全文获取类型
收费全文 | 5428篇 |
免费 | 7篇 |
国内免费 | 18篇 |
专业分类
航空 | 2840篇 |
航天技术 | 1922篇 |
综合类 | 23篇 |
航天 | 668篇 |
出版年
2021年 | 30篇 |
2019年 | 32篇 |
2018年 | 57篇 |
2017年 | 31篇 |
2014年 | 81篇 |
2013年 | 131篇 |
2012年 | 108篇 |
2011年 | 158篇 |
2010年 | 114篇 |
2009年 | 178篇 |
2008年 | 247篇 |
2007年 | 136篇 |
2006年 | 141篇 |
2005年 | 143篇 |
2004年 | 114篇 |
2003年 | 170篇 |
2002年 | 102篇 |
2001年 | 177篇 |
2000年 | 109篇 |
1999年 | 140篇 |
1998年 | 164篇 |
1997年 | 123篇 |
1996年 | 171篇 |
1995年 | 215篇 |
1994年 | 183篇 |
1993年 | 120篇 |
1992年 | 127篇 |
1991年 | 76篇 |
1990年 | 62篇 |
1989年 | 138篇 |
1988年 | 62篇 |
1987年 | 66篇 |
1986年 | 60篇 |
1985年 | 195篇 |
1984年 | 150篇 |
1983年 | 129篇 |
1982年 | 133篇 |
1981年 | 176篇 |
1980年 | 58篇 |
1979年 | 41篇 |
1978年 | 50篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1975年 | 58篇 |
1974年 | 39篇 |
1973年 | 42篇 |
1972年 | 51篇 |
1971年 | 42篇 |
1970年 | 44篇 |
1969年 | 37篇 |
排序方式: 共有5453条查询结果,搜索用时 140 毫秒
871.
The denial of effective communications by enemy forces during hostile military operations has been a primary concern for military commanders since the inception of radio communications on the battlefield before World War II. Since then, the electromagnetic environment has been in a constant state of evolution toward more sophisticated jam-resistant and convert forms of modulation. For example, exotic modulation techniques employing spread spectrum (SS) signaling are routinely used by our adversaries to provide their communication links an advantage over US and Allied jammers. These same spread spectrum modulation techniques are being refined to provide convert, low probability-of-intercept (LPI) features to the unintended interceptor. The thrust of this paper focuses on developments in the theory and algorithms for detection, characterization, and exploitation of advanced waveforms using new mathematical signal processing tools. Specifically, quadratic time-frequency signal representations, wavelet transforms, and cyclostationary signal processing are introduced. This overview demonstrates the importance of these advanced techniques in a clear and concise manner. Applications and future research activities are described 相似文献
872.
J. E. Grindlay 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):597-609
In the past three years, a new era of study of globular clusters has begun with multiwavelength observations from the current generation of astronomical telescopes in space. We review the recent results obtained from our studies of compact binaries and x-ray sources in globulars with ROSAT and HST as well as our balloon-borne hard x-ray telescope EXITE and ground-based observations (CTIO). With ROSAT, we have obtained the most sensitive high resolution soft x-ray images of clusters which show multiple low luminosity sources in cluster cores that are likely indicative of the long-sought population of cataclysmic variables (CVs). We have obtained deep H images of two clusters with HST and found CV candidates for 3 of the ROSAT sources in the core of NGC 6397. New CTIO imaging and spectroscopy of two ‘dim source’ fields in ω-Cen are also described. With EXITE we carried out the first hard x-ray imaging observations of the cluster 47 Tuc; such studies can ultimately limit the populations of millisecond pulsars and pulsar emission mechanisms. A long ROSAT exposure on 47 Tuc also shows probable cluster diffuse emission, possibly due to hot gas from ablating millisecond pulsars. Multiwavelength studies of globular clusters may provide new constraints on problems as diverse as the origin of CVs and LMXBs and the origin of hot gas in globulars. 相似文献
873.
The functional approach to studying human motor systems attempts to give a better understanding of the processes behind planning movements and their coordinated performance by relying on weightlessness as a particularly enlightening experimental condition. Indeed, quantitative monitoring of sensorimotor adaptation of subjects exposed to weightlessness outlines the functional role of gravity in motor and postural organization. The recent accessibility of the MIR Space Station has allowed for the first time experimental quantitative kinematic analysis of long-term sensorimotor and postural adaptation to the weightless environment though opto-electronic techniques. In the frame of the EUROMIR'95 Mission, two protocols of voluntary posture perturbation (erect posture, EP; forward trunk bending, FTB) were carried out during four months of microgravity exposure. Results show that postural strategies for quasistatic body orientation in weightlessness are based on the alignment of geometrical body axes (head and trunk) along external references. A proper whole body positioning appears to be recovered only after months of microgravity exposure. By contrast, typically, terrestrial strategies of co-ordination between movement and posture are promptly restored and used when performing motor activities in the weightless environment. This result is explained under the assumption that there may be different sensorimotor integration processes for static and dynamic postural function and that the organisation of coordinated movement might rely stably on egocentric references and kinematics synergies for motor control. 相似文献
874.
In automatic radar detection, digital integration of the envelope detector outputs is often used as a good approximation to the optimum. This requires quantizing the envelope detector outputs. In this paper, quantizer structures for narrowband signal detection are considered. Quantizer characteristics are derived to optimize performance as measured by the detector efficacy?an asymptotic performance measure. Asymptotic and finite sample performance results are presented. The results obtained are not limited in their application to Gaussian noise only, although this important case is given specific consideration. 相似文献
875.
Some of the problems in the development of pulsewidth power converters when the switching frequency is increased from hundreds of kilohertz to tens of megahertz are highlighted. The major goal is the establishment of a ten megahertz power chain designed to operate in a distributed system from a DC power bus of 24 to 48 V. Available components and materials are discussed, and possible topologies-flyback, forward, push-pull forward, and buck converters-are examined. The performance criteria chosen for this analysis are, in the order of preference, output requirements, size and weight, efficiency, and cost 相似文献
876.
The concept of designing a high-speed, permanent magnet, brushless DC motor aircraft fuel pump drive using a cycloconverter link is examined. A combination of sinusoidal and DC steady-state analysis is used to produce a simple model of the system. A closed-loop control system with an outer loop based on speed and an inner loop based on current is postulated wherein a proportional-plus-integral controller is placed in the forward path to assure minimum speed error. Gains are then set to assure that the eigenvalues of the linearized control system lie within the left half s -plane over the entire full range 相似文献
877.
D Odom T Yamrom J Oro 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(9):55-59
Deoxynucleoside 5'-monophosphates were condensed by cyanamide or by l-ethyl-3-(3-dimethylaminopropyl) carbodiimide in the presence of ammonium chloride at 0 degree, 37 degrees or 60 degrees C through several cycles of evaporation to dryness with replenishment of all reactants at each cycle. We found that at 37 degrees or 60 degrees cyanamide gives distinctly more high molecular weight material than does carbodiimide. Indeed, the yield of condensed products for the cyanamide reaction (dimers and higher oligomers) was found to be between 60 percent and 80 percent. The molecular weight distribution of the product shifts to higher molecular weights as cycling continues at 37 degrees or 60 degrees for both condensing agents. The water soluble carbodiimide gives higher yields of low molecular weight product but much lower yields of the higher molecular weight products. At 0 degree yields of high molecular weight product were low for both condensing agents. Results of characterization of the products demonstrate the synthesis of oligodeoxynucleotides including tetramers, with 3'-5' phosphodiester linkages. 相似文献
878.
Dias J.M.B. Marques P.A.C. 《IEEE transactions on aerospace and electronic systems》2003,39(2):604-624
A novel methodology is presented for determining the velocity and location of multiple moving targets using a single strip-map synthetic aperture radar (SAR) sensor. The so-called azimuth position uncertainty problem is therefore solved. The method exploits the structure of the amplitude and phase modulations of the returned echo from a moving target in the Fourier domain. A crucial step in the whole processing scheme is a matched filtering, depending on the moving target parameters, that simultaneously accounts for range migration and compresses two-dimensional signatures into one-dimensional ones without losing moving target information. A generalized likelihood ratio test approach is adopted to detect moving targets and derive their trajectory parameters. The effectiveness of the method is illustrated with synthetic and real data covering a wide range of targets velocities and signal-to-clutter ratios (SCRs). Even in the case of parallel to platform moving target motion, the most unfavorable scenario, the proposed method yields good results for, roughly, SCR > 10 dB. 相似文献
879.
An overview of military avionics data buses and their applications, with the emphasis on optical fiber networking techniques. The evolution of military avionics data buses is discussed. The development trend actually reflects an increasing demand on such data buses, which requires the change from low-speed to high-speed transmissions, from single-rate to dual-rate operations, and from centralized control to distributed control. Recent progress in military avionics networks is described. 相似文献
880.
K Masuda J Takahashi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,19(7):1019-1022
In the present work, we investigate the sticking process of a hydrogen atom on the surface of dust grains. As a realistic model for the icy mantle of dust grains, we produced slab-shaped amorphous water ice with infinite area by classical molecular dynamics (MD) computational simulation using two-dimensional periodic boundary condition. The resulting amorphous water ice slabs at 10 K and 70 K were found to be in good agreement with the experimental high-density and low-density amorphous water ice, respectively. Then, we investigated the dynamical behaviors of an impinging H atom on the surface of it by MD simulations. The sticking probabilities of incident H atoms with several initial temperatures on 10 K and 70 K ice were obtained. It was found that most of H atoms colliding with the 10 K ice stuck on the surface of it. After having stuck, the impinging H atoms diffused on the surface of ice and became trapped in one of potential wells on the surface. The mobility of a H atom on the surface of the amorphous water ice was found to depend only upon the temperature of ice. 相似文献