全文获取类型
收费全文 | 5560篇 |
免费 | 24篇 |
国内免费 | 24篇 |
专业分类
航空 | 2782篇 |
航天技术 | 2020篇 |
综合类 | 22篇 |
航天 | 784篇 |
出版年
2021年 | 41篇 |
2019年 | 41篇 |
2018年 | 79篇 |
2017年 | 57篇 |
2016年 | 52篇 |
2014年 | 104篇 |
2013年 | 124篇 |
2012年 | 120篇 |
2011年 | 177篇 |
2010年 | 130篇 |
2009年 | 215篇 |
2008年 | 288篇 |
2007年 | 129篇 |
2006年 | 122篇 |
2005年 | 145篇 |
2004年 | 143篇 |
2003年 | 175篇 |
2002年 | 108篇 |
2001年 | 187篇 |
2000年 | 112篇 |
1999年 | 141篇 |
1998年 | 169篇 |
1997年 | 106篇 |
1996年 | 148篇 |
1995年 | 184篇 |
1994年 | 168篇 |
1993年 | 102篇 |
1992年 | 137篇 |
1991年 | 63篇 |
1990年 | 64篇 |
1989年 | 143篇 |
1988年 | 64篇 |
1987年 | 61篇 |
1986年 | 59篇 |
1985年 | 185篇 |
1984年 | 149篇 |
1983年 | 123篇 |
1982年 | 130篇 |
1981年 | 180篇 |
1980年 | 60篇 |
1979年 | 48篇 |
1978年 | 49篇 |
1977年 | 50篇 |
1976年 | 34篇 |
1975年 | 62篇 |
1974年 | 41篇 |
1973年 | 39篇 |
1972年 | 46篇 |
1970年 | 35篇 |
1969年 | 37篇 |
排序方式: 共有5608条查询结果,搜索用时 709 毫秒
631.
The approach of the Year 2000 (Y2K) has called into question the condition of readiness of computer systems upon which organizations depend. Will these systems operate reliably after midnight, January 1, 2000, or will problems associated with date calculations cause failures that adversely affect users? This question has caused organizations to identify their critical systems, test these systems for Y2K problems, and take corrective action where necessary to ensure Y2K readiness. This paper examines various technical and management issues that Southwest Research Institute has dealt with in solving these problems 相似文献
632.
The factors which affect the target-handling capacity of an agile-beamelectronically scanned multitarget-tracking radar are reviewed and analyze.Consideration is given to the choice of strategy for dwell allocation amongtargets, the choice of trajectory extrapolation algorithms for target tracking, and the determination of saturation target acquisition rates. Emphasisis placed upon radar systems with control computers having limited speed and memory, and recommendations are made for allocation strategy and targetytracking algorithms, based on uniform sampling, to be used in maximizing thetarget-handling capacity in this situation. 相似文献
633.
Active Spacecraft Potential Control Investigation 总被引:1,自引:0,他引:1
K. Torkar R. Nakamura M. Tajmar C. Scharlemann H. Jeszenszky G. Laky G. Fremuth C. P. Escoubet K. Svenes 《Space Science Reviews》2016,199(1-4):515-544
634.
D Massimino M Andre 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(3):293-296
Wheat plants were grown in twin closed growth chambers under normal and reduced atmospheric pressures. For the first 22 days from sowing, the reduced pressure was maintained at 200 hPa, and at 100 hPa for the remaining 27 days until harvest. These pressures were obtained by evacuation of the chamber and adding oxygen (170 and 79 hPa respectively) and carbon dioxide (0.65 and 1.0 hPa respectively; about 2 and 3 times above the control). Eighty-seven per cent of the final dry mass was produce under 100 hPa treatment. Growth and development of wheat are not negatively affected by low pressure treatment. Compared to the control, final dry mass increased by 76%, leaf number by 133%, and ear number by 35%, probably due to elevation of CO2. Shortening of shoot parts and increases in chlorophyll and proteins content are not in accordance with a predicted CO2 effect and could be attributed to the N2 removal and the subsequent alteration in gas diffusion rate. 相似文献
635.
636.
Large-scale Coronal Heating, Clustering of Coronal Bright Points, and Concentration of Magnetic Flux
By combining quiet-region Fe XII coronal images from SOHO/EIT with magnetograms from NSO/Kitt Peak and from SOHO/MDI, we show
that the population of network coronal bright points and the magnetic flux content of the network are both markedly greater
under the bright half of the large-scale quiet corona than under the dim half. These results (1) support the view that the
heating of the entire corona in quiet regions and coronal holes is driven by fine-scale magnetic activity (microflares, explosive
events, spicules) seated low in the magnetic network, and (2) suggest that this large-scale modulation of the magnetic flux
and coronal heating is a signature of giant convection cells.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
637.
The calculation of particle trajectories in the Earth's magnetic field has been a subject of interest since the time of Störmer. The fundamental problem is that the trajectory-tracing process involves using mathematical equations that have `no solution in closed form'. This difficulty has forced researchers to use the `brute force' technique of numerical integration of many individual trajectories to ascertain the behavior of trajectory families or groups. As the power of computers has improved over the decades, the numerical integration procedure has grown more tractable and while the problem is still formidable, thousands of trajectories can be computed without the expenditure of excessive resources. As particle trajectories are computed and the characteristics analyzed we can determine the cutoff rigidity of a specific location and viewing direction and direction and deduce the direction in space of various cosmic ray anisotropies. Unfortunately, cutoff rigidities are not simple parameters due to the chaotic behavior of the cosmic-ray trajectories in the cosmic ray penumbral region. As the computational problem becomes more manageable, there is still the issue of the accuracy of the magnetic field models. Over the decades, magnetic field models of increasing complexity have been developed and utilized. The accuracy of trajectory calculations employing contemporary magnetic field models is sufficient that cosmic ray experiments can be designed on the basis of trajectory calculations. However, the Earth's magnetosphere is dynamic and the most widely used magnetospheric models currently available are static. This means that the greatest uncertainly in the application of charged particle trajectories occurs at low energies. 相似文献
638.
Leung H. Zhijian Hu Blanchette M. 《IEEE transactions on aerospace and electronic systems》1999,35(2):663-674
This paper evaluates the performance of multiple target tracking (MTT) algorithms in real-life stressful radar tracking environments. Real closely spaced maneuver radar data, generated by six F-18 fighters and other targets, were collected jointly by the defence departments of Canada and United States to support this practical MTT algorithm evaluation study. A set of performance metrics was defined here to compare the suboptimal nearest neighbor (SNN), global nearest neighbor (GNN), and various variants of the joint probabilistic data association (JPDA) MTT trackers. Results reveal an interesting result that all these MTT algorithms exhibited very close performance. In addition, the weighted sum approach of the PDA/JPDA trackers which are theoretically effective were observed to perform poorly in tracking closely spaced targets. Overall speaking, the GNN filter based on the Munkres algorithm had the best performance in terms of both tracking performance and robustness 相似文献
639.
T. Aramaki S.E. Boggs W.W. Craig H. Fuke F. Gahbauer C.J. Hailey J.E. Koglin N. Madden K. Mori R.A. Ong T. Yoshida 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The General AntiParticle Spectrometer (GAPS) is a novel approach for indirect dark matter searches that exploits cosmic antideuterons. GAPS complements existing and planned direct dark matter searches as well as other indirect techniques, probing a different and unique region of parameter space in a variety of proposed dark matter models. The GAPS method involves capturing antiparticles into a target material with the subsequent formation of an excited exotic atom. The exotic atom decays with the emission of atomic X-rays and pions from the nuclear annihilation, which uniquely identifies the captured antiparticle. This technique has been verified through the accelerator testing at KEK in 2004 and 2005. The prototype flight is scheduled from Hokkaido, Japan in 2011, preparatory for a long duration balloon flight from the Antarctic in 2014. 相似文献
640.
K. Boubaker A.H. Bhrawy 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012,49(6):1062-1066
In this paper, Bonnor–Ebert gas sphere model of polytropic stars has been investigated through an analytical approach. Two confirmed and well-established methods have been used: the Enhanced Lagrangian Formulation Method ELFM and the Boubaker Polynomials Expansion Scheme BPES. Solutions to the related generalized Lane–Emden equation of the second kind have been expressed and plotted. Results have given evidence to the relevance of the dimensionless Bonnor–Ebert radius, in good agreement with some recently proposed profiles. 相似文献