全文获取类型
收费全文 | 5563篇 |
免费 | 9篇 |
国内免费 | 25篇 |
专业分类
航空 | 2563篇 |
航天技术 | 2075篇 |
综合类 | 191篇 |
航天 | 768篇 |
出版年
2021年 | 31篇 |
2019年 | 36篇 |
2018年 | 79篇 |
2017年 | 44篇 |
2016年 | 47篇 |
2014年 | 111篇 |
2013年 | 135篇 |
2012年 | 120篇 |
2011年 | 190篇 |
2010年 | 122篇 |
2009年 | 212篇 |
2008年 | 285篇 |
2007年 | 134篇 |
2006年 | 130篇 |
2005年 | 148篇 |
2004年 | 151篇 |
2003年 | 166篇 |
2002年 | 204篇 |
2001年 | 227篇 |
2000年 | 90篇 |
1999年 | 137篇 |
1998年 | 162篇 |
1997年 | 108篇 |
1996年 | 151篇 |
1995年 | 181篇 |
1994年 | 152篇 |
1993年 | 93篇 |
1992年 | 128篇 |
1991年 | 59篇 |
1990年 | 56篇 |
1989年 | 123篇 |
1988年 | 45篇 |
1987年 | 51篇 |
1986年 | 55篇 |
1985年 | 203篇 |
1984年 | 155篇 |
1983年 | 111篇 |
1982年 | 135篇 |
1981年 | 188篇 |
1980年 | 59篇 |
1979年 | 44篇 |
1978年 | 42篇 |
1977年 | 44篇 |
1975年 | 46篇 |
1974年 | 43篇 |
1973年 | 38篇 |
1972年 | 39篇 |
1971年 | 34篇 |
1970年 | 39篇 |
1969年 | 45篇 |
排序方式: 共有5597条查询结果,搜索用时 0 毫秒
101.
Spectral emission lines created in the solar chromosphere — corona transition region show net red-shifts. It has been proposed that this may be the result of the return of spicular material. We simulate a spicule numerically using the rebound shock model and find that the resulting hydrodynamic evolution leads to a perceived up-flow in transition region spectral lines even though the average velocity in the line forming region is directed downward. The explanation for this apparent paradox is found in the correlation between density and velocity in the waves generated by the rebound shock spicule. 相似文献
102.
G V Dalrymple P K Leichner K A Harrison A B Cox K A Hardy Y L Salmon J C Mitchell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):267-270
Protons of a specific energy, 55 MeV, have been found to induce primary high grade astrocytomas (HGA) in the Rhesus monkey (Macaca mulatta). Brain tumors of this type were not induced by protons of other energies (32-2,300 MeV). Induction of HGA has been identified in human patients who have had radiation therapy to the head. We believe that the induction of HGA in the monkey is a consequence of dose distribution, not some unique "toxic" property of protons. Comparison of the human experience with the monkey data indicates the RBE for induction of brain tumors to be about one. It is unlikely that protons cause an unusual change in oncogenic expression, as compared to conventional electromagnetic radiation. 相似文献
103.
Timothy G. Hawarden Robert Crane Harley A. Thronson Jr. Alan J. Penny Anna H. Orlowska Thomas W. Bradshaw 《Space Science Reviews》1995,74(1-2):45-56
The designs of cold space telescopes, cryogenic and radiatively cooled, are similar in most elements and both benefit from orbits distant from the Earth. In particular such orbits allow the anti-sunward side of radiatively-cooled spacecraft to be used to provide large cooling radiators for the individual radiation shields. Designs incorporating these features have predictedT
tel near 20 K. The attainability of such temperatures is supported by limited practical experience (IRAS, COBE). Supplementary cooling systems (cryogens, mechanical coolers) can be advantageously combined with radiative cooling in hybrid designs to provide robustness against deterioration and yet lower temperatures for detectors, instruments, and even the whole telescope. The possibility of such major additional gains is illustrated by the Very Cold Telescope option under study forEdison, which should offerT
tel5 K for a little extra mechanical cooling capacity. 相似文献
104.
Miller RH 《Acta Astronautica》1995,36(8-12):581-587
Human productivity during assembly operations in-orbit is dependent on limits set by fatigue, metabolic rates, learning, and assembly techniques. In order to quantify these effects, tests were conducted in the NASA MSFC Neutral Buoyancy Simulator, in the NASA KC-135 in parabolic flight, and in space with the EASE program during the Shuttle Atlantis mission 61-B. A separate program attempted to relate productivity to system costs. Because of the surprisingly high productivity which had been demonstrated in orbit, it was shown that assembly operations would have only a small effect on system costs at the present level of launch costs. The results of these continuing studies have been reported in a recent paper(1). They will be briefly summarized here and the results updated to include additional cost elements and to examine the effects of reductions in transportation costs, resulting from advances in technology and from increased demand, on system costs. It is shown that, as launch costs are reduced, the assembly costs could become an increasingly important component of the total system costs. 相似文献
105.
Seven healthy subjects were submitted to a 42-day head down bedrest, where leg venous compliance (venous distensibity index VDI) and leg volumes were assessed by mercury strain gauge plethysmography with venous occlusion and optoelectronic plethysmography, respectively. Plethysmographic and volometric measurements were made, before, during (at days 1, 4, 7, 14, 21, 26, 34 and 41), and after bedrest (days 1, 4, 7, 11 and 30 of the recovery period). Results showed a continuous decrease in leg volumes throughout bedrest, when VDI increased until day 26 of bedrest, and then decreased afterwards. The recovery period was characterized by a rapid return of VDI to prebedrest levels while leg volumes progressively normalised. These results showed that leg venous compliance changes are not always dependant upon skeletal muscle changes, and that factors other than size of muscle compartment are able to determine increases in leg venous compliance during long-term bedrest. 相似文献
106.
U. Villante M. De Lauretis P. Francia M. Vellante A. Piancatelli 《Space Science Reviews》2006,122(1-4):107-117
We review the results obtained in the frequency range of Pc3 (22-100 mHz) and Pc4 (7-22 mHz) pulsations at Italian Antarctic
stations in the southern polar cap (“Mario Zucchelli”, at Terra Nova Bay, TNB, 80˚.S; “Concordia”, the Italian/French base at Dome C, DMC, 89˚.S). The absence of a midnight enhancement in the pulsation power suggests a negligible substorm influence at extreme latitudes,
while the sharp noon enhancement, which appears only at TNB, is determined by the closer proximity of the station to cusp
related phenomena. The relationship between the frequency of the band-limited signals and the interplanetary magnetic field
strength, the cone angle influence, and the higher correlation of the Pc3 power with the solar wind speed in the morning hours
suggest a global scenario in which upstream waves would be mainly responsible for the mid-frequency activity in the polar
cap. However, the polarization pattern is odd with respect to the predictions for tailward propagating modes. 相似文献
107.
Andrew P. Ingersoll 《Space Science Reviews》1976,18(5-6):603-639
Current information on the neutral atmosphere of Jupiter is reviewed, with approximately equal emphasis on composition and thermal structure on the one hand, and markings and dynamics on the other. Studies based on Pioneer 10 and 11 data are used to refine the atmospheric model. Data on the interior are reviewed for the information they provide on the deep atmosphere. The markings and dynamics are discussed with emphasis on qualitative relationships and analogies with phenomena in the Earth's atmosphere.Contribution No. 2652 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif. 91125, U.S.A. 相似文献
108.
Bruckner J.M.H. Scott R.W. Rea F.G. 《IEEE transactions on aerospace and electronic systems》1973,(6):883-888
The estimation of a multimodal linear system whose mode-to-mode transitions are described by a finite-state Markov chain is described. The problem has application in studying separation standards in an air traffic control environment. An optimal solution is formulated which is computationally infeasible. A suboptimal estimator is then derived which closely approximates the optimal estimator. An example is presented to illustrate the technique. 相似文献
109.
The application of existing estimation theory to the problem of specification and performance of passive sonar spectral estimators is considered. The classification function is addressed, so that the signal is assumed to be present, and so that the energy arrival angle is known. The spatial filter considered is a line array of M equally spaced omnidirectional hydrophones. Signal and ambient noise are both zero-mean, wide-sense, stationary Gaussian random processes that differ in their spatial correlation across the face of the array. The signal is a plane wave that can be made totally spacially corrected between array elements by inserting delays between sensors to invert the signal propagation delay. The noise correlation is a function of frequency, bandwidth, element separation, and the relative time delay between sensors. Under these assumptions, the Cramer-Rao lower bound is derived for the class of unbiased estimates of signal power in a narrow frequency band at the hydrophone in the presence of correlated ambient noise of known power. The bound is examined numerically, resulting in a threshold phenomenon with M that constitutes a new design consideration. In addition, there is a striking insensitivity to realistic values of ambient noise correlation, and there are ranges in signal-to-noise ratio for which one gains more by increasing M than by increasing the bandwidth-time product. Specific processors, including a new unbiased estimator when noise power is unknown, are developed. 相似文献
110.
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