全文获取类型
收费全文 | 4311篇 |
免费 | 6篇 |
国内免费 | 8篇 |
专业分类
航空 | 2203篇 |
航天技术 | 1641篇 |
综合类 | 18篇 |
航天 | 463篇 |
出版年
2018年 | 58篇 |
2017年 | 47篇 |
2016年 | 33篇 |
2014年 | 66篇 |
2013年 | 91篇 |
2012年 | 74篇 |
2011年 | 130篇 |
2010年 | 91篇 |
2009年 | 148篇 |
2008年 | 228篇 |
2007年 | 85篇 |
2006年 | 72篇 |
2005年 | 99篇 |
2004年 | 125篇 |
2003年 | 153篇 |
2002年 | 79篇 |
2001年 | 147篇 |
2000年 | 89篇 |
1999年 | 109篇 |
1998年 | 141篇 |
1997年 | 101篇 |
1996年 | 114篇 |
1995年 | 140篇 |
1994年 | 164篇 |
1993年 | 82篇 |
1992年 | 116篇 |
1991年 | 48篇 |
1990年 | 58篇 |
1989年 | 115篇 |
1988年 | 40篇 |
1987年 | 56篇 |
1986年 | 63篇 |
1985年 | 142篇 |
1984年 | 88篇 |
1983年 | 97篇 |
1982年 | 105篇 |
1981年 | 134篇 |
1980年 | 56篇 |
1979年 | 46篇 |
1978年 | 42篇 |
1977年 | 38篇 |
1976年 | 26篇 |
1975年 | 40篇 |
1974年 | 36篇 |
1973年 | 32篇 |
1972年 | 36篇 |
1971年 | 35篇 |
1969年 | 35篇 |
1967年 | 25篇 |
1966年 | 26篇 |
排序方式: 共有4325条查询结果,搜索用时 0 毫秒
681.
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
682.
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
683.
684.
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
685.
The effect of random errors in the steering vector of an Applebaum adaptive array is examined. Each component of the steering vector is assumed to have a random error component uncorrelated between elements. The array output signal-to-interferenceplus-noise ratio (SINR) is computed as a function of the error variance. It is shown that the array output SINR becomes more sensitive to steering vector errors as more elements are added to the array and as the received desired signal power becomes larger. The variance of the steering vector error that may be tolerated depends on the required desired signal dynamic range. The larger the dynamic range that must be accommodated, the smaller the error variance must be. 相似文献
686.
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
687.
W.A. Gault G.G. Shepherd 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(7):111-114
A CCD imaging device has been designed which combines a spatial resolution of 0.1° with a spectral resolving power of 105, sufficient to measure Doppler shifts and line widths of individual atmospheric emission lines. It will be flown aboard Spacelab and will be used to study neutral winds and temperatures as functions of height, latitude and time of day. 相似文献
688.
Johnson S.D. Witulski A.F. Erickson R.W. 《IEEE transactions on aerospace and electronic systems》1988,24(3):263-274
Because of their tolerance of transformer nonidealities, resonant converters are considered to be well-suited to high-voltage applications. The series and parallel resonant topologies, as well as a newly discovered hybrid resonant topology are compared for high-voltage applications. Design criteria which incorporate transformer nonidealities are developed and used in the construction of high voltage prototypes for each topology. It is found that the parallel topology leads to the lowest peak switch current and the most ideal behavior 相似文献
689.
M. Grewing A. Vidal-Madjar A. Labeque C. Laurent A. Boksenberg C. Burgio S. di Serego Alighieri G.P. Whitcomb W. Burton C.I. Coleman R. Hoekstra C. Jamar J. Lequeux P. Rafanelli 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(4):147-151
Magellan is one of the future space projects being studied by the European Space Agency. The aim is to provide high resolution (λ/Δλ ≥ 2.5 × 104) spectra in the far and extreme UV (between 500 and 1550 Å) of faint galactic and extragalactic objects (V ≤ 16m). The instrument consists of a mechanical collimator, a concave holographic grating and a bidimensional photon-counting detector. A low resolution mode (λ/Δλ ≥ 103) will provide spectra of objects as faint as 18m.5. Magellan is planned as an observatory, operated in real time, and allowing interaction with the observer. 相似文献
690.
W.B. Hubbard 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(2):103-106
We have collected information about Martian atmospheric scale heights derived from observations of the occultation of ? Gem by Mars on April 8, 1976. The observations give data in the altitude range ~50 to 80 km. A rough, unweighted average of results so far available yields a temperature of ~165°K. Excursions of ~±40°K about this mean may be present as a function of both altitude and areographic coordinates. 相似文献