全文获取类型
收费全文 | 21907篇 |
免费 | 583篇 |
国内免费 | 840篇 |
专业分类
航空 | 12036篇 |
航天技术 | 6397篇 |
综合类 | 981篇 |
航天 | 3916篇 |
出版年
2021年 | 248篇 |
2019年 | 175篇 |
2018年 | 277篇 |
2017年 | 162篇 |
2016年 | 222篇 |
2015年 | 184篇 |
2014年 | 632篇 |
2013年 | 707篇 |
2012年 | 640篇 |
2011年 | 842篇 |
2010年 | 698篇 |
2009年 | 1037篇 |
2008年 | 1057篇 |
2007年 | 596篇 |
2006年 | 600篇 |
2005年 | 586篇 |
2004年 | 545篇 |
2003年 | 610篇 |
2002年 | 546篇 |
2001年 | 688篇 |
2000年 | 501篇 |
1999年 | 621篇 |
1998年 | 635篇 |
1997年 | 487篇 |
1996年 | 522篇 |
1995年 | 584篇 |
1994年 | 557篇 |
1993年 | 466篇 |
1992年 | 430篇 |
1991年 | 342篇 |
1990年 | 306篇 |
1989年 | 478篇 |
1988年 | 263篇 |
1987年 | 279篇 |
1986年 | 270篇 |
1985年 | 658篇 |
1984年 | 531篇 |
1983年 | 424篇 |
1982年 | 498篇 |
1981年 | 618篇 |
1980年 | 247篇 |
1979年 | 190篇 |
1978年 | 189篇 |
1976年 | 156篇 |
1975年 | 192篇 |
1974年 | 183篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1969年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Els Peeters Nieves Leticia Martín-HernáNdez Nemesio J. RodríGuez-FernáNdez Xander Tielens 《Space Science Reviews》2005,119(1-4):273-292
An overview is given of ISO results on regions of high excitation ISM and gas, i.e. HII regions, the Galactic Centre and Supernova Remnants. IR emission due to fine-structure lines, molecular hydrogen, silicates,
polycyclic aromatic hydrocarbons and dust are summarised, their diagnostic capabilities illustrated and their implications
highlighted.
Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries:
France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA. 相似文献
102.
The complex angle (CA) method for resolving a low angle target from its multipath signal is evaluated in the presence of system noise. It is shown that standard deviation improvements of around 3-to-1 can be achieved at a 20-dB signal-to-noise power ratio relative to a normal monopulse system without the CA. It is also shown that the CA method is unbiased, giving bias improvements of as much as 100 times relative to normal monopulse. Evaluation of the assumptions in the technique shows very little sensitivity to knowledge of the reflecting surface's conductivity or dielectric constant. However, the method is somewhat sensitive to knowledge of surface roughness. 相似文献
103.
104.
Optimum estimation (tracking) of the polarization plane of a linearly polarized electromagnetic wave is determined when the signal is a narrow-band Gaussian random process with a polarization plane angle which is also a Gaussian random process. This model is Compared to previous work and is applicable to space communication. The estimator performs a correlation operation similar to an amplitude -comparison monopulse angle tracker, giving the name correlation polarimeter. Under large signal-to-noise ratio (SNR), the estimator is causal. Performance of the causal correlation polarimeter is evaluated for arbitrary SNR. Optimum precorrelation filtering is determined. With low SNR, the performance of this system is far better than that of previously developed systems. Practical implementation is discussed. A scheme is given to reduce the effect of linearly polarized noise. 相似文献
105.
Many modern telemetry systems which use phase-shift keying (PSK) have receivers which derive a coherent reference from the fully modulated PSK signal itself and thus conserve the energy which otherwise would be allocated to a discrete reference signal. In this paper, an optimum receiver structure for estimating a phase reference from the PSK signal itself is derived and its realization discussed. It is shown that at low signal-to-noise ratios, the optimum detector can be realized with a Costas loop. Since a Costas loop and squaring loop exhibit identical performance, it follows that either of these simple devices gives optimum performance for low-input signal-to-noise ratios. 相似文献
106.
Electron concentration and Hall mobility have been investigated in pure zinc oxide single crystals, while in the dark and under ultraviolet (UV) excitation. Mode of scattering in ZnO has been determined by studying the temperature dependence of various electrical parameters in the range from -170 to 120°C. It has been observed that the degradation to the crystals by UV irradiation is limited only to the surface layer and shows up only at relatively lower temperatures, which is in contrast to the degradation behavior in thin films and powders. 相似文献
107.
The theoretical and experimental work performed since 1960 in the area of high-intensity and high-temperature operation of silicon and gallium arsenide photovoltaic devices is reviewed. Test results for conventional 5-grid silicon cells, for specially designed 13-grid silicon cells, and for a GaAs cell are presented parametrically for the illumination intensity range from 0.07 to 2.8 W/cm2 and the temperature range from 30 to 1 50°C. The data cover the 3 points on the currentvoltage characteristic required to reconstruct the full characteristic in the power-generating quadrant. The 13-grid silicon cells showed much better performance than the GaAs cell. 相似文献
108.
It is shown that the errors associated with radio elevation measurements may be investigated systematically using a variational technique. The error occurring when spaced antennas are used is compared with that for a single directional antenna. Integral expressions are obtained for the refractive errors. 相似文献
109.
The magnetotail and substorms 总被引:5,自引:0,他引:5
The tail plays a very active and important role in substorms. Magnetic flux eroded from the dayside magnetosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary of the tail flares more, the field strength in the tail increases, and the currents strengthen and move closer to the Earth. Further, the plasma sheet thins and the magnetic flux crossing the neutral sheet lessens. At the onset of the expansion phase, the stored magnetic flux is returned from the tail and energy is deposited in the magnetosphere and ionosphere. During the expansion phase of isolated substorms, the flaring angle and the lobe field strength decrease, the plasma sheet thickens and more magnetic flux crosses the neutral sheet.In this review, we discuss the experimental evidence for these processes and present a phenomenological or qualitative model of the substorm sequence. In this model, the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields. During the growth phase of substorms the merging rate on the dayside magnetosphere exceeds the reconnection rate in the neutral sheet. In order to remove the oversupply of magnetic flux in the tail, a neutral point forms in the near earth portion of the tail. If the new reconnection rate exceeds the dayside merging rate, then an isolated substorm results. However, a situation can occur in which dayside merging and tail reconnection are in equilibrium. The observed polar cap electric field and its correlation with the interplanetary magnetic field is found to be in accord with open magnetospheric models. 相似文献
110.
Quintessence – the energy density of a slowly evolving scalar field – may constitute a dynamical form of the homogeneous dark energy in the universe. We review the basic idea and indicate observational tests which may distinguish quintessence from a cosmological constant. 相似文献