全文获取类型
收费全文 | 4640篇 |
免费 | 20篇 |
国内免费 | 12篇 |
专业分类
航空 | 2117篇 |
航天技术 | 1757篇 |
综合类 | 25篇 |
航天 | 773篇 |
出版年
2021年 | 37篇 |
2019年 | 26篇 |
2018年 | 60篇 |
2017年 | 53篇 |
2016年 | 50篇 |
2014年 | 83篇 |
2013年 | 120篇 |
2012年 | 110篇 |
2011年 | 183篇 |
2010年 | 128篇 |
2009年 | 186篇 |
2008年 | 265篇 |
2007年 | 130篇 |
2006年 | 104篇 |
2005年 | 149篇 |
2004年 | 160篇 |
2003年 | 150篇 |
2002年 | 104篇 |
2001年 | 161篇 |
2000年 | 74篇 |
1999年 | 114篇 |
1998年 | 139篇 |
1997年 | 87篇 |
1996年 | 98篇 |
1995年 | 125篇 |
1994年 | 146篇 |
1993年 | 79篇 |
1992年 | 104篇 |
1991年 | 40篇 |
1990年 | 53篇 |
1989年 | 91篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 47篇 |
1985年 | 143篇 |
1984年 | 132篇 |
1983年 | 109篇 |
1982年 | 96篇 |
1981年 | 173篇 |
1980年 | 37篇 |
1979年 | 38篇 |
1978年 | 43篇 |
1977年 | 39篇 |
1976年 | 33篇 |
1975年 | 37篇 |
1974年 | 30篇 |
1973年 | 32篇 |
1972年 | 42篇 |
1970年 | 25篇 |
1969年 | 25篇 |
排序方式: 共有4672条查询结果,搜索用时 640 毫秒
751.
McCormick W.S. Tsui J.B.Y. Bakkie V.L. 《IEEE transactions on aerospace and electronic systems》1989,25(5):729-732
The instantaneous frequency measurement (IFM) receiver is capable of measuring the center frequency of single frequency pulses over a wide range (bandwidth) of center frequencies. Because of various constraints, the frequency resolution requirement results in long correlator delay times that reduce the single correlator bandwidth. A large bandwidth can be achieved only if two or more correlators are used. The problem of estimating frequency is then reduced to the simultaneous congruence problem of number theory. A design procedure is presented for solving the congruence problem for a given amount of noise protection, a stated frequency resolution, a minimum bandwidth, and a fixed level of precision (bits) in the IFM receiver 相似文献
752.
Buongiorno C. Vallerani E. Borriello G. 《Aerospace and Electronic Systems Magazine, IEEE》1989,4(5):17-25
The authors present the plan under discussion in Italy between the government entities and Aeritalia, the leading Italian aerospace company, for the technology development needed to support advanced transportation systems. The plan centers around the following areas: thermal protection and control; advanced structures and materials; guidance, navigation, and control; and tests and computation methods. Experimental activities needed to support the development of such systems and the difficulties involved are examined 相似文献
753.
This paper is devoted to an experimental study of swept wing leading edge contamination by the turbulence emanating from the wing-wall junction. The main objective is to delay the contamination onset by applying surface suction along the attachment line. Two series of experiments are described; the first one was performed in a small wind tunnel at CERT ONERA, the second one was carried out in the F2 wind tunnel at Le Fauga Mauzac centre. Hot film measurements showed that leading edge contamination could be delayed up to very large Reynolds numbers. We also studied the behaviour of the relaminarized boundary layer downstream of the sucked region, along the span as well as in the chordwise direction. 相似文献
754.
Sekine M. Ohatani S. Musha T. Irabu T. Kiuchi E. Hagisawa T. Tomita Y. 《IEEE transactions on aerospace and electronic systems》1982,(6):729-730
It is shown that the Weibull-distributed ground clutter obeys a Weibull distribution after processing by the double canceler moving target indicator (MTI). 相似文献
755.
G Horneck H Bucker K Dose K D Martens A Bieger H D Mennigmann G Reitz H Requardt P Weber 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(10):19-27
Bacterial spores are proper test organisms for studying problems of space biology and exobiology. During the Spacelab 1 mission, studies on the limiting factors for survival of Bacillus subtilis spores in free space have been performed. An exposure tray on the pallet of Spacelab 1 accomodated 316 samples of dry spores for treatment with space vacuum and/or the following selected wavelengths of solar UV: > 170 nm, 220 nm, 240nm, 260nm and 280 nm. After recovery, inactivation, mutation induction, reparability, and photochemical damages in DNA and protein have been studied. The results contribute to the understanding of the mechanisms of increased UV sensitivity of bacterial spores in vacuo and to a better assessment of the chance of survival of resistant forms in space and of interplanetary transfer of life. 相似文献
756.
Rockah Y. Messer H. Schultheiss P.M. 《IEEE transactions on aerospace and electronic systems》1988,24(4):402-410
The passive localisation of radiating sources using an array subject to random perturbations in sensor phases is presented. All source signals as well as additive noises observed at the sensors are assumed to be independent identically distributed zero-mean Gaussian random processes. Cramer-Rao bounds are derived for source bearings and ranges for the phase errors at each sensor. It is shown that accurate phase calibration can be achieved when the number of sources exceeds a certain minimum. The locations of the calibrating sources need not be known a priori and need only satisfy mild regularity conditions. A calibration procedure is proposed which uses maximum-likelihood techniques 相似文献
757.
G.S.R. Sarma 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(5):55-58
Asymptotic stability characteristics with respect to the onset of Marangoni convection in a liquid layer under the simultaneous action of a large transverse magnetic field and a large rate of rotation are investigated. In certain parameter ranges there is a decrease in critical Marangoni number for increasing magnetic field and rotation depending on the coupling between interfacial perturbations and rotation. 相似文献
758.
B.V. Dementjev S.G. Kukin M.N. Markov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):107-110
The radiation spectra of the ionosphere in the 4–5 μm region have been determined from stratospheric balloons by means of the specially elaborated method of the registration of angular and spectral distribution of the radiation. The radiation bands of 4.4 μm and 4.8 μm which have 0.1–0.2 erg cm?2 sr?1s?1. brightness obtained in two flights, are identified with the vibration-rotation transitions of the ion NO+ (0 - 1) and the molecules N14N15 (0 - 1) and CO (0 - 1) and (3 - 2) from altitudes between 120 and 500 km. 相似文献
759.
G. Erdós J. Kóta 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(3):139-142
We consider regular motion of 50 – 200 GV particles in a large-scale interplanetary magnetic field model which contains a wavy neutral sheet responsible for the sector-structure. Numerical calculations based upon energy losses along various trajectories are carried out to obtain the predicted omni-directional density and anisotropy of cosmic rays at various solar latitudes. A marked difference is found between odd and even solar cycles. The post-1969 field configuration gives small radial and large latitudinal gradient: cosmic ray density increases toward the poles. The latitudinal gradient turns out smaller and of opposite sense for the pre-1969 epoch. Anisotropy changes dramatically as we move off the solar equator: corotation appears to be restricted to low latitudes. 相似文献
760.