全文获取类型
收费全文 | 3304篇 |
免费 | 9篇 |
国内免费 | 5篇 |
专业分类
航空 | 1228篇 |
航天技术 | 1087篇 |
综合类 | 20篇 |
航天 | 983篇 |
出版年
2022年 | 15篇 |
2021年 | 40篇 |
2019年 | 17篇 |
2018年 | 132篇 |
2017年 | 103篇 |
2016年 | 117篇 |
2015年 | 42篇 |
2014年 | 120篇 |
2013年 | 138篇 |
2012年 | 123篇 |
2011年 | 169篇 |
2010年 | 134篇 |
2009年 | 198篇 |
2008年 | 197篇 |
2007年 | 125篇 |
2006年 | 82篇 |
2005年 | 102篇 |
2004年 | 97篇 |
2003年 | 111篇 |
2002年 | 83篇 |
2001年 | 115篇 |
2000年 | 38篇 |
1999年 | 53篇 |
1998年 | 61篇 |
1997年 | 42篇 |
1996年 | 45篇 |
1995年 | 80篇 |
1994年 | 61篇 |
1993年 | 35篇 |
1992年 | 45篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 42篇 |
1988年 | 12篇 |
1987年 | 17篇 |
1986年 | 16篇 |
1985年 | 70篇 |
1984年 | 59篇 |
1983年 | 51篇 |
1982年 | 44篇 |
1981年 | 84篇 |
1980年 | 26篇 |
1979年 | 17篇 |
1978年 | 14篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1974年 | 11篇 |
1972年 | 13篇 |
1971年 | 11篇 |
排序方式: 共有3318条查询结果,搜索用时 31 毫秒
91.
GTD Terrain Reflection Model Applied to ILS Glide Scope 总被引:1,自引:0,他引:1
Luebbers R. Ungvichian V. Mitchell L. 《IEEE transactions on aerospace and electronic systems》1982,(1):11-20
The capability of calculating the reflection of electromagnetic signals from uneven terrain has many applications. One of these is the determination of instrument landing system (ILS) glide slope performance. For this application the wavelength is approximately 1 m, incidence angles are usually near grazing, and the fields are horizontally polarized, so that gross irregularities such as dropoffs and hills are more important than surface roughness. Past approaches used to calculate the ground reflections for this application have been three-dimensional physical optics models which were very cumbersome and time consuming and which neglected important diffraction and shadowing phenomenon; a two-dimensional physical optics model which was faster than the three-dimensional models but ignored many shadowing and transverse terrain variation effects; and a half-plane diffraction model which is applicable only to a specified type of terrain geometry. In this paper a terrain reflection model based on the geometrical theory of diffraction (GTD) is described which can accommodate any piecewise linear terrain profile, requires less computer time than the physical optics models, is capable of including transverse terrain effects, and determines the reflected fields with all important diffraction and blockage effects included. 相似文献
92.
In the theory of signal detectability, the signal-to-noise ratio (SNR), defined as the quotient of the average received signal energy and the spectral density of the white Gaussian noise, is a fundamental parameter. For a signal which is exactly known, or known except for a random phase, this ratio uniquely defines the detection performance which can be achieved with a matched filter receiver. However, when the signal amplitude is a random parameter, the detection performance is changed and must be determined from the probability density function (pdf) of the amplitude. Relative to the case of a constant signal amplitude, such signal amplitude fluctuation usually degrades performance when a high probability of detection (Pd) is required, but improves performance at low values of Pd; the corresponding change in the required SNR is the so-called signal fluctuation loss Lf. Thus, since Lf in some cases represents an improvement in performance for low values of Pd, a question of at least theoretical interest is: how large might this improvement be, when the class of all signal amplitude pdf's is considered. The solution, presented here, results in a lower bound on the signal fluctuation loss Lf as a function of Pd, or equivalently an upper bound on Pd as a function of SNR. The corresponding most favorable pdf was determined using the Lagrange multiplier technique and results of a numerical maximization are included to provide insight into the general properties of the solution. 相似文献
93.
94.
Vanjari S.V. Krogmeier J.V. Bell M.R. 《IEEE transactions on aerospace and electronic systems》2007,43(4):1426-1440
In sensor networks distributed over large areas, communication by means of active transmitters on sensor nodes is inherently energy expensive and poses a significant bottleneck to achieve a long battery life. We propose modulated reradiation of radar illumination as a means to transmit information from a group of sensors to an airborne radar. This puts the communications energy burden on the radar transmitter rather than on the sensor nodes, thus increasing their battery lifetimes. To distinguish the sensor return from the clutter return, the modulation on the sensors is done by switching a nonlinear load on the sensor antenna and processing the harmonic reradiation. We present techniques to transmit information from the sensors, which use stripmap mode synthetic aperture radar (SAR) ideas to decode the information and to simultaneously obtain a geographic map of the sensor locations. 相似文献
95.
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
96.
Theoretical pressure balance arguments have implied that steady convection is hardly possible in the terrestrial magnetotail and that steady energy input necessarily generates a cyclic loading-unloading sequence, i.e., repetitive substorms. However, observations have revealed that enhanced solar wind energy input to the magnetospheric system may either lead to substorm activity or enhanced but steady convection. This topic is reviewed with emphasis on several recent case studies of the Steady Magnetospheric Convection (SMC) events. In these cases extensive data sets from both satellite and ground-based instruments from various magnetospheric and ionospheric regions were available.Accurate distinction of the spatial and temporal scales of the magnetospheric processes is vital for correct interpretation of the observations during SMC periods. We show that on the large scale, the magnetospheric configuration and plasma convection are stable during SMC events, but that both reveal considerable differences from their quiet-time assemblies. On a shorter time scale, there are numerous transient activations which are similar to those found during substorms, but which presumably originate from a more distant tail reconnection process, and map to the poleward boundary of the auroral oval. The available observations and the unresolved questions are summarized here.The tail magnetic field during SMC events resembles both substorm growth and recovery phases in the neartail and midtail, respectively, but this configuration may remain stable for up to ten hours. Based on observations and model results we discuss how the magnetospheric system avoids pressure balance problems when the plasma convects earthward.Finally, the importance of further coordinated studies of SMC events is emphasized. Such studies may shed more light on the substorm dynamics and help to verify quantitatively the theoretical models of the convecting magnetosphere. 相似文献
97.
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
98.
The solar wind termination shock is described as a multi-fluid phenomenon taking into account the magnetohydrodynamic self-interaction of a multispecies plasma consisting of solar wind ions, pick-up ions and shock-generated anomalous cosmic ray particles. The spatial diffusion of these high energy particles relative to the resulting, pressure-modified solar wind flow structure is described by a coupled system of differential equations describing mass-, momentum-, and energy-flow continuities for all plasma components. The energy loss due to escape of energetic particles (MeV) from the precursor into the inner heliosphere is taken into account. We determine the integrated properties of the anomalous cosmic ray gas and the low-energy solar wind. Also the variation of the compression ratio of the shock structure is quantitatively determined and is related to the pick-up ion energization efficiency and to the mean energy of the downstream anomalous cosmic ray particles. The variation of the resulting shock structure and of the solar wind sheath plasma extent beyond the shock is discussed with respect to its consequences for the LISM neutral gas filtration and the threedimensional shape of the heliosphere. 相似文献
99.
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
100.
N. Gopalswamy Z. Mikić D. Maia D. Alexander H. Cremades P. Kaufmann D. Tripathi Y.-M. Wang 《Space Science Reviews》2006,123(1-3):303-339
The coronal mass ejection (CME) phenomenon occurs in closed magnetic field regions on the Sun such as active regions, filament regions, transequatorial interconnection regions, and complexes involving a combination of these. This chapter describes the current knowledge on these closed field structures and how they lead to CMEs. After describing the specific magnetic structures observed in the CME source region, we compare the substructures of CMEs to what is observed before eruption. Evolution of the closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the eruption. We describe this pre-eruption evolution and attempt to link them to the observed features of CMEs. Small-scale energetic signatures in the form of electron acceleration (signified by nonthermal radio bursts at metric wavelengths) and plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. We survey these pre-eruptive energy releases using observations taken before and during the eruption of several CMEs. Finally, we discuss how the observations can be converted into useful inputs to numerical models that can describe the CME initiation. 相似文献