全文获取类型
收费全文 | 5146篇 |
免费 | 27篇 |
国内免费 | 9篇 |
专业分类
航空 | 2658篇 |
航天技术 | 1798篇 |
综合类 | 23篇 |
航天 | 703篇 |
出版年
2021年 | 36篇 |
2019年 | 32篇 |
2018年 | 80篇 |
2017年 | 59篇 |
2016年 | 51篇 |
2014年 | 86篇 |
2013年 | 110篇 |
2012年 | 102篇 |
2011年 | 156篇 |
2010年 | 116篇 |
2009年 | 178篇 |
2008年 | 264篇 |
2007年 | 113篇 |
2006年 | 101篇 |
2005年 | 128篇 |
2004年 | 131篇 |
2003年 | 167篇 |
2002年 | 96篇 |
2001年 | 162篇 |
2000年 | 111篇 |
1999年 | 101篇 |
1998年 | 149篇 |
1997年 | 108篇 |
1996年 | 138篇 |
1995年 | 177篇 |
1994年 | 168篇 |
1993年 | 100篇 |
1992年 | 136篇 |
1991年 | 60篇 |
1990年 | 65篇 |
1989年 | 143篇 |
1988年 | 59篇 |
1987年 | 59篇 |
1986年 | 81篇 |
1985年 | 165篇 |
1984年 | 132篇 |
1983年 | 122篇 |
1982年 | 121篇 |
1981年 | 144篇 |
1980年 | 56篇 |
1979年 | 46篇 |
1978年 | 52篇 |
1977年 | 44篇 |
1975年 | 53篇 |
1974年 | 45篇 |
1973年 | 39篇 |
1972年 | 51篇 |
1971年 | 45篇 |
1969年 | 35篇 |
1967年 | 30篇 |
排序方式: 共有5182条查询结果,搜索用时 546 毫秒
811.
D. Cunnold 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(6):47-56
Stratospheric ozone observations by the SAGE and SBUV satellite instruments in March and April, 1979 have been analyzed. All SAGE profiles have been smoothed vertically over 8 km to provide some compatibility with the SBUV vertical resolution. Comparing the zonal mean ozone mixing ratios against smoothed LIMS profiles, it is inferred that SAGE is systematically overestimating ozone by approximately 20% at tropical latitudes at pressures lower than 5 mb and that SBUV is underestimating ozone by approximately 15% at 50–70° latitude at 10 mb. A comparison of the longitudinal variations of ozone by SBUV and SAGE is made and the detectability of planetary waves in ozone is emphasized. The uncorrelated portion of the SAGE variances are found to be approximately consistent with the SAGE noise model. Based on the correlated variances, the amplitudes of the smoothed SAGE planetary waves in ozone are found to be the same, on average, as in the SBUV experiment at mid-latitudes between 1 and 10 mb. Planetary wave detectability is illustrated during two several day periods at mid-latitudes and a persistent and theoretically-consistent relationship between ozone and temperature is noted. These examples, however, indicate that differences between ozone planetary wave amplitudes derived from the two sensors may occur when there is a strong vertical gradient in wave amplitude. 相似文献
812.
Organisms use gravity for spatial orientation, and differentiation into species during evolution follows geological processes which are caused by gravity. On the other hand, the task of most organismic functions which have or may have a relation to gravity is to compensate gravity. Furthermore, today it is very obvious that organisms do not disintegrate under the conditions of weightlessness, at least for the currently tested durations. These previous statements indicate a large field of still unknown regulation and adaptation mechanisms. Experiments to simulate weightlessness on the fast clinostat and with hyper-g show a highly developed ability of the genetic chain and of differentiating cells in being autonomous against mechanical stresses caused by outer accelerations. Nevertheless, different strong and slight changes of different tested end points were found. The question remains if the cells react actively or only passively. 相似文献
813.
The problem of bearing estimation for active systems is examined from the point of view of the generalized wideband ambiguity function (GAF). The maximum likelihood (ML) estimator is derived and its local and global properties are discussed. A structure is proposed which searches the three-dimensional ambiguity surface in two stages first, in range-Doppler, and then, in bearing with the goal of reducing search complexity when utilizing highly resolvent waveforms. Comparisons are made between the ML estimators and structures utilizing phase information which generate closed form estimators. The beneficial results of full bandwidth utilization are discussed in terms of both local and global properties of the GAF. 相似文献
814.
The relationships of type Pi (broadband) pulsations to various other substorm-related phenomena are reviewed. Several of the more popular mechanisms for the origin of Pi activity are discussed in the light of the observations. There is only one mechanism in sight that tentatively accounts for observed characteristics of Pi 1–2 activity at auroral oval and polar cap latitudes and that is the three-dimensional current loop mechanism. If two or more mechanisms are involved in the generation of Pi noise, then it is possible that the garden-hose overstability and/or a drift Alfvén wave mechanism operating in the plasma sheet contribute to the observed pulsations.The common feature of all Pi 1–2 events is not the presence of temporal precipitation pulsations but the presence of an E-region, suggesting that enhanced conductivity and E-region currents are required. Pi activity appears to be closely related to unsteady convection in progress. Pi data promise to provide useful information on convection and field-aligned and ionospheric currents. 相似文献
815.
816.
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
817.
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
818.
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
819.
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
820.
The leading edge estimator (LEE) of a pulse signal is defined as the instant at which a filtered version of the received noisy signal passes a preset threshold. A rigorous analysis for a rectangular pulse model of the signal results in an exact probability density function for the LEE, valid within the time interval of the leading edge of the filtered pulse. Possible occurrence of the threshold crossing outside of this interval is considered to be an anomalous estimate, since it leads to a gross error in comparison with the regular cases. It is found that the density function of the LEE error is asymmetrical and therefore biased, that the probability PA of anomalous estimation increases with the filter bandwidth, thus setting a well definable limit to the latter and that, for prespecified PA, the minimum bias and variance are proportional, respectively, to R-1 and R-2, minima being obtained by allowing for the largest bandwidth compatible with PA. On the other hand, for given bandwidth the variance decreases only as R-1. Here R is the signal-to-noise energy ratio. Results are presented in form of parameterized graphs. 相似文献