全文获取类型
收费全文 | 5415篇 |
免费 | 9篇 |
国内免费 | 18篇 |
专业分类
航空 | 2836篇 |
航天技术 | 1920篇 |
综合类 | 23篇 |
航天 | 663篇 |
出版年
2021年 | 30篇 |
2019年 | 32篇 |
2018年 | 57篇 |
2017年 | 31篇 |
2014年 | 82篇 |
2013年 | 132篇 |
2012年 | 107篇 |
2011年 | 155篇 |
2010年 | 114篇 |
2009年 | 177篇 |
2008年 | 245篇 |
2007年 | 136篇 |
2006年 | 140篇 |
2005年 | 143篇 |
2004年 | 114篇 |
2003年 | 170篇 |
2002年 | 101篇 |
2001年 | 176篇 |
2000年 | 109篇 |
1999年 | 140篇 |
1998年 | 164篇 |
1997年 | 123篇 |
1996年 | 169篇 |
1995年 | 215篇 |
1994年 | 182篇 |
1993年 | 120篇 |
1992年 | 127篇 |
1991年 | 76篇 |
1990年 | 62篇 |
1989年 | 138篇 |
1988年 | 62篇 |
1987年 | 66篇 |
1986年 | 60篇 |
1985年 | 195篇 |
1984年 | 150篇 |
1983年 | 130篇 |
1982年 | 133篇 |
1981年 | 176篇 |
1980年 | 58篇 |
1979年 | 41篇 |
1978年 | 50篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1975年 | 58篇 |
1974年 | 38篇 |
1973年 | 42篇 |
1972年 | 51篇 |
1971年 | 42篇 |
1970年 | 44篇 |
1969年 | 37篇 |
排序方式: 共有5442条查询结果,搜索用时 15 毫秒
841.
A unified approach to model-reference adaptive control systems is presented. A linearized error equation for each adaptive system is derived about some operating point and converted into the standard form for plotting root loci. The contribution made here is to convert the selection of an adaptive technique and the choosing of adaptive coefficients for some desired system performance from a "mystical art" to standard control techniquess. 相似文献
842.
Approximate equations are developed for converting geodetic coordinates to a local north-east-down (NED) Cartesian coordinate system, and it is shown that good accuracy can be obtained if the ellipsoidal shape of the Earth is taken into account. 相似文献
843.
J.P. Matthews K.H. Yearby 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(2):209-212
Halley (75.5°S, 26.7°W, L = 4.3) is well placed for reception of subionospheric signals from the Siple transmitter. Occasionally a two-hop magnetospheric response to such signals can be observed. The data presented here are relevant to the problems of the radiation and propagation of VLF waves in the polar earth/ionosphere waveguide, duct stimulation and ducted wave amplification and the growth of triggered emissions. An upper sideband emission 20–40 Hz higher in frequency than the transmitted pulse was often received at a similar strength to, and simultaneously with the two-hop whistler mode echo. 相似文献
844.
Two attractive real-time implementation tests are presented that discriminate between correlated and uncorrelated clutter. A clutter model is assumed in which the envelope distribution within a cell is Rayleigh, but the mean clutter level fluctuates from cell to cell. Both the tests utilize observations made in pairs on two clutter envelopes corresponding to two consecutive azimuth sweeps. The results are applicable to real-time testing of the effectiveness of various decorrelation techniques employed by radar systems. 相似文献
845.
M.J. Smith J.B. Gregory A.H. Manson C.E. Meek R. Schminder D. Ku¨rschner K. Labitzke 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):173-176
The global developments of the four stratospheric events (~20–50 km) are described, using balloon, satellite and rocket data. Winds data for heights of 60–95 km, derived from L.F. drift (52°N, 15°E; Europe) and M.F. radar (52°N, 107°W; Canada), are then compared with the stratospheric data. There is clear evidence that the preliminary planetary waves often penetrate strongly to ~90 km, and that mean wind reversals also occur. However, there are dramatic differences between European and Canadian mesospheric responses. 相似文献
846.
M.G. Kivelson K.W. Behannon T.E. Cravens I. de Pater T.V. Johnson D.L. Matson H. Masursky D.J. Southwood V.M. Vasyliunas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(3):5-14
A was presented in conjunction with the Twenty-fourth Plenary Meeting of the Committee on Space Research. This paper summarizes the talks presented and places the remaining papers of this volume in context. 相似文献
847.
Thin-film rechargeable Li-LiMn2O4 batteries have been fabricated and characterized. Following deposition by electron beam evaporation of LiMn2O4, the amorphous as-deposited cathode films 1 cm2 in area by 0.3to 4-μm thick were annealed at 700°C to 800°C in oxygen in order to form the crystalline spinet phase. The specific capacity of the cells between 4.5 V to 3.8 V ranged from 50 μAh/mg to 120 μAh/mg. When cycled over this range, the batteries exhibited excellent secondary performance with capacity losses as low as 0.001% per cycle. On charging to 5.3 V, a plateau with a median voltage of 5.1 V was observed. The total charge extracted between 3.8 V to 5.3 V corresponded to about 1 Li/Mn2 O4 相似文献
848.
J Kiefer S Rase F Schopfer E Schneider K Weber G Kraft 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(8):115-125
The action of heavy ions (Ar to U) accelerated to specific energies up to about 10 MeV/u (u=atomic mass unit) on different functions of yeast cells was studied. Ribosomal-RNA synthesis is inhibited according to a single-hit mechanism. Inactivation cross-sections were linearly related to the ratio of the squares of the effective charge Z* and the velocity of the ions. It is concluded from the analysis that the range of the most energetic delta-electrons is larger than previously assumed. There is no such dependence for survival and induction of mutants. In both cases cross-sections increase with the ion's specific-energy indicating an important contribution of long-range delta-electrons. The analysis shows that diploid yeast is not killed by a single-hit mechanism even by very heavy ions if the track width is too small. The relative importance of the penumbral region is even more pronounced with the more sensitive strains. 相似文献
849.
A.J. Dean 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):73-82
The advent of improved γ-ray telescopes which incorporate high angular resolution imaging properties and adequate sensitivity will advance this branch of astronomy from the discovery phase to the exploratory phase. As in other fields, such as radio and X-ray astronomy, which have recently undergone this change, it will prove a fascinating era. The recent development of position sensitive γ-ray detection planes operated in conjunction with a suitable coded aperture mask have made γ-ray telescopes feasible which are capable of generating γ-ray images of the sky with a precision of 1 arc minute over the photon energy range 0.1 to 10 MeV. With a sensitivity of at least 1–10 milliCrab and scintillation standard spectral resolution not only can a large number of discrete γ-ray objects be identified and studied in detail but nuclear γ-ray line images of extended objects such as the Galactic Plane, Cloud Complexes, and supernovae remnants may be generated by this class of astronomical instrument. 相似文献
850.
J.M. Malherbe G. Simon P. Mein N. Mein B. Schmieder J.C. Vial 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(11):53-56
Disappearances of preflare filaments have been observed on June 22, 1980 (S07,W13) (flare at 13.04 U.T.) and September 3, 1980 (flare at 7.52 U.T.). The analysis of MSDP data (MEUDON) leads to the followings conclusions : - Disappearances begin a few minutes before the Hα impulsive phase. - The filaments can be seen again after the flares. - Upwards motions occur in several points, without disturbing significantly preëxisting downflows.Velocity maps suggest shears or velocity loops.The filament disappearance seems to be due to a heating mechanism beginning before the flare maximum. 相似文献