全文获取类型
收费全文 | 7425篇 |
免费 | 46篇 |
国内免费 | 38篇 |
专业分类
航空 | 3687篇 |
航天技术 | 2490篇 |
综合类 | 250篇 |
航天 | 1082篇 |
出版年
2021年 | 57篇 |
2018年 | 98篇 |
2017年 | 88篇 |
2016年 | 64篇 |
2015年 | 42篇 |
2014年 | 131篇 |
2013年 | 170篇 |
2012年 | 169篇 |
2011年 | 281篇 |
2010年 | 190篇 |
2009年 | 290篇 |
2008年 | 339篇 |
2007年 | 200篇 |
2006年 | 180篇 |
2005年 | 179篇 |
2004年 | 162篇 |
2003年 | 215篇 |
2002年 | 249篇 |
2001年 | 283篇 |
2000年 | 157篇 |
1999年 | 185篇 |
1998年 | 215篇 |
1997年 | 171篇 |
1996年 | 184篇 |
1995年 | 233篇 |
1994年 | 224篇 |
1993年 | 136篇 |
1992年 | 173篇 |
1991年 | 83篇 |
1990年 | 97篇 |
1989年 | 171篇 |
1988年 | 80篇 |
1987年 | 81篇 |
1986年 | 75篇 |
1985年 | 229篇 |
1984年 | 168篇 |
1983年 | 158篇 |
1982年 | 161篇 |
1981年 | 216篇 |
1980年 | 73篇 |
1979年 | 66篇 |
1978年 | 79篇 |
1977年 | 52篇 |
1976年 | 60篇 |
1975年 | 76篇 |
1974年 | 57篇 |
1972年 | 72篇 |
1971年 | 61篇 |
1970年 | 46篇 |
1969年 | 47篇 |
排序方式: 共有7509条查询结果,搜索用时 286 毫秒
921.
The objective of this work is to explore ways in which performance of battery systems can be enhanced through the use of energy-efficient battery management techniques. The phenomenon of charge recovery that takes place under pulsed discharge conditions is identified as a mechanism that can be exploited to enhance the capacity of a cell in a portable communication device. The bursty nature of many data traffic sources suggests that data transmissions in communication devices may provide natural opportunities for charge recovery. We model the data source as a stochastic process and let the cell discharge be driven by such a process. We use a model of a dual lithium ion insertion cell to identify the improvement to cell capacity that results from the stochastic discharge. The insight from this study leads us to propose discharge shaping techniques that tradeoff energy efficiency with delay in the required current supply 相似文献
922.
Acquisition of the idling first-order phase-locked loop (PLL) is investigated. An analytical solution for the Fokker-Planck equation is found if the noise term is neglected. Furthermore, the modulation signal that optimizes the acquisition is obtained by use of phase-plane techniques and Pontryagin's minimum principle. 相似文献
923.
L H Fraser 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(3):297-302
Since 1986, a series of microcosm experiments has been conducted at the Unit of Comparative Plant Ecology (UCPE) in an attempt to test our understanding of the principles controlling the structure and dynamics of plant communities and ecosystems. In each experiment microcosms have been seeded with a common pool of organisms, and systems have been allowed to assemble under replicated controlled conditions. Experiment variables have included mineral nutrient supply, temperature, moisture supply, soil depth, carbon dioxide concentration, mycorrhizas, rhizobia, herbivores and carnivores. Results from these experiments are presented to illustrate the value of synthesised ecosystems in ecological research. 相似文献
924.
In the past, Air Force technical manuals have been stored in libraries containing many volumes of such manuals. This article details the creation and development of an Electronic Technical Manual for the F-16 Analog Test Station Sustainment (FATSS) Project. The manual will allow users to search a single CD-ROM to access data on maintenance, operational procedures, self-test, calibration, schematics, and illustrated parts breakdowns 相似文献
925.
A possible variant presented for solving the equation of motion of a rotating flow is more general with respect to the variants known in the literature. On the basis of the simultaneous solution of the equations of motion and energy, the temperature fields are determined under changes of the rotation nature. 相似文献
926.
Aeronautical telemetry using multiple-antenna transmitters 总被引:1,自引:0,他引:1
Jensen M.A. Rice M.D. Anderson A.L. 《IEEE transactions on aerospace and electronic systems》2007,43(1):262-272
The placement of multiple antennas on an air vehicle is one possible practice for overcoming signal obstruction created by vehicle maneuvering during air-to-ground transmission. Unfortunately, for vehicle attitudes where more than one of these antennas has a clear path to the receiving station, this practice also leads to self-interference nulls, resulting in dramatic degradation in the average signal integrity. This paper discusses application of unitary space-time codes such as the Alamouti transmit diversity scheme and unitary differential space-time codes to overcome the self-interference effect observed in such systems. The mathematical foundations of these techniques within the context of this application as well as computational performance gains associated with their implementation are provided. Issues such as the cost of channel estimation for trained techniques as well as the throughput performance of nondifferential and differential schemes for realistic air-vehicle motion are analyzed 相似文献
927.
L. G. Blomberg J. A. Cumnock 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,33(12):2161-2165
Mercury has a small but intriguing magnetosphere. In this brief review, we discuss some similarities and differences between Mercury’s and Earth’s magnetospheres. In particular, we discuss how electric and magnetic field measurements can be used as a diagnostic tool to improve our understanding of the dynamics of Mercury’s magnetosphere. These points are of interest to the upcoming ESA-JAXA BepiColombo mission to Mercury. 相似文献
928.
Solar wind turbulence during the solar cycle deduced from Galileo coronal radio-sounding experiments
A.I. Efimov L.N. Samoznaev M.K. Bird I.V. Chashei D. Plettemeier 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
Seven coronal radio-sounding campaigns were carried out during the active lifetime of the Galileo spacecraft in the years 1994–2002. The observational data analyzed in the present work are S-band frequency fluctuation measurements recorded during the solar conjunctions at different phases of solar activity cycle #23, specifically: periods near solar maximum (three conjunctions), near solar minimum (three conjunctions) and during the ascending phase (one conjunction). These data are all applicable to low heliographic latitudes, i.e. to the slow solar wind. The rms frequency fluctuation and power-law index of the frequency fluctuation temporal spectra are determined as a function of heliocentric distance. The turbulence power spectrum tends to be flatter inside ca. 20 solar radii during all phases of the solar cycle. This coincides with a transition in the flow from the inner acceleration region to the outer region of constant velocity. The radial falloff rate and absolute level of the rms frequency fluctuation are essentially invariant over the solar cycle. 相似文献
929.
R. Musci T. SchildknechtT. Flohrer G. Beutler 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(7):1071-1076
A new population of uncatalogued objects in geosynchronous Earth orbits (GEO), with a mean motion of about 1 rev/day and eccentricities up to 0.6, has been identified recently. The first observations of this new type of objects were acquired in the framework of the European Space Agency’s (ESA) search for space debris in GEO and the geostationary transfer orbit (GTO) using the ESA 1-m telescope on Tenerife. Earlier studies have postulated that the perturbations due to the solar radiation pressure can lead to such large eccentricities for GEO objects with a high area-to-mass ratio (A/M). The simulations showed that the eccentricities of GEO objects with large A/M exhibit periodic variations with periods of about one year and amplitudes depending on the value of A/M. The findings of these studies could be confirmed by observations from the ESA 1-m telescope on Tenerife. 相似文献
930.
We examine X-ray maps of the whole sky covering energies from 0.1 to 6 keV. We model Loop I X-ray emission as being caused by an old supernova that exploded into an already warm interstellar medium. After comparison with Loop III we deduce that there may be a general temperature gradient in the gas as we move away from the plane in the northern galactic hemisphere. 相似文献