全文获取类型
收费全文 | 3253篇 |
免费 | 23篇 |
国内免费 | 13篇 |
专业分类
航空 | 1573篇 |
航天技术 | 1147篇 |
综合类 | 4篇 |
航天 | 565篇 |
出版年
2021年 | 26篇 |
2019年 | 23篇 |
2018年 | 54篇 |
2017年 | 51篇 |
2016年 | 45篇 |
2015年 | 26篇 |
2014年 | 71篇 |
2013年 | 91篇 |
2012年 | 82篇 |
2011年 | 120篇 |
2010年 | 79篇 |
2009年 | 137篇 |
2008年 | 153篇 |
2007年 | 99篇 |
2006年 | 71篇 |
2005年 | 92篇 |
2004年 | 90篇 |
2003年 | 99篇 |
2002年 | 72篇 |
2001年 | 115篇 |
2000年 | 65篇 |
1999年 | 68篇 |
1998年 | 87篇 |
1997年 | 58篇 |
1996年 | 93篇 |
1995年 | 112篇 |
1994年 | 111篇 |
1993年 | 48篇 |
1992年 | 71篇 |
1991年 | 34篇 |
1990年 | 32篇 |
1989年 | 76篇 |
1988年 | 28篇 |
1987年 | 24篇 |
1986年 | 33篇 |
1985年 | 90篇 |
1984年 | 90篇 |
1983年 | 65篇 |
1982年 | 66篇 |
1981年 | 99篇 |
1980年 | 19篇 |
1979年 | 25篇 |
1978年 | 29篇 |
1977年 | 27篇 |
1975年 | 29篇 |
1974年 | 23篇 |
1973年 | 21篇 |
1972年 | 20篇 |
1969年 | 20篇 |
1968年 | 18篇 |
排序方式: 共有3289条查询结果,搜索用时 93 毫秒
681.
Wahl D.E. Eichel P.H. Ghiglia D.C. Jakowatz C.V. Jr. 《IEEE transactions on aerospace and electronic systems》1994,30(3):827-835
The phase gradient autofocus (PGA) technique for phase error correction of spotlight mode synthetic aperture radar (SAR) imagery is examined carefully in the context of four fundamental signal processing steps that constitute the algorithm. We demonstrate that excellent results over a wide variety of scene content, and phase error function structure are obtained if and only if all of these steps are included in the processing. Finally, we show that the computational demands of the fun PGA algorithm do not represent a large fraction of the total image formation problem, when mid to large size images are involved 相似文献
682.
Howard E. McCurdy 《Space Policy》1994,10(4):277-289
Estimates of the cost of human space flight continue to generate controversy in the effort to set US space policy. Estimates vary widely, depending upon the position of the observer. This article identifies the real cost of major space flight programs and traces the heterofore unsuccessful efforts to cut the expense of space operations. 相似文献
683.
The pioneer venus orbiter ultraviolet spectrometer experiment: Analysis of hydrogen lyman alpha data
L.J. Paxton D.E. Anderson A.I.F. Stewart 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(9):129-132
Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS) HI 1216Å data from six (6) orbits are analyzed. Analysis of subsolar region periapsis data show that for an exobase temperature of 305K, the exobase density is and the column abundance of atomic hydrogen between 110 and 200 km is 2.4 ± 0.8(13) cm?2. The upward flux through the exobase is determined to be 7.5 ± 2.5(7)/cm2s. Apoapsis data were analyzed for both evening and morning geometries. We conclude: (1) the observed limb profiles show a diurnal variation consistent with Brinton et al.; (2) the model temperature field provides a good fit to the morning data, but the morning temperature field must be used to match the evening data; and (3) the theoretical Ly α limb intensity profiles are sensitive to small changes in the shape and magnitude of the variation of exobase hydrogen with solar zenith angle. The solar Ly α flux at line center required to fit the magnitude of the data is 8(11) photons/cm2s Å at Venus. 相似文献
684.
M. Morini D. Maccagni L. Maraschi D. Molteni E. G. Tanzi A. Treves 《Space Science Reviews》1985,40(3-4):601-606
We have observed the BL Lac object PKS 2155-304 with the EXOSAT observatory and with IUE on 1983 October 31 and November 29. At the same time, optical photometry has been obtained. Variability has been observed both on a time scale of hours during the October 31 observation and on the time scale of one month. During the November 29 observation the source was 2 times brighter than in the previous observation, but no spectral modification is found between the two observations. The source has been also detected in the ME instrument and an energy spectrum has been fitted with a power law. A marginal detection in the ME Xenon counters suggests a flattening of the spectrum in the high energy band. These X-ray data extending over three decades in energy are compared with the simultaneous UV and optical observations. 相似文献
685.
E.H. Lemke 《Acta Astronautica》1985,12(3):149-154
The idea of stabilizing the shape of a reflector dish for a space-based radiotelescope by an extensive magnetic field is considered. We outline a construction where the axes of rotational symmetry of the dish lie on the axes of rotational symmetry of a large circular solenoid. Its magnetic field produces a stabilizing pressure by acting on a distribution of dipole moment on the dish. In cosmic regions where the gravity gradient is smaller than the solar radiation pressure, large receiver apertures and large area-to-mass ratios of the dish are attainable. The latter might lead to a higher shape accuracy of the reflecting surface compared to the truss frame stabilization. We calculate the distribution of dipole moment that gives a parabolic surface in the approximation of no dish elasticity and no magnetic dish-dish interaction. A disadvantage is the relative large mass of the solenoid. However, we find that it can be about three times smaller in diameter than the dish. 相似文献
686.
E.S. Kazimirovsky E.I. Zhovty 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(1):149-151
Methods for constructing a dynamical part of the IRI model are described. 相似文献
687.
The paper reviews the main elements and major issues affecting human productivity and discusses some selected factors which might not be sufficiently identified and/or considered from the Bioastronautics point of view at the present stage of planning Space Station design and operation. 相似文献
688.
O. Kjeldseth Moe Ø. Andreassen P. Maltby J.-D.F. Bartoe G.E. Brueckner K.R. Nicolas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(8):63-66
Intensities and profiles of ion emission lines between 1170 A and 1700 A from an active region on the Sun are measured from spectra obtained with the Naval Research Laboratory's High Resolution Telescope and Spectrograph - HRTS. The measurements provide simultaneous determination of line intensities, wavelength shifts and Doppler widths at 50 separate positions in the active region, with spatial resolution of 1 arc second and spectral resolution 0.07 A. Fine structure variation of intensities and gas flow velocities in the temperature range 20,000–200,000 K are determined. The density sensitive line pair I(1486 N IV)/I(1548 C IV) has been used to measure electron pressures. Derived emission measures imply filling factors of 0.05–0.1 to balance the divergence of conductive flux width radiative losses above 60,000 K. 相似文献
689.
The detection search problem, one of distributing limited resources so as to maximize the detection probability in the single-try search for a concealed target with known probability density, is analyzed. Under fairly general assumptions, the optimal search density uniquely exists when the detection index is governed by the law of diminishing returns and another simple regularity condition. Numerical procedure based on the bisection method, which is guaranteed to converge if the solution uniquely exists, may be used to solve for the optimal search density and the associated Lagrange multiplier. When it is not possible to confidently estimate the target a priori probability density, the minimax solution guarantees a positive detection probability at the expense of degradation in performance 相似文献
690.
D R Morrison M L Lewis G H Barlow P Todd M E Kunze B E Sarnoff Z Li 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):77-79
Suspensions of cultured primary human embryonic kidney cells were subjected to continuous flow electrophoresis on Space Shuttle flight STS-8. The objectives of the experiments were to obtain electrophoretically separated fractions of the original cell populations and to test these fractions for the amount and kind of urokinase (a kidney plasminogen activator that is used medically for digesting blood clots), the morphologies of cells in the individual fractions, and their cellular electrophoretic mobilities after separation and subsequent proliferation. Individual fractions were successfully cultured after return from orbit, and they were found to differ substantially from one another and from the starting sample with respect to all of these properties. 相似文献