全文获取类型
收费全文 | 3159篇 |
免费 | 5篇 |
国内免费 | 11篇 |
专业分类
航空 | 1669篇 |
航天技术 | 1077篇 |
综合类 | 9篇 |
航天 | 420篇 |
出版年
2018年 | 37篇 |
2017年 | 19篇 |
2016年 | 22篇 |
2014年 | 54篇 |
2013年 | 77篇 |
2012年 | 64篇 |
2011年 | 106篇 |
2010年 | 83篇 |
2009年 | 114篇 |
2008年 | 175篇 |
2007年 | 90篇 |
2006年 | 60篇 |
2005年 | 77篇 |
2004年 | 78篇 |
2003年 | 100篇 |
2002年 | 52篇 |
2001年 | 99篇 |
2000年 | 48篇 |
1999年 | 79篇 |
1998年 | 93篇 |
1997年 | 74篇 |
1996年 | 91篇 |
1995年 | 124篇 |
1994年 | 98篇 |
1993年 | 66篇 |
1992年 | 101篇 |
1991年 | 42篇 |
1990年 | 36篇 |
1989年 | 87篇 |
1988年 | 32篇 |
1987年 | 28篇 |
1986年 | 32篇 |
1985年 | 87篇 |
1984年 | 91篇 |
1983年 | 59篇 |
1982年 | 82篇 |
1981年 | 92篇 |
1980年 | 34篇 |
1979年 | 42篇 |
1978年 | 32篇 |
1977年 | 22篇 |
1976年 | 21篇 |
1975年 | 37篇 |
1974年 | 24篇 |
1973年 | 18篇 |
1972年 | 29篇 |
1969年 | 17篇 |
1968年 | 16篇 |
1967年 | 20篇 |
1966年 | 16篇 |
排序方式: 共有3175条查询结果,搜索用时 15 毫秒
931.
Y. C. Whang 《Space Science Reviews》1991,57(3-4):339-388
Observations of plasma and magnetic fields by Pioneer 10 and 11 and Voyager 1 and 2 reveal that MHD shocks are an important component of the large-scale solar wind structures in the outer heliosphere. This review discusses recent progress in simulation studies of the nonlinear evolution of the solar wind structures, and in particular concentrates on the theoretical development and applications of the shock interactions model. Various stream propagation models, which do not use the Rankine-Hugoniot relations to calculate the jump conditions at shock crossings, have been used to simulate the essential evolution process of isolated streams and the formation and propagation of corotating and transient shocks. They produce fairly good results in the region up to a few AU. In 1984, the shock interactions model was introduced to study the evolution of large-scale solar wind structures in the region outside 1 AU up to several tens of AU. The model uses the exact Rankine-Hugoniot relations to calculate the shock speed and shock strength at all shock crossings. So that the model can more accurately calculate the shock speeds and the accumulated irreversible shock heating of plasma at several tens of AU. The applications of the shock interactions model are presented in three groups. (a) The first group covers the basic interaction of a shock with the ambient solar wind, the formation and propagation of shock pairs, and the collision and merging of shocks. (b) The second group covers the use of the shock interactions model to simulate the nonlinear evolution of large-scale solar wind structures in the outer heliosphere. These simulation results can provide the detailed evolution process for large-scale solar wind structures in the vast region not directly observed. Two selected studies are reported. (c) Finally, the shock interactions model is applied to studying the heating of the solar wind in the outer heliosphere. The model calculations support shocks being chiefly responsible for the heating of the solar wind plasma in the outer heliosphere at least up to 30 AU. 相似文献
932.
A knowledge-based system for air traffic control (ATC) in terminal control regions is designed for improving service and operation. Domain knowledge is established following rules, regulations, and comments from controllers. An expert system is adopted for approach control and airport control in the terminal control region. The proposed system was implemented in PC-AT with Turbo Prolog. The air terminal control monitor was checked with regard to flight information input, real-time data refreshment, air traffic rescheduling, constraint violation double checking, emergency responses, and normal monitoring. Simulation results are given 相似文献
933.
The signal format and spectral properties of the 406-MHz emergency locator transmitter (ELT) used in the search and rescue satellite aided tracking (SARSAT) system are examined. The ELT improves location estimate accuracies and can relay information about the particular aircraft and its problem, by means of the digitally modulated message fields. It is shown that due to the RF signal frequency characteristics and the Doppler shift, processing must be performed over a frequency band of approximately 25 kHz. Through the use of the fast Fourier transformation (FFT), the frequency spectrum of the ELT is analyzed, taking account of effects due to noise, multiple simultaneously received signals, and Doppler shift. It is demonstrated that the FFT provides an effective means for detecting and recognizing the presence of one or more ELT signals over this 25-kHz frequency band. Some recommendations are made to improve the spectral characteristics and the performance of the ELT 相似文献
934.
The Heavy Ion Counter on the Galileo spacecraft will monitor energetic heavy nuclei of the elements from C to Ni, with energies from 6 to 200 MeV nucl-1. The instrument will provide measurements of trapped heavy ions in the Jovian magnetosphere, including those high-energy heavy ions with the potential for affecting the operation of the spacecraft electronic circuitry. We describe the instrument, which is a modified version of the Voyager CRS instrument. 相似文献
935.
THE DIGITAL WAVE-PROCESSING EXPERIMENT ON CLUSTER 总被引:1,自引:0,他引:1
L. J. C. Woolliscroft H. St. C. Alleyne C. M. Dunford A. Sumner J. A. Thompson S. N. Walker K. H. Yearby A. Buckley S. Chapman M. P. Gough 《Space Science Reviews》1997,79(1-2):209-231
The wide variety of geophysical plasmas that will be investigated by the Cluster mission contain waves with a frequency range from DC to over 100 kHz with both magnetic and electric components. The characteristic duration of these waves extends from a few milliseconds to minutes and a dynamic range of over 90 dB is desired. All of these factors make it essential that the on-board control system for the Wave-Experiment Consortium (WEC) instruments be flexible so as to make effective use of the limited spacecraft resources of power and telemetry-information bandwidth. The Digital Wave Processing Experiment, (DWP), will be flown on Cluster satellites as a component of the WEC. DWP will coordinate WEC measurements as well as perform particle correlations in order to permit the direct study of wave/particle interactions. The DWP instrument employs a novel architecture based on the use of transputers with parallel processing and re-allocatable tasks to provide a high-reliability system. Members of the DWP team are also providing sophisticated electrical ground support equipment, for use during development and testing by the WEC. This is described further in Pedersen et al. (this issue). 相似文献
936.
The ACE Magnetic Fields Experiment 总被引:2,自引:0,他引:2
Smith C.W. L'Heureux J. Ness N.F. Acuña M.H. Burlaga L.F. Scheifele J. 《Space Science Reviews》1998,86(1-4):613-632
The magnetic field experiment on ACE provides continuous measurements of the local magnetic field in the interplanetary medium.
These measurements are essential in the interpretation of simultaneous ACE observations of energetic and thermal particles
distributions. The experiment consists of a pair of twin, boom- mounted, triaxial fluxgate sensors which are located 165 inches
(=4.19 m) from the center of the spacecraft on opposing solar panels. The electronics and digital processing unit (DPU) is
mounted on the top deck of the spacecraft. The two triaxial sensors provide a balanced, fully redundant vector instrument
and permit some enhanced assessment of the spacecraft's magnetic field. The instrument provides data for Browse and high-level
products with between 3 and 6 vector s−1 resolution for continuous coverage of the interplanetary magnetic field. Two high-resolution
snapshot buffers each hold 297 s of 24 vector s−1 data while on- board Fast Fourier Transforms extend the continuous data
to 12 Hz resolution. Real-time observations with 1-s resolution are provided continuously to the Space Environmental Center
(SEC) of the National Oceanographic and Atmospheric Association (NOAA) for near- instantaneous, world-wide dissemination in
service to space weather studies. As has been our team's tradition, high instrument reliability is obtained by the use of
fully redundant systems and extremely conservative designs. We plan studies of the interplanetary medium in support of the
fundamental goals of the ACE mission and cooperative studies with other ACE investigators using the combined ACE dataset as
well as other ISTP spacecraft involved in the general program of Sun-Earth Connections.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
937.
Murphy R. T. Rannells David C. 《IEEE transactions on aerospace and electronic systems》1967,(2):236-241
A practical approach for meeting the immediate and future needs of communication systems that require very wideband transmitters capable of operating in the S-, C-, and X-band frequency ranges is described. Included are: 1) a review of the basic studies conducted to determine the most practical transmitter approach for wideband applications; 2) a discussion of a prototype 10-watt frequency-modulated transmitter developed to demonstrate the feasibility of a wideband system; and 3) a review of the preliminary computer analyses and empirical results obtained on the development of a unit capable of wide deviation with reasonable linearity at baseband frequencies up to 10 Megahertz. 相似文献
938.
We have developed a 2D semi-empirical model (Sittler and Guhathakurta 1999) of the corona and the interplanetary medium using
the time independent MHD equations and assuming azimuthal symmetry, utilizing the SOHO, Spartan and Ulysses observations.
The model uses as inputs (1) an empirically derived global electron density distribution using LASCO, Mark III and Spartan
white light observations and in situ observations of the Ulysses spacecraft, and (2) an empirical model of the coronal magnetic
field topology using SOHO/LASCO and EIT observations. The model requires an estimate of solar wind velocity as a function
of latitude at 1 AU and the radial component of the magnetic field at 1 AU, for which we use Ulysses plasma and magnetic field
data results respectively. The model makes estimates as a function of radial distance and latitude of various fluid parameters
of the plasma such as flow velocity V, temperature Teff, and heat flux Qeff which are derived from the equations of conservation of mass, momentum and energy, respectively, in the rotating frame of
the Sun. The term "effective" indicates possible wave contributions. The model can be used as a planning tool for such missions
as Solar Probe and provide an empirical framework for theoretical models of the solar corona and solar wind.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
939.
N.U. Crooker J.T. Gosling V. Bothmer R.J. Forsyth P.R. Gazis A. Hewish T.S. Horbury D.S. Intriligator J.R. Jokipii J. Kóta A.J. Lazarus M.A. Lee E. Lucek E. Marsch A. Posner I.G. Richardson E.C. Roelof J.M. Schmidt G.L. Siscoe B.T. Tsurutani R.F. Wimmer-Schweingruber 《Space Science Reviews》1999,89(1-2):179-220
Corotating interaction regions (CIRs) in the middle heliosphere have distinct morphological features and associated patterns
of turbulence and energetic particles. This report summarizes current understanding of those features and patterns, discusses
how they can vary from case to case and with distance from the Sun and possible causes of those variations, presents an analytical
model of the morphological features found in earlier qualitative models and numerical simulations, and identifies aspects
of the features and patterns that have yet to be resolved.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
940.