全文获取类型
收费全文 | 7186篇 |
免费 | 11篇 |
国内免费 | 33篇 |
专业分类
航空 | 3332篇 |
航天技术 | 2745篇 |
综合类 | 24篇 |
航天 | 1129篇 |
出版年
2021年 | 60篇 |
2019年 | 47篇 |
2018年 | 134篇 |
2017年 | 97篇 |
2016年 | 80篇 |
2014年 | 148篇 |
2013年 | 198篇 |
2012年 | 182篇 |
2011年 | 271篇 |
2010年 | 183篇 |
2009年 | 326篇 |
2008年 | 369篇 |
2007年 | 195篇 |
2006年 | 167篇 |
2005年 | 205篇 |
2004年 | 210篇 |
2003年 | 259篇 |
2002年 | 154篇 |
2001年 | 242篇 |
2000年 | 155篇 |
1999年 | 191篇 |
1998年 | 209篇 |
1997年 | 164篇 |
1996年 | 192篇 |
1995年 | 247篇 |
1994年 | 243篇 |
1993年 | 117篇 |
1992年 | 177篇 |
1991年 | 65篇 |
1990年 | 76篇 |
1989年 | 167篇 |
1988年 | 57篇 |
1987年 | 69篇 |
1986年 | 72篇 |
1985年 | 217篇 |
1984年 | 155篇 |
1983年 | 138篇 |
1982年 | 167篇 |
1981年 | 207篇 |
1980年 | 67篇 |
1979年 | 50篇 |
1978年 | 53篇 |
1977年 | 46篇 |
1976年 | 41篇 |
1975年 | 41篇 |
1974年 | 44篇 |
1973年 | 32篇 |
1972年 | 36篇 |
1970年 | 33篇 |
1969年 | 36篇 |
排序方式: 共有7230条查询结果,搜索用时 375 毫秒
861.
862.
This paper describes one solution to the problem of providing Perimeter Intruder Detection, (PIDS), for sites whose boundary includes direct access from uncontrolled water. Two surveillance heads were proposed, each consisting of a TI camera and a Colour CCTV camera mounted on a pan and tilt unit 相似文献
863.
The separation of motions into slow (precession) and fast (nutation) components in the problem of the entry of a spacecraft (SC) with a small asymmetry into the atmosphere is considered. For the separation of the slow and fast motions the method of integral manifolds is used together with the asymptotic method for singularly perturbed systems. The separation of motions allows one to isolate the frequencies that are functions of the slow variables of a system, and further on, after determining the integer relations between them, to construct the resonant curves (surfaces). This method gives the possibility to analyze the conditions of the emergence of resonances for a SC at angles of attack that are not small and when aerodynamic characteristics are nonlinear. Examples of the construction of resonant curves for a SC with typical aerodynamic characteristics are considered. 相似文献
864.
Blue Jets: Upward Lightning 总被引:1,自引:0,他引:1
Blue jets are beams of blue light propagating from the tops of active thunderclouds up to altitudes of ~50 km. They resemble tall trees with quasi-vertical trunk and filamentary branches. Their apparent speeds are in the range of 10 s to 100 s km/s. Other events, having essentially lower terminal altitudes (<26 km), are named blue starters. These phenomena represent the first documented class of upward electrical discharges in the stratosphere. Some of upward discharges, termed gigantic jets, propagate into the lower ionosphere at much higher speeds in the final phase. We describe salient features of the upward discharges in the atmosphere, give an assessment of the theories of their development, and discuss the consequences for the electrodynamics and chemistry of the stratosphere. We argue that this upward lightning phenomenon can be understood in terms of the bi-directional leader, emerging from the anvil. 相似文献
865.
A.A. Arellano-Baeza R.V. García M. Trejo-Soto A. Martinez Bringas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Mexico is one of the most volcanically active regions in North America. Volcanic activity in central Mexico is associated with the subduction of the Cocos and Rivera plates beneath the North American plate. Periods of enhanced microseismic activity, associated with the volcanic activity of the Popocatépetl volcano are compared with periods, during which the microseismic activity was low. We detected systematical changes in the number of lineaments, associated with the microseismic activity due to lineament analysis of a temporal sequence of high resolution satellite images of the Popocatépetl volcano, provided by the ASTER/VNIR instrument. The Lineament Extraction and Stripes Statistic Analysis (LESSA) software package was used for the lineament extraction. In the future it would allow develop a methodology for detection of possible elevation of pressure in volcano edifice. 相似文献
866.
Cheng Hu Tao Zeng M. Cherniakov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Bistatic SAR (BSAR) systems have recently been the subject of several studies, but little attention has been given to the potential of the location of targets. In the paper, the performance of the estimation of 3-D position of a target (TPE) in parasitic SAR is obtained analytically and illustrated by computer simulation using ambiguity function analysis, and the maximum likelihood estimate (MLE) approach. It was shown that by using a multi look parasitic BSAR all three coordinates of an isolated point target (IPT) could be evaluated. Analytical closed form equations that characterize the measurement accuracy were derived. In the final these equations will be used to demonstrate various cases of more practical, including optimal trajectories choice under any geometry configuration case, moreover, the conclusion is verified via Matlab. 相似文献
867.
L. Foschini L. Maraschi F. Tavecchio G. Ghisellini M. Gliozzi R.M. Sambruna 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
It has been suggested that some radio-loud narrow-line Seyfert 1 contain relativistic jets, on the basis of their flat-spectrum radio nuclei and studies on variability. We present preliminary results of an ongoing investigation of the X-ray and multiwavelength properties of 5 radio-loud NLS1 based on archival data from Swift and XMM-Newton. Some sources present interesting characteristics, very uncharacteristic for a radio-quiet narrow-line Seyfert 1, such as very hard X-ray spectra, and correlated optical and ultraviolet variability. However, none of the studied sources show conclusive evidence for relativistic jets. γ-Ray observations with Fermi are strongly recommended to definitely decide on the presence or not of relativistic jets. 相似文献
868.
John M. Gonzales Jr. Brett A. Lowry Paul B. Brown Caula A. Beyl Leopold Nyochemberg 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Utilization of bio-regenerative life support systems (BLSS) plant waste residues as a nutritional source by Nile tilapia (Oreochromis niloticus) has proven problematic as a result of high concentrations of fibrous compounds in the plant waste residues. Nutritional improvement of plant waste residues by composting with the oyster mushroom (Pleurotus ostreatus), and the effects on growth and nutrient utilization of Nile tilapia fed such residues were evaluated. Five Nile tilapia (mean weight = 70.9 ± 3.1 g) were stocked in triplicate aquaria and fed one of two experimental diets, cowpea (CP) and composted cowpea (CCP), twice daily for a period of 8 weeks. Composting of cowpea residue resulted in reduced concentrations of nitrogen-free extract, hemi-cellulose and trypsin inhibitor activity, though trypsin inhibitor activity remained high. Composting did not reduce crude fiber, lignin, or cellulose concentrations in the diet. No significant differences (P < 0.05) were observed in weight gain, specific growth rate, survival rate, daily consumption, and food conversion ratio between tilapia fed CP and CCP. These results suggest that P. ostreatus is not a suitable candidate for culture in conjunction with the culture of Nile tilapia. Additional work is needed to determine what, if any, benefit can be obtained from incorporating composted residue as feed for Nile tilapia. 相似文献
869.
M. Yamashita K. Tomita-Yokotani H. Hashimoto N. Sawaki M. Notoya 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
The treatment of sodium, Na, and potassium, K, presents a challenge in space agriculture material recycling, as humans require Na and plants cannot grow at high Na concentrations. Hence, we are proposing the use of marine macro-algae to harvest K and other minerals from composted human waste. Ulva was selected for this feasibility study, since it tolerates a wide range of salinity levels. Growth capability of Ulva was examined under various total salinity levels and proportions of Na and K in the incubation medium. A homeostatic feature of Ulva was found in its intra-cellular concentration of Na and K, and in the intra-cellular ratio between Na and K (at 0.58 ± 0.30, lower than that of human metabolic waste). Intracellular concentration of K in Ulva is 20 times higher than seawater. Because of these characteristics, Ulva is a good candidate species for space agriculture. 相似文献
870.
F.F. Badavi M.A. Xapsos J.W. Wilson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
A rapid analytical procedure for the prediction of a micro-dosimeter response function in low Earth orbit (LEO), correlated with the Space Transportation System (STS, shuttle) Tissue Equivalent Proportional Counter (TEPC) measurements is presented. The analytical model takes into consideration the energy loss straggling and chord length distribution of the detector, and is capable of predicting energy deposition fluctuations in a cylindrical micro-volume of arbitrary aspect ratio (height/diameter) by incoming ions through both direct and indirect (δ ray) events. At any designated (ray traced) target point within the vehicle, the model accepts the differential flux spectrum of Galactic Cosmic Rays (GCRs) and/or trapped protons at LEO as input. On a desktop PC, the response function of TEPC for each ion in the GCR/trapped field is computed at the average rate of 30 s/ion. The ionizing radiation environment at LEO is represented by O’Neill’s GCR model (2004), covering charged particles in the 1 ? Z ? 28 range. O’Neill’s free space GCR model is coupled with the Langley Research Center (LaRC) angular dependent geomagnetic cutoff model to compute the transmission coefficient in LEO. The trapped proton environment is represented by a LaRC developed time dependent procedure which couples the AP8MIN/AP8MAX, Deep River Neutron Monitor (DRNM) and F10.7 solar radio frequency measurements. The albedo neutron environment is represented by the extrapolation of the Atmospheric Ionizing Radiation (AIR) measurements. The charged particle transport calculations correlated with STS 51 and 114 flights are accomplished by using the most recent version (2005) of the LaRC deterministic High charge (Z) and Energy TRaNsport (HZETRN) code. We present the correlations between the TEPC model predictions (response function) and TEPC measured differential/integral spectra in the lineal energy (y) domain for both GCR and trapped protons, with the conclusion that the model correctly accounts for the increase in flux at low y values where energetic ions are the primary contributor. We further discuss that, even with the incorporation of angular dependency in the cutoffs, comparison of the GCR differential/integral flux between STS 51 and 114 TEPC measured data and current calculations indicates that there still exists an underestimation by the simulations at low to mid range y values. This underestimation is partly related the exclusion of the secondary pion particle production from the current version of HZETRN. 相似文献