全文获取类型
收费全文 | 6117篇 |
免费 | 651篇 |
国内免费 | 486篇 |
专业分类
航空 | 3365篇 |
航天技术 | 1901篇 |
综合类 | 440篇 |
航天 | 1548篇 |
出版年
2023年 | 47篇 |
2022年 | 107篇 |
2021年 | 167篇 |
2020年 | 130篇 |
2019年 | 133篇 |
2018年 | 239篇 |
2017年 | 185篇 |
2016年 | 161篇 |
2015年 | 168篇 |
2014年 | 243篇 |
2013年 | 254篇 |
2012年 | 267篇 |
2011年 | 322篇 |
2010年 | 335篇 |
2009年 | 380篇 |
2008年 | 386篇 |
2007年 | 274篇 |
2006年 | 220篇 |
2005年 | 216篇 |
2004年 | 164篇 |
2003年 | 203篇 |
2002年 | 217篇 |
2001年 | 194篇 |
2000年 | 159篇 |
1999年 | 159篇 |
1998年 | 145篇 |
1997年 | 129篇 |
1996年 | 123篇 |
1995年 | 156篇 |
1994年 | 138篇 |
1993年 | 90篇 |
1992年 | 116篇 |
1991年 | 69篇 |
1990年 | 56篇 |
1989年 | 85篇 |
1988年 | 42篇 |
1987年 | 38篇 |
1986年 | 38篇 |
1985年 | 111篇 |
1984年 | 79篇 |
1983年 | 68篇 |
1982年 | 83篇 |
1981年 | 103篇 |
1980年 | 31篇 |
1979年 | 41篇 |
1978年 | 28篇 |
1977年 | 18篇 |
1974年 | 17篇 |
1973年 | 16篇 |
1972年 | 15篇 |
排序方式: 共有7254条查询结果,搜索用时 562 毫秒
241.
Very Long Baseline Interferometry (VLBI) is a differential technique observing radiation of compact extra-galactic radio sources with pairs of radio telescopes. For these observations, the frequency standards at the telescopes need to have very high stability. In this article we discuss why this is, and we investigate exactly how precise the frequency standards need to be. Four areas where good clock performance is needed are considered: coherence, geodetic parameter estimation, correlator synchronization, and UT1 determination. We show that in order to ensure the highest accuracy of VLBI, stability similar to that of a hydrogen maser is needed for time-scales up to a few hours. In the article, we are considering both traditional VLBI where extra-galactic radio sources are observed, as well as observation of man-made artificial radio sources emitted by satellites or spacecrafts. 相似文献
242.
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
243.
244.
245.
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
246.
Cosmic Research - An Erratum to this paper has been published: https://doi.org/10.1134/S0010952521120017 相似文献
247.
248.
本文通过对该型教练机现有航空电子系统构型、驾驶员操作程序、接口控制文件深入研究,结合未来功能扩展需求,给出了合理的系统构型分析、功能分配、数据流规划、原相关系统软件改动分析,在此基础上还设计出了嵌入式训练系统技术方案.依据方案研制的嵌入式训练系统原理样机通过系统动态试验、航电系统地面综合试验和飞机地面通电试验,证明系统... 相似文献
249.
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
250.
An algorithm of analyzing the design parameters for a thin-walled wing from composite material based on the efficient distribution
of structural material between load-carrying elements is described. We consider the problems of convergence acceleration for
the iteration design analysis. Also presented are the examples of determining the design parameters for a superlight aircraft
wing and the rational parameters of the filler in the form of a three-dimensional rod structure. 相似文献