全文获取类型
收费全文 | 6404篇 |
免费 | 14篇 |
国内免费 | 25篇 |
专业分类
航空 | 3057篇 |
航天技术 | 2331篇 |
综合类 | 24篇 |
航天 | 1031篇 |
出版年
2021年 | 53篇 |
2018年 | 118篇 |
2017年 | 80篇 |
2016年 | 80篇 |
2015年 | 41篇 |
2014年 | 154篇 |
2013年 | 165篇 |
2012年 | 175篇 |
2011年 | 238篇 |
2010年 | 164篇 |
2009年 | 275篇 |
2008年 | 367篇 |
2007年 | 167篇 |
2006年 | 151篇 |
2005年 | 188篇 |
2004年 | 174篇 |
2003年 | 208篇 |
2002年 | 127篇 |
2001年 | 207篇 |
2000年 | 110篇 |
1999年 | 153篇 |
1998年 | 181篇 |
1997年 | 119篇 |
1996年 | 142篇 |
1995年 | 181篇 |
1994年 | 182篇 |
1993年 | 100篇 |
1992年 | 144篇 |
1991年 | 76篇 |
1990年 | 62篇 |
1989年 | 150篇 |
1988年 | 61篇 |
1987年 | 66篇 |
1986年 | 63篇 |
1985年 | 195篇 |
1984年 | 142篇 |
1983年 | 125篇 |
1982年 | 132篇 |
1981年 | 207篇 |
1980年 | 72篇 |
1979年 | 57篇 |
1978年 | 53篇 |
1977年 | 47篇 |
1976年 | 45篇 |
1975年 | 50篇 |
1974年 | 40篇 |
1973年 | 44篇 |
1972年 | 42篇 |
1970年 | 40篇 |
1969年 | 40篇 |
排序方式: 共有6443条查询结果,搜索用时 140 毫秒
991.
It has been shown in the literature that the linear-phase constraint of finite-duration impulse-response (FIR) digital filters can, under certain circumstances, be effectively traded either for a better filter amplitude response or a reduction in the number of filter coeficients. It is shown that such a tradeoff can be exploited for moving target indicator (MTI) radar signal processors to increase the usable bandwidth for target detection. Although it is demonstrated that the increase is significant for narrowband (ground) clutter, it is negligible for wideband (weather) clutter. 相似文献
992.
T. Natarajan Y.S. Rajan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(11):31-34
The paper deals with the Indian experience in training the resources scientists in the fast developing field of remote sensing and the facilities available for obtaining such training in India. The necessity of imparting training to the scientists at different levels is brought out. An outline of the training curriculum of a leading institution is described in some detail. 相似文献
993.
Z.I. Tsvetanov S.A. Zhekov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,3(10-12)
The kinematics of gas clouds in broad-line region of active galactic nuclei and quasars is considered. The motion of the clouds is governed by three forces — gravitational influence from the central supermassive body, radiational pressure from the continuum radiation, and resistance of the intercloud medium. Clouds moves radial but only outward motion gives a velocity field, which is in accordance with the observational data. The profiles of the permited lines are obtained in some simplify assumptions for the emissive capacity of the gas in clouds, and are in good agreement with the observational data. In the framework of the model under consideration there is a possibility to estimate some physical parameters of the nuclei such as mass of the central body and density of the intercloud medium. 相似文献
994.
G. Stanev M. Petrunova D. Teodosiev I. Kutiev K. Serafimov S. Chapkunov V. Chmyrev N. Isaev P. Puschaev I. Pimenov S. Bilichanko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(7):43-47
The instrument IESP-IPMP represents the complex unit measuring the vector of the DC electric field, the vectors of the electric and magnetic field in the frequency range of 0.2 – 6.5 Hz (wave form), autocorrelation functions of waves with frequencies of 0.1 – 5 kHz, and wave amplitudes in 8 bandpass channels. Some results are shown and compared in the various frequency ranges. 相似文献
995.
G.H. Schwehm R.H. Giese F. Giovane D.W. Schuerman J.L. Weinberg 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(8):121-125
The 1985 International Solar Polar Mission will provide the first opportunity to perform measurements from out of the ecliptic and from above the solar poles. Included in this mission is the Zodiacal Light/Background Starlight Experiment of the Ruhr-University Bochum, FRG (in collaboration with the State University of New York at Albany). The experiment is based on the use of a multicolor, sky-scanning photopolarimeter, which will be carried on the NASA S/C.The scientific objectives of the experiment are to investigate the intensity, polarization, and color of the diffuse sky brightness and determine the spatial distribution and physical properties of the interplanetary dust, including a possible interstellar component, as a function of S/C position in and out of the ecliptic.The instrument will allow simultaneous measurements in two separate wavelength bands. Measurements will be performed at 180, 220, 240, 280, 320, 350, 360, 440, 540, 640, 800 nm, including the state of polarization. Two different fields-of-view (1 sq deg and 5.6 sq deg) can be selected.The use of a microprocessor system in the instrument electronics will provide flexibility in measurement sequences and programs, on-board data processing and data quality control. 相似文献
996.
H.S. Hudson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(4):307-314
The Pinhole/Occulter Facility concept uses a remote occulting mask to provide high resolution observations of the solar corona and of astronomical X-ray sources. With coded-aperture and Fourier-transform techniques, the Pinhole/Occulter makes images at a resolution of 0.2 arc sec for 2 - 120 keV X-rays, using a 50-m boom erected from the payload bay of the Space Shuttle or mounted on a free-flying platform. The remote occulter also creates a large shadow area for solar coronal observations; the Pinhole/Occulter concept includes separate optical and ultraviolet telescopes with 50-cm apertures. These large telescopes will provide a new order of resolution and sensitivity for diagnostic observations of faint structures in the solar corona. The Pinhole/Occulter is a powerful and versatile tool for general-purpose X-ray astronomy, with excellent performance in a broad spectral band complementary to that accessible with AXAF. The large collecting area of 1.5 m2 results in a 5σ detection threshold of about 0.02 μJy for the 2 - 10 keV band, or about 10?5 ph(cm2sec keV)?1 at 20 keV. 相似文献
997.
998.
R.G. Walker H.H. Aumann 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(9):197-201
The large beam size of the Infrared Astronomy Satellite (IRAS) focal plane detector array is well suited to measuring the low level thermal emission from cometary dust. Eight comets discovered in 1983 and nine previously known periodic comets were observed by IRAS during its ten month active lifetime. Dust production rates are derived for a wide range of heliocentric distances. Grain properties are inferred from application of simple models to the long wavelength spectral energy distribution. 相似文献
999.
Johannsen K. Tustioson G. Egli S. 《IEEE transactions on aerospace and electronic systems》1980,(5):683-711
In many domestic satellite communication systems single-channel per-carrier FM (SCPC-FM) has become the accepted transmission method for thin route communication. The SCPC-FM modem operates over a wide range of carrier frequencies and is designed to use satellite power and bandwidth efficiently. It incorporates elements which make the signal transfer complex, so the system behavior is not immediately obvious. This article investigates the signal processes and analyzes the performance of a companded and preemphasized SCPC-FM system and it furnishes insight into the choice of crucial system parameters. 相似文献
1000.
Many radio and sonar systems require signal outputs in complex low-pass form. To achieve this, it is possible to use uniform sampling of the bandpass signal, together with computation of the quadrature component by way of a Hilbert transform. The bandpass to low-pass translation is accomplished by undersampling. A hardware implementation is described which achieves 70 dB spurious-free dynamic range and a bandwidth of 30 kHz. 相似文献