全文获取类型
收费全文 | 3209篇 |
免费 | 6篇 |
国内免费 | 13篇 |
专业分类
航空 | 1695篇 |
航天技术 | 1087篇 |
综合类 | 11篇 |
航天 | 435篇 |
出版年
2019年 | 17篇 |
2018年 | 39篇 |
2017年 | 22篇 |
2016年 | 21篇 |
2014年 | 57篇 |
2013年 | 81篇 |
2012年 | 62篇 |
2011年 | 108篇 |
2010年 | 84篇 |
2009年 | 115篇 |
2008年 | 178篇 |
2007年 | 94篇 |
2006年 | 66篇 |
2005年 | 83篇 |
2004年 | 78篇 |
2003年 | 100篇 |
2002年 | 54篇 |
2001年 | 100篇 |
2000年 | 49篇 |
1999年 | 80篇 |
1998年 | 93篇 |
1997年 | 74篇 |
1996年 | 92篇 |
1995年 | 125篇 |
1994年 | 98篇 |
1993年 | 66篇 |
1992年 | 101篇 |
1991年 | 42篇 |
1990年 | 36篇 |
1989年 | 87篇 |
1988年 | 34篇 |
1987年 | 31篇 |
1986年 | 32篇 |
1985年 | 88篇 |
1984年 | 91篇 |
1983年 | 60篇 |
1982年 | 82篇 |
1981年 | 92篇 |
1980年 | 36篇 |
1979年 | 42篇 |
1978年 | 32篇 |
1977年 | 22篇 |
1976年 | 21篇 |
1975年 | 37篇 |
1974年 | 24篇 |
1973年 | 18篇 |
1972年 | 29篇 |
1969年 | 17篇 |
1968年 | 17篇 |
1967年 | 21篇 |
排序方式: 共有3228条查询结果,搜索用时 836 毫秒
501.
Global positioning by means of satellites requires simultaneous observation by at least four satellites. The problem is to determine the minimum number of satellites and the corresponding orbital geometry necessary to satisfy this requirement on a continuous basis. To model the problem, a fixed number of users are assumed uniformly distributed in a known manner over the surface of the earth, and the satellites are restricted to exist in either three or four orbital planes. However, the orbit radius and inclination angle are left as variables. Under these assumptions, and starting with a small number of satellites which will be increased afterwards, an algorithm is developed to determine the visibility of satellites at each surface location. In this way it is possible to specify the minimum number of satellites needed by any desired orbital geometry. It is found that the number of satellites required for three-dimensional continuous worldwide coverage decreases as the orbit radius is increased. There appears to be no general trend regarding the effect of the inclination angle on the minimum number of satellites. 相似文献
502.
D.A. Usher M.C. Needels 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(12):163-166
Aminoacylation of the “internal” 2′-hydroxyl groups of poly(A) with the imidazolide of racemic DL-leucine resulted in the formation of an enantiomeric excess of the D-ester, whereas aminoacylation with the imidazolide of racemic N-3,5-dinitrobenzoyl-DL-leucine gave an enantiomeric excess of the L-ester. Comparison of these results with those obtained earlier for alanine shows that the larger side chain tends to favor the formation of the D-ester. High resolution proton magnetic resonance spectroscopy was performed on the D- and L-alanyl “internal” 2′-esters of 3′,5′-ApA. The resonance of the side chain methyl group of the D-ester is upfield from that of the L-ester, and the separation of the peaks increases with a decrease in temperature. 相似文献
503.
C.S. Zerefos 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(5):177-181
The results of a cross-spectral analysis between monthly mean temperatures at 100 mb, 50 mb and 30 mb over the equator and the corresponding monthly mean BUV total ozone at different latitude zones are presented for the period 1970–1977. Significant squared coherences between total ozone and 50 mb equatorial temperatures at 26 months are only found between 5 degrees on each side of the equator, between 45 deg N and 55 deg N and at 45 deg S. At latitudes were the QBO in stratospheric temperature diminishes so does the QBO in total ozone (i.e. close to 35 deg N and 15 deg S). Over subtropical latitudes there is a tendency towards a more biennial oscillation in total ozone (not correlated with the equatorial QBO in stratospheric temperatures) and at 65 deg N and 65 deg S total ozone oscillates at periods greater than the equatorial QBO. 相似文献
504.
An algorithm is described for detecting moving optical targets against spatially nonstationary Poisson background and noise. The algorithm has applications in optical detection of objects such as meteors, asteroids, and satellites against a stellar background. A maximum-likelihood approach is used which results in reducing interference from stars. It is shown that by choosing a detection threshold to provide a constant false alarm rate, the resulting algorithm is independent of the signal strength of the target. An analysis of this algorithm is presented, showing the probability of detection for several false-alarm rates 相似文献
505.
The radar payload on a space-based radar (SBR) satellite could require tens of kilowatts of power distributed to many small loads over a large area. This poses special problems for the power distribution and control system (PDCS). A study that examined the power requirements of an SBR spacecraft is reported. A baseline prime power system, generating about 30 kW, was derived. The proposed distribution network would transmit 240 V at 20 kHz. The voltage would be downconverted in one converter for about 100 transmit/receive modules. The design considerations are discussed, and the baseline PDCS is described 相似文献
506.
507.
A formula is derived for interpolation between output samples of a fast Fourier transform (FFT), i.e., in the frequency domain. Such a formula is useful for obtaining greater frequency resolution when two coarse FFT outputs are available. Consideration is also given to the effect of such interpolation on a weighted FFT. 相似文献
508.
C.R. Philbrick M.E. Garnder P. Lämmerzhal 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(1):95-98
Measurements of the density and composition of the thermosphere between 150 and 500 km, which were obtained by the S3-1 satellite, have been compared with the Jacchia and MSIS models. The measurements of the densities of O, N2, N and Ar show some differences from the current models which should be considered during the preparation of the next CIRA model. The Ar measurements are particularly useful in examining the response of the neutral atmosphere to geomagnetic heating. These results are useful in establishing the appropriate lower boundary conditions for modeling of the thermosphere. 相似文献
509.
M K Wallis N C Wickramasinghe F Hoyle 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(4):281-285
Comet Halley studies indicate most of the nucleus is covered by an insulating crust, presumed of pyrolysed organic material. The subcrust is warmed and percolated by gases within 2AU, so provides one habitat for primitive replicating organisms. Cracks and crevices within contaminated ice in the craters provides a habitat for photosynthesising organisms. Subsurface lakes on the Europa model, though insulated by some metres of ice, would require a trigger (perhaps meteorite impact and energy source (chemical or metabolic energy) to initiate and maintain a suitable-habitat on short period comets. Constraints on transfer between comets and other planetary bodies implies that radiation-resistant species with lengthy hibernation potential would be expected. 相似文献
510.