全文获取类型
收费全文 | 3470篇 |
免费 | 32篇 |
国内免费 | 16篇 |
专业分类
航空 | 1689篇 |
航天技术 | 1202篇 |
综合类 | 11篇 |
航天 | 616篇 |
出版年
2021年 | 28篇 |
2019年 | 31篇 |
2018年 | 65篇 |
2017年 | 52篇 |
2016年 | 47篇 |
2015年 | 32篇 |
2014年 | 74篇 |
2013年 | 106篇 |
2012年 | 91篇 |
2011年 | 138篇 |
2010年 | 89篇 |
2009年 | 148篇 |
2008年 | 154篇 |
2007年 | 111篇 |
2006年 | 78篇 |
2005年 | 105篇 |
2004年 | 95篇 |
2003年 | 100篇 |
2002年 | 74篇 |
2001年 | 120篇 |
2000年 | 74篇 |
1999年 | 72篇 |
1998年 | 88篇 |
1997年 | 59篇 |
1996年 | 95篇 |
1995年 | 117篇 |
1994年 | 115篇 |
1993年 | 48篇 |
1992年 | 76篇 |
1991年 | 35篇 |
1990年 | 32篇 |
1989年 | 78篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 33篇 |
1985年 | 103篇 |
1984年 | 96篇 |
1983年 | 69篇 |
1982年 | 68篇 |
1981年 | 108篇 |
1980年 | 19篇 |
1979年 | 25篇 |
1978年 | 30篇 |
1977年 | 28篇 |
1975年 | 32篇 |
1974年 | 23篇 |
1973年 | 22篇 |
1972年 | 21篇 |
1969年 | 22篇 |
1967年 | 18篇 |
排序方式: 共有3518条查询结果,搜索用时 218 毫秒
911.
A family of 4-wire electronic equipments is being developed for use in a tactical switching system for the field army. The characteristics and design of two automatic switchboards to be used in the system are discussed, as well as the signaling methods and numbering plan that are employed. 相似文献
912.
Hart Walter G. McGowan Gerald F. DuPont T. E. 《IEEE transactions on aerospace and electronic systems》1968,(4):592-606
The design of an adaptive tracking filter control system and its application to large flexible booster vehicles is described. The problem of elastic vehicle stability is considered along with its solution by conventional and adaptive techniques. The improvement in vehicle stability, which may be achieved with the adaptive tracking filter technique as compared to conventional techniques, is shown. The control system evolved uses two adaptive tracking filters to phase stabilize the first and second bending modes, in addition to conventional compensation techniques. The application of this adaptive technique to a vehicle in which the modal frequencies are in close proximity (<2.5 percent separation) is discussed. The mechanization of the adaptive control system involves the selection of design techniques and components that are electrically and physically compatible with the intended airborne application. The development of a suitable frequency tracking technique and tracking filter is described, in addition to the electronic and mechanical design of the adaptive control system prototype. 相似文献
913.
A prominent neuroscientist A. Damasio has observed that “More may have been learned about the brain and the mind in the 1990s... than during the entire previous history of psychology and neuroscience” (1999). This progress, which should continue at an exponentially increasing pace as we move into the 21st century, sets the stage for another revolution-in the scientific formulation of intentional behavior. Just as applied sciences such as fluid mechanics and solid-state electronics benefited from breakthroughs in physics, so it may be expected that social psychology and economics, for example, will similarly benefit from our expanding knowledge of psychosomatic and psychological function. Economics, however, with its basic concepts dating from the nineteenth century, must endure an extensive renewal. Toward this end, a new mathematical theory of behavior was formulated in the early 1990s based on the contributions of prominent psychologists and economists over the past two centuries. Presentation of the new approach at international conferences continued in 2000 with attention moving beyond the fundamentals into macroeconomic concerns including business cycles, productivity, and unemployment 相似文献
914.
The Coronal Diagnostic Spectrometer (CDS) on SOHO is a grazing/normal incidence spectrograph, aimed to produce stigmatic spectra of selected regions of the solar surface in six spectral windows of the extreme ultraviolet from 150 Å to 785 Å (Harrison et al. 1995). In the present work, CDS, EIT, MDI and Yohkoh observations of active region lops have been analyzed. These observations are part of JOP 54. CDS monochromatic images from lines at different temperatures have been co-aligned with EIT and MDI images, and loop structures have been clearly identified using Fe XVI emission lines. Density sensitive lines and lines from adjacent stages of ionization of Fe ions have been used to measure electron density and temperature along the loop length; these measurements have been used to determine the electron pressure along the loop and test the constant pressure assumption commonly used in loop modeling. The observations have been compared with a static, isobaric loop model (Landini and Monsignori Fossi 1975) assuming a temperature-constant heating function in the energy balance equation. Good agreement is found for the temperature distribution along the loop at the coronal level. The model pressure is somewhat higher than obtained from density sensitive line ratios. 相似文献
915.
Margarita Karovska Brian E. Wood John W. Cook Russell A. Howard Guenter E. Brueckner 《Space Science Reviews》1999,87(1-2):219-222
We present the results from a study of the dynamical properties of polar jets in the Sun's polar regions using LASCO C2 coronagraph
and EIT observations. In the simplest impulsive acceleration scenarios for jets, gravity is expected to be the dominant force
on the jet following the initial acceleration, especially in the Sun's polar regions where the open magnetic fields should
not significantly impede the jet's motion. Our analysis shows that although at low heights the kinematics of the jets could
be consistent with a gravitational deceleration of the ejected plasma, at higher heights their motions may simply follow the
ambient solar wind outflow. If so, the polar jets can be used as tracers of the solar wind in coronal holes.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
916.
Test instrumentation is described for use in the measurement of threshold sensitivities and noise bandwidths of narrow-band phase-locked receivers. The method is statistical in nature with the data being recorded digitally by a counter and printer. Scale factors can be selected for convenient relation of the data to the standard deviations, 1,2, or 3 sigma. Curves are included showing the results of a typical experiment. 相似文献
917.
The atomic physics relevant to the interpretation of solar spectra produced by plasmas at temperatures ≳ 105 K are discussed.
Methods for determining relative abundance ratios are presented and examples provided from the Coronal Diagnostic Spectrometer
on board SOHO. In particular, the Fe/Si ratio in the corona is found to be close to photospheric; the Mg/Ne ratio in the transition
region is found to vary by an order of magnitude in different solar features. The Mg/Ne ratios in supergranule cell centres
and the network are separated for the first time, although no significant differences are found.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
918.
Thermal drill sampling system onboard high-velocity impactors for exploring the subsurface of Europa
P. Weiss K.L. Yung N. Kömle S.M. Ko E. Kaufmann G. Kargl 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The planned Europa Jupiter System Mission (EJSM) will provide a unique opportunity to place scientific instruments onto the surface of Jupiter’s moon Europa in the late 2020s. After the Galileo mission, this will be a long awaited chance to have a close glimpse into some of the mysteries of this moon. Care must be taken in the choice of in-situ science that will be undertaken on the surface. 相似文献
919.
The numerical simulation of the flow for the VFE-2 delta wing configuration with rounded leading edges is presented using the Cobalt Navier–Stokes solver. Cobalt uses a cell-centered unstructured hybrid mesh approach, and several numerical results are presented for the steady RANS equations as well as for the unsteady DES and DDES hybrid approaches. Within this paper the focus is related to the dual primary vortex flow topology, especially the sensitivity of the flow to angle of attack and Reynolds number effects. Reasonable results are obtained with both steady RANS and SA-DDES simulations. The results are compared and verified by experimental data, including surface pressure and pressure sensitive paint results, and recommendations for improving future simulations are made. 相似文献
920.
The Solar System includes two planets—Mercury and Mars—significantly less massive than Earth, and all evidence indicates that planets of similar size orbit many stars. In fact, one of the first exoplanets to be discovered is a lunar-mass planet around a millisecond pulsar. Novel classes of exoplanets have inspired new ideas about planet formation and evolution, and these “sub-Earths” should be no exception: they include planets with masses between Mars and Venus for which there are no Solar System analogs. Advances in astronomical instrumentation and recent space missions have opened the sub-Earth frontier for exploration: the Kepler mission has discovered dozens of confirmed or candidate sub-Earths transiting their host stars. It can detect Mars-size planets around its smallest stellar targets, as well as exomoons of comparable size. Although the application of the Doppler method is currently limited by instrument stability, future spectrographs may detect equivalent planets orbiting close to nearby bright stars. Future space-based microlensing missions should be able to probe the sub-Earth population on much wider orbits. A census of sub-Earths will complete the reconnaissance of the exoplanet mass spectrum and test predictions of planet formation models, including whether low-mass M dwarf stars preferentially host the smallest planets. The properties of sub-Earths may reflect their low gravity, diverse origins, and environment, but they will be elusive: Observations of eclipsing systems by the James Webb Space Telescope may give us our first clues to the properties of these small worlds. 相似文献