全文获取类型
收费全文 | 5577篇 |
免费 | 31篇 |
国内免费 | 21篇 |
专业分类
航空 | 2482篇 |
航天技术 | 2026篇 |
综合类 | 20篇 |
航天 | 1101篇 |
出版年
2021年 | 50篇 |
2019年 | 38篇 |
2018年 | 113篇 |
2017年 | 91篇 |
2016年 | 81篇 |
2015年 | 33篇 |
2014年 | 119篇 |
2013年 | 158篇 |
2012年 | 139篇 |
2011年 | 248篇 |
2010年 | 186篇 |
2009年 | 258篇 |
2008年 | 295篇 |
2007年 | 168篇 |
2006年 | 126篇 |
2005年 | 167篇 |
2004年 | 172篇 |
2003年 | 182篇 |
2002年 | 123篇 |
2001年 | 172篇 |
2000年 | 86篇 |
1999年 | 119篇 |
1998年 | 148篇 |
1997年 | 113篇 |
1996年 | 104篇 |
1995年 | 154篇 |
1994年 | 148篇 |
1993年 | 87篇 |
1992年 | 113篇 |
1991年 | 41篇 |
1990年 | 53篇 |
1989年 | 99篇 |
1988年 | 46篇 |
1987年 | 39篇 |
1986年 | 50篇 |
1985年 | 171篇 |
1984年 | 149篇 |
1983年 | 119篇 |
1982年 | 102篇 |
1981年 | 190篇 |
1980年 | 62篇 |
1979年 | 42篇 |
1978年 | 51篇 |
1977年 | 41篇 |
1976年 | 42篇 |
1975年 | 48篇 |
1974年 | 39篇 |
1973年 | 37篇 |
1972年 | 47篇 |
1969年 | 28篇 |
排序方式: 共有5629条查询结果,搜索用时 15 毫秒
251.
Qiang Guo Volodymyr G. Galushko Andriy V. Zalizovski Sergiy B. Kashcheyev Yu Zheng 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(9):2267-2274
A modification of the Doppler Interferometry Technique is suggested to enable estimating angles of arrival of comparatively broadband HF signals scattered by random irregularities of the ionospheric plasma with the use of small-size weakly directional antennas. The technique is based on the measurements of cross-spectra phases of the probe radiation recorded at least in three spatially separated points. The developed algorithm has been used to investigate the angular and frequency-time characteristics of HF signals propagating at frequencies above the maximum usable one (MUF) for the direct radio path Moscow-Kharkiv. The received signal spectra show presence of three families of spatial components attributed, respectively, to scattering by plasma irregularities near the middle point of the radio path, ground backscatter signals and scattering of the sounding signals by the intense plasma turbulence associated with auroral activations. It has been shown that the regions responsible for the formation of the third family components are located well inside the auroral oval. The drift velocity and direction of the auroral ionosphere plasma have been determined. The obtained estimates are consistent with the classical conception of the ionospheric plasma convection at high latitudes and do not contradict the results of investigations of the auroral ionosphere dynamics using the SuperDARN network. 相似文献
252.
253.
R.V. Semenyshyn A.N. VeklichI.L. Babich V.F. Boretskij 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Plasma of the free burning electric arc between Ag–SnO2–ZnO composite electrodes as well as brass electrodes were investigated. The plasma temperature distributions were obtained by Boltzmann plot method involving Cu I, Ag I or Zn I spectral line emissions. The electron density distributions were obtained from the width and from absolute intensity of spectral lines. The laser absorption spectroscopy was used for measurement of copper atom concentration in plasma. Plasma equilibrium composition was calculated using two independent groups of experimental values (temperature and copper atom concentration, temperature and electron density). It was found that plasma of the free burning electric arc between brass electrodes is in local thermodynamical equilibrium. The experimental verification of the spectroscopic data of Zn I spectral lines was carried out. 相似文献
254.
A.K. Sharma O.B. Gurav Anindya Bose H.P. Gaikwad G.A. Chavan Atanu Santra S.S. Kamble R.S. Vhatkar 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(10):3131-3138
Indian Space Research Organization (ISRO) has developed an indigenous system named Indian Regional Navigation Satellite System (IRNSS) or NavIC (Navigation with Indian Constellation), that consists of 7 satellites and transmits navigation signal in L and S bands. ISRO, for validation of the system, has installed many IGS (IRNSS/GPS/SBAS) receivers scattered over the Indian region. Using preliminary data from two geographically widely separated stations over India, this paper presents the results on studies on parameters of IRNSS signal quality and discusses how these parameters may be used to study the ionospheric behavior over the Indian region. The results show the importance and advantages of using IRNSS data for such studies. 相似文献
255.
Based on an integrated approach, we solve the optimization problem for energy content and weight-dimensional characteristics of the electric servo drive by synthesis of structure and analysis of characteristics for different power supplies. The generalized power supply model, developed for different power supply of drive control systems, is given. 相似文献
256.
Q. Wang G. Xu S. Petrovic U. Schaefer U. Meyer T. Xu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
A regional tropospheric model can be constructed using surveys from GPS ground networks. Using this model the tropospheric delays of a kinematic station within the region can be interpolated. However, such a model is generally not suitable for an airborne platform high above the ground networks. In this paper, a method of constructing a regional tropospheric model for airborne GPS applications is described. First, the kinematic station in the air is projected onto the ground. Then the tropospheric delays at projected point are interpolated from those of the ground networks. Finally, the tropospheric delays at projected point are extended upward to the airborne platform using pressure and temperature gradients and humidity exponential function. For validation of this method, the data of airborne campaign carried out by BKG in cooperation with GFZ and BGR 2008 in the northern Alps are used. The results show that GPS kinematic positioning precision in height component can be improved using this method. 相似文献
257.
This paper presents the results of analyzing the shaping of thin-walled conical parts with the uniform thickness along generatrix. 相似文献
258.
Devajyoti Dutta Sanjay Sharma G.K. Sen B.A.M. Kannan S. Venketswarlu R.M. Gairola J. Das G. Viswanathan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
By using a Doppler Weather Radar (DWR) at Shriharikota (13.66°N & 80.23°E), an Artificial Neural Network (ANN) based technique is proposed to improve the accuracy of rain intensity estimation. Three spectral moments of a Doppler spectra are utilized as an input data to an ANN. Rain intensity, as measured by the tipping bucket rain gauges around the DWR station, are considered as a target values for the given inputs. Rain intensity as estimated by the developed ANN model is validated by the rain gauges measurements. With the help of a developed technique, reasonable improvement in the estimation of rain intensity is observed. By using the developed technique, root mean square error and bias are reduced in the range of 34–18% and 17–3% respectively, compared to Z–R approach. 相似文献
259.
260.
V. G. Dmitriev V. I. Biryukov O. V. Egorova S. I. Zhavoronok L. N. Rabinskii 《Russian Aeronautics (Iz VUZ)》2017,60(2):169-176
The discretization of the boundary value problem for laminated composite shells is based on the finite difference approach using the regular mesh with the constant grid step and the difference operators of the second order of accuracy. The dynamic relaxation method is proposed for the solution of the nonlinear problem. The evolutionary equations of the dynamic relaxation are constructed, and the optimum parameters of the converging linear iterative process are estimated. 相似文献