全文获取类型
收费全文 | 3697篇 |
免费 | 128篇 |
国内免费 | 115篇 |
专业分类
航空 | 1874篇 |
航天技术 | 1296篇 |
综合类 | 80篇 |
航天 | 690篇 |
出版年
2021年 | 33篇 |
2019年 | 32篇 |
2018年 | 60篇 |
2017年 | 54篇 |
2016年 | 53篇 |
2015年 | 33篇 |
2014年 | 103篇 |
2013年 | 124篇 |
2012年 | 109篇 |
2011年 | 154篇 |
2010年 | 120篇 |
2009年 | 168篇 |
2008年 | 174篇 |
2007年 | 118篇 |
2006年 | 87篇 |
2005年 | 117篇 |
2004年 | 99篇 |
2003年 | 113篇 |
2002年 | 83篇 |
2001年 | 131篇 |
2000年 | 95篇 |
1999年 | 110篇 |
1998年 | 120篇 |
1997年 | 87篇 |
1996年 | 108篇 |
1995年 | 131篇 |
1994年 | 126篇 |
1993年 | 66篇 |
1992年 | 90篇 |
1991年 | 46篇 |
1990年 | 43篇 |
1989年 | 82篇 |
1988年 | 34篇 |
1987年 | 28篇 |
1986年 | 35篇 |
1985年 | 92篇 |
1984年 | 90篇 |
1983年 | 66篇 |
1982年 | 67篇 |
1981年 | 100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3940条查询结果,搜索用时 15 毫秒
131.
Light scattering in planetary atmospheres 总被引:45,自引:0,他引:45
This paper reviews scattering theory required for analysis of light reflected by planetary atmospheres. Section 1 defines the radiative quantities which are observed. Section 2 demonstrates the dependence of single-scattered radiation on the physical properties of the scatterers. Section 3 describes several methods to compute the effects of multiple scattering on the reflected light. 相似文献
132.
This paper deals with the application of modern estimation techniques to the problem of speech data rate reduction. It is desirable to adaptively identify and quantitize the parameters of the speech model. These paramaters cannot be identified and quantized exactly; the performance of the predictor is thereby degraded and this could prevent data reduction. In many cases it is desirable to emply a suboptimal predictor in order to simplify the algorithms, and predictor performance is again degraded. This paper develops sensitivity and error analysis as a potential method for determining quantitatively how speech data reduction system performance is degraded by imprecise parameter knowledge or suboptimal filtering. An intended use of the sensitivity and error analysis algorithms is to determine parameter identification and model structure requirements of configuration concepts for adaptive speech digitizers. First, sensitivity and error analysis algorithms are presented that form the basis for the remainder of the work. The algorithms are then used to determine how imprecise knowledge of vocal tract parameters degrades predictor performance in speech. Transversal filters have previously been proposed for this application. The sensitivity analysis algorithms are then used to determine when and by how much the transverse filter is suboptimal to the Kalman filter. In particular, the question of how effectively a higher order of all-pole model approximates a system with zeros is answered, as this question is of considerable importance in speech. Finally, the physical significance of the innovations process in speech data rate reduction is studied. 相似文献
133.
134.
The modulation of galactic cosmic rays in the heliosphere seems to be dominated by four major mechanisms: convection, diffusion,
drifts (gradient, curvature and current sheet), and adiabatic energy losses. In this regard the global structure of the solar
wind, the heliospheric magnetic field (HMF), the current sheet (HCS), and that of the heliosphere itself play major roles.
Individually, the four mechanisms are well understood, but in combination, the complexity increases significantly especially
their evolvement with time - as a function of solar activity. The Ulysses observations contributed significantly during the past solar minimum modulation period to establish the relative importance
of these major mechanisms, leading to renewed interest in developing more sophisticated numerical models, and in the underlying
physics, e.g., what determines the diffusion tensor. With increased solar activity, the relative contributions of the mentioned
mechanisms change, but how they change and what causes these changes over an 11-year solar cycle is not well understood. It
can therefore be expected that present and forthcoming observations during solar maximum activity will again produce very
important insights into the causes of long-term modulation. In this paper the basic theory of solar modulation is reviewed
for galactic cosmic rays. The influence of the Ulysses observations on the development of the basic theory and numerical models are discussed, especially those that have challenged
the theory and models. Model-based predictions are shown for what might be encountered during the next solar minimum. Lastly,
modulation theory and modelling are discussed for periods of maximum solar activity when a global reorganization of the HMF,
and the HCS, occurs.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
135.
F.B. Rizzato A.C.-L. Chian M.V. Alves R. Erichsen S.R. Lopes G.I. de Oliveira R. Pakter E.L. Rempel 《Space Science Reviews》2003,107(1-2):507-514
Langmuir waves and turbulence resulting from an electron beam-plasma instability play a fundamental role in the generation
of solar radio bursts. We report recent theoretical advances in nonlinear dynamics of Langmuir waves. First, starting from
the generalized Zakharov equations, we study the parametric excitation of solar radio bursts at the fundamental plasma frequency
driven by a pair of oppositely propagating Langmuir waves with different wave amplitudes. Next, we briefly discuss the emergence
of chaos in the Zakharov equations. We point out that chaos can lead to turbulence in the source regions of solar radio emissions.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
136.
Blind adaptive decision fusion for distributed detection 总被引:3,自引:0,他引:3
Mirjalily G. Zhi-Quan Luo Davidson T.N. Bosse E. 《IEEE transactions on aerospace and electronic systems》2003,39(1):34-52
We consider the problem of decision fusion in a distributed detection system. In this system, each detector makes a binary decision based on its own observation, and then communicates its binary decision to a fusion center. The objective of the fusion center is to optimally fuse the local decisions in order to minimize the final error probability. To implement such an optimal fusion center, the performance parameters of each detector (i.e., its probabilities of false alarm and missed detection) as well as the a priori probabilities of the hypotheses must be known. However, in practical applications these statistics may be unknown or may vary with time. We develop a recursive algorithm that approximates these unknown values on-line. We then use these approximations to adapt the fusion center. Our algorithm is based on an explicit analytic relation between the unknown probabilities and the joint probabilities of the local decisions. Under the assumption that the local observations are conditionally independent, the estimates given by our algorithm are shown to be asymptotically unbiased and converge to their true values at the rate of O(1/k/sup 1/2/) in the rms error sense, where k is the number of iterations. Simulation results indicate that our algorithm is substantially more reliable than two existing (asymptotically biased) algorithms, and performs at least as well as those algorithms when they work. 相似文献
137.
J. L. Phillips S. J. Bame S. P. Gary J. T. Gosling E. E. Scime R. J. Forsyth 《Space Science Reviews》1995,72(1-2):109-112
Ulysses plasma measurement from 1.15 to 5.31 AU and from S6.4° to S48.3° solar latitude are used to assess the trends in the solar wind thermal electron temperature and anisotropy. Improved spacecraft potential corrections and data products have been incorporated. The radial temperature gradient is steeper than in previous determinations, but flatter than adiabatic. When normalized to 1 AU, temperature decrease with increasing latitude. Little change in the average thermal anisotropy has been seen during the mission. 相似文献
138.
J. T. Gosling S. J. Bame D. J. McComas J. L. Phillips V. J. Pizzo B. E. Goldstein M. Neugebauer 《Space Science Reviews》1995,72(1-2):99-104
Ulysses plasma observations reveal that the forward shocks that commonly bound the leading edges of corotating interaction regions (CIRs) beyond 2 AU from the Sun at low heliographic latitudes nearly disappeared at a latitude of S26°. On the other hand, the reverse shocks that commonly bound the trailing edges of the CIRs were observed regularly up to S41.5°, but became weaker with increasing latitude. Only three CIR shocks have been observed poleward of S41.5°; all of these were weak reverse shocks. The above effects are a result of the forward waves propagating to lower heliographic latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a global model of solar wind flows that originate in a simple tilted-dipole geometry back at the Sun. 相似文献
139.
K. C. Hansen T. Bagdonat U. Motschmann C. Alexander M. R. Combi T. E. Cravens T. I. Gombosi Y.-D. Jia I. P. Robertson 《Space Science Reviews》2007,128(1-4):133-166
The plasma environment of comet 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, is explored over a range of heliocentric
distances throughout the mission: 3.25 AU (Rosetta instruments on), 2.7 AU (Lander down), 2.0 AU, and 1.3 AU (perihelion).
Because of the large range of gas production rates, we have used both a fluid-based magnetohydrodynamic (MHD) model as well
as a semi-kinetic hybrid particle model to study the plasma distribution. We describe the variation in plasma environs over
the mission as well as the differences between the two modeling approaches under different conditions. In addition, we present
results from a field aligned, two-stream transport electron model of the suprathermal electron flux when the comet is near
perihelion. 相似文献
140.
Boland J.S. Pinson L.J. Peters E.G. Kane G.R. Malcolm W.W. 《IEEE transactions on aerospace and electronic systems》1979,(1):11-20
The problem of locating a reference image within a larger image using a correlation technique is discussed. Although the particular application discussed is that of locating a reference image obtained from one video sensor or a photograph, within the larger field of view obtained from a second video sensor in real time (i.e., at the TV field rate), the results are general and useful for a number of applications. The tradeoffs necessary to obtain real time correlat are discussed and their effect on correlation accuracy is given. 相似文献