全文获取类型
收费全文 | 3199篇 |
免费 | 10篇 |
国内免费 | 5篇 |
专业分类
航空 | 1192篇 |
航天技术 | 1041篇 |
综合类 | 19篇 |
航天 | 962篇 |
出版年
2022年 | 15篇 |
2021年 | 37篇 |
2019年 | 16篇 |
2018年 | 128篇 |
2017年 | 102篇 |
2016年 | 116篇 |
2015年 | 41篇 |
2014年 | 111篇 |
2013年 | 135篇 |
2012年 | 118篇 |
2011年 | 158篇 |
2010年 | 127篇 |
2009年 | 192篇 |
2008年 | 190篇 |
2007年 | 122篇 |
2006年 | 74篇 |
2005年 | 101篇 |
2004年 | 94篇 |
2003年 | 106篇 |
2002年 | 83篇 |
2001年 | 113篇 |
2000年 | 36篇 |
1999年 | 53篇 |
1998年 | 61篇 |
1997年 | 42篇 |
1996年 | 43篇 |
1995年 | 80篇 |
1994年 | 61篇 |
1993年 | 34篇 |
1992年 | 45篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 41篇 |
1988年 | 11篇 |
1987年 | 17篇 |
1986年 | 16篇 |
1985年 | 70篇 |
1984年 | 56篇 |
1983年 | 48篇 |
1982年 | 42篇 |
1981年 | 82篇 |
1980年 | 25篇 |
1979年 | 16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3214条查询结果,搜索用时 15 毫秒
261.
262.
The main goal of the paper is to demonstrate the presence of chaotic trajectories in the gaseous disk of a real spiral galaxy. As an example we have chosen NGC 3631. First, we show the stationarity of the 3-D velocity field restored from the observed line-of-sight velocity field of the gaseous disk. That allows to analyse behaviour of the trajectories of the fluid particles (gas clouds) in the disk, calculating the corresponding observed streamlines. We estimate the Lyapunov characteristic numbers using their independence of the metrics and show the existence of chaotic trajectories outside the vortices which are present in the velocity field, and in the vicinity of the saddle point. Related spectra of the stretching numbers for some trajectories are also calculated. 相似文献
263.
Jurewicz A.J.G. Burnett D.S. Wiens R.C. Friedmann T.A. Hays C.C. Hohlfelder R.J. Nishiizumi K. Stone J.A. Woolum D.S. Becker R. Butterworth A.L. Campbell A.J. Ebihara M. Franchi I.A. Heber V. Hohenberg C.M. Humayun M. McKeegan K.D. McNamara K. Meshik A. Pepin R.O. Schlutter D. Wieler R. 《Space Science Reviews》2003,105(3-4):535-560
Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be
exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure
materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’),
with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically
placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.
Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for
solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector
arrays and elsewhere targeted for the analyses of specific solar-wind components.
Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the
ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface
and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability.
A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis
website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma
data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community
throughout the 21st Century.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
264.
Alexeev Igor I. Belenkaya Elena S. Bobrovnikov Sergey Yu. Kalegaev Vladimir V. 《Space Science Reviews》2003,107(1-2):7-26
A magnetohydrodynamic model of the solar wind flow is constructed using a kinematic approach. It is shown that a phenomenological
conductivity of the solar wind plasma plays a key role in the forming of the interplanetary magnetic field (IMF) component
normal to the ecliptic plane. This component is mostly important for the magnetospheric dynamics which is controlled by the
solar wind electric field. A simple analytical solution for the problem of the solar wind flow past the magnetosphere is presented.
In this approach the magnetopause and the Earth's bow shock are approximated by the paraboloids of revolution. Superposition
of the effects of the bulk solar wind plasma motion and the magnetic field diffusion results in an incomplete screening of
the IMF by the magnetopause. It is shown that the normal to the magnetopause component of the solar wind magnetic field and
the tangential component of the electric field penetrated into the magnetosphere are determined by the quarter square of the
magnetic Reynolds number. In final, a dynamic model of the magnetospheric magnetic field is constructed. This model can describe
the magnetosphere in the course of the severe magnetic storm. The conditions under which the magnetospheric magnetic flux
structure is unstable and can drive the magnetospheric substorm are discussed. The model calculations are compared with the
observational data for September 24–26, 1998 magnetic storm (Dst
min=−205 nT) and substorm occurred at 02:30 UT on January 10, 1997.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
265.
Cantrell B.H. Gordon W.B. Trunk G.V. 《IEEE transactions on aerospace and electronic systems》1981,(2):213-221
The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface. 相似文献
266.
The performance of electronically scanned radar systems is evaluated for clear and barrage jamming environments. Radar figures of merit in jamming are derived for search and track modes and are directly related to antenna pattern relative sidelobe levels. A random sidelobe model is analyzed with various numbers of independent jammers. Probability distributions of detection probability are derived to determine detection performance versus average jamming level and number of jammers. 相似文献
267.
SAR ATR performance using a conditionally Gaussian model 总被引:1,自引:0,他引:1
O'Sullivan J.A. DeVore M.D. Kedia V. Miller M.I. 《IEEE transactions on aerospace and electronic systems》2001,37(1):91-108
A family of conditionally Gaussian signal models for synthetic aperture radar (SAR) imagery is presented, extending a related class of models developed for high resolution radar range profiles. This signal model is robust with respect to the variations of the complex-valued radar signals due to the coherent combination of returns from scatterers as those scatterers move through relative distances on the order of a wavelength of the transmitted signal (target speckle). The target type and the relative orientations of the sensor, target, and ground plane parameterize the conditionally Gaussian model. Based upon this model, algorithms to jointly estimate both the target type and pose are developed. Performance results for both target pose estimation and target recognition are presented for publicly released data from the MSTAR program 相似文献
268.
Clarkson I.V.L. Pollington A.D. 《IEEE transactions on aerospace and electronic systems》2007,43(2):645-650
The case is considered in which a frequency-agile receiver (FAR) for electronic support (ES) attempts to intercept radar emissions over a wide search bandwidth. It was recently shown [1,2] that a random strategy exists in which the expected intercept time can be made arbitrarily close to linear as a function of the scan period of the radar. Can a deterministic strategy be devised in which a similar linear relationship exists for the maximum intercept time? By applying the celebrated arithmetic results of van der Waerden [3] and Szemeredi [4], we show that no such strategy is possible. 相似文献
269.
Maskery M. Krishnamurthy V. 《IEEE transactions on aerospace and electronic systems》2007,43(3):843-863
The problem of deploying countermeasures (CM) against antiship missiles is investigated from a network centric perspective in which multiple ships coordinate to defend against a known missile threat. Using the paradigm of network enabled operations (NEOPS), the problem is formulated as a transient stochastic game with communication where the appropriate strategy takes the form of an optimal stationary correlated equilibrium. Under this strategy, ships cooperate through real-time communication to satisfy both local and collective interests. The use of communication results in a performance improvement over the noncommunicating, Nash equilibrium scenario. This framework allows us to develop a theoretical foundation for NEOPS and captures the trade-off between information exchange and performance, while generalizing the standard Nash equilibrium solution for the missile deflection game given in [1], The NEOPS equilibrium strategy is characterized as the solution to an optimization problem with linear objective and bilinear constraints, which can be solved calculating successive improvements starting from an initial noncooperative (Nash) solution. The communication overhead required to implement this strategy is associated with the mutual information between individual action probability distributions at equilibrium. Numerical results illustrate the trade-off between communication and performance. 相似文献
270.
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