEEE transactions on aerospace and electronic systems》1992,28(3):666-674
When the basic step transform algorithm is used to compress synthetic-aperture radar (SAR) signals in azimuth, the linear FM rate and sampling rate must satisfy certain tight constraints. In practice, these constraints cannot be satisfied and errors are introduced into the compressed SAR image. A modification is described of the basic step transform which incorporates interpolation and resampling into the algorithm. These changes allow the removal of the constraints and make the step transform more useful for the compression of real data. An autofocusing capability is also included, without introducing much additional complexity 相似文献
805.
806.
807.
808.
介绍了俄罗斯铝锂合金铸锭熔炼和铸造方法的发展和改进,评述了铝锂合金熔铸特点,熔体氧化,防护添加剂,熔剂和氩气防护,氧、碳、氮和氢等非金属夹杂,铸造裂纹(冷裂和热裂)敏感性,结晶状态,不均匀组织等方面的研究结果。 相似文献
809.
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
810.
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