全文获取类型
收费全文 | 7793篇 |
免费 | 17篇 |
国内免费 | 38篇 |
专业分类
航空 | 3703篇 |
航天技术 | 2616篇 |
综合类 | 27篇 |
航天 | 1502篇 |
出版年
2021年 | 65篇 |
2019年 | 45篇 |
2018年 | 223篇 |
2017年 | 166篇 |
2016年 | 107篇 |
2015年 | 46篇 |
2014年 | 159篇 |
2013年 | 194篇 |
2012年 | 209篇 |
2011年 | 373篇 |
2010年 | 280篇 |
2009年 | 398篇 |
2008年 | 426篇 |
2007年 | 286篇 |
2006年 | 171篇 |
2005年 | 244篇 |
2004年 | 217篇 |
2003年 | 237篇 |
2002年 | 144篇 |
2001年 | 223篇 |
2000年 | 149篇 |
1999年 | 169篇 |
1998年 | 197篇 |
1997年 | 146篇 |
1996年 | 185篇 |
1995年 | 241篇 |
1994年 | 216篇 |
1993年 | 125篇 |
1992年 | 183篇 |
1991年 | 70篇 |
1990年 | 67篇 |
1989年 | 154篇 |
1988年 | 60篇 |
1987年 | 57篇 |
1986年 | 73篇 |
1985年 | 226篇 |
1984年 | 173篇 |
1983年 | 140篇 |
1982年 | 173篇 |
1981年 | 202篇 |
1980年 | 62篇 |
1979年 | 50篇 |
1978年 | 59篇 |
1977年 | 54篇 |
1976年 | 46篇 |
1975年 | 42篇 |
1974年 | 52篇 |
1972年 | 34篇 |
1970年 | 40篇 |
1969年 | 39篇 |
排序方式: 共有7848条查询结果,搜索用时 17 毫秒
31.
Shing F Fung 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(6):1441-1450
The study of Earth's radiation belts is one of the oldest subjects in space physics. Despite the tremendous progress made in the last four decades, we still lack a complete understanding of the radiation belts in terms of their configurations, dynamics, and detailed physical accounts of their sources and sinks. The static nature of early empirical trapped radiation models, for examples, the NASA AP-8 and AE-8 models, renders those models inappropriate for predicting short-term radiation belt behaviors associated with geomagnetic storms and substorms. Due to incomplete data coverage, these models are also inaccurate at low altitudes (e.g., <1000 km) where many robotic and human space flights occur. The availability of radiation data from modern space missions and advancement in physical modeling and data management techniques have now allowed the development of new empirical and physical radiation belt models. In this paper, we will review the status of modern radiation belt modeling. 相似文献
32.
Wang L. Dawson J.M. Hornak L.A. Famouri P. Ghaffarian R. 《IEEE transactions on aerospace and electronic systems》2004,40(2):567-575
Closed-loop control has been successfully applied to a microelectromechanical systems (MEMS) lateral comb resonator device in real-time to perform impulse disturbance damping and sinusoidal position control, enabled by the use of a through-wafer optical microprobe to obtain position feedback. This result leverages the application of lifetime, in-situ control of MEMS in order to provide quality assurance of microsystems in safety critical applications. A position feedback signal produced by a through-wafer optical microprobe has been used for comb resonator system model identification by two independent methods to accurately determine the effective mass, damping, and spring constant values of the device. After accurate determination of system parameters, closed-loop impulse disturbance damping and proportional-integral-differential (PID) translational control were applied. Closed-loop control results presented indicate controllability of such microstructures and response times on the order of the natural frequency of the device. 相似文献
33.
The results of the experimental studies of damping characteristics of a composite torsion bar in the helicopter main rotor
hingeless hub are presented. A variant of their improvement is considered. 相似文献
34.
We present an algorithm for calculating the shells of revolution with the branching meridian using the triangular finite elements, the rigidity matrices of which are formed based on the vector method of displacement interpolation [1]. The correct kinematic and static conditions of shell conjugation on the line of their coupling have been developed. The shell structure consisting of a cylinder and adjacent shells is calculated under various conditions of support. 相似文献
35.
Lenz R. Schuler K. Younis M. Wiesbeck W. 《Aerospace and Electronic Systems Magazine, IEEE》2006,21(5):30-33
In April 2006, the TerraSAR-X satellite was launched. This paper describes the development of a novel and highly integrated, digitally-controlled active SAR system calibrator (DARC). It consists of both an active transponder path for absolute radiometric calibration and a calibrated receiver chain for antenna pattern evaluation of the satellite antenna. A total of 16 active transponder and receiver systems and 17 receiver-only systems will be fabricated for a calibration campaign in 2006. 相似文献
36.
Ulander L.M.H. Hellsten H. Stenstrom G. 《IEEE transactions on aerospace and electronic systems》2003,39(3):760-776
Exact synthetic aperture radar (SAR) inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, back-projection integration in time domain can also be used. This technique has the benefit of handling a general aperture geometry. In the past, however, back-projection has seldom been used due to heavy computational burden. We show that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization. By representing images in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms. The algorithm is applied to data from the airborne ultra-wideband CARABAS SAR and shown to give a reduction in processing time of two to three orders of magnitude. 相似文献
37.
I. S. Akhmed’yanov 《Russian Aeronautics (Iz VUZ)》2009,52(4):383-389
The application of the quadrature method [1] to analyzing the shells of revolution under symmetrical loading relative to the
zero meridian is considered. The method can be treated as a generalization of the method presented in [2] to solve the Volterra
integral equation of the second kind. 相似文献
38.
A problem of determining a shape of the airfoil being streamlined by a potential incompressible inviscid flow is solved by
the successive approximation method using a specified chord velocity diagram. It is shown that a closed airfoil that possesses
a specified chord velocity diagram can be constructed with a sufficient accuracy; if the chord diagram is unsuccessfully specified,
the closed airfoil may prove to be not univalent, that is, physically unrealizable. 相似文献
39.
Garenaux K. Merlet T. Alouini M. Lopez J. Vodjdani N. Boula-Picard R. Fourdin C. Chazelas J. 《Aerospace and Electronic Systems Magazine, IEEE》2007,22(2):3-8
Radar systems require transmission of very high purity signals. Photonics is now mature enough to achieve analog transmission with very low noise, strong immunity, and wide-bandwidth even in harsh environments. We present our recent developments of optimized optical links dedicated to radar and multifunction systems 相似文献
40.
Search and rescue satellite aided tracking (SARSAT) depends on the processing of emergency locator transmitter (ELT) signals which are received by a satellite in low polar orbit. Since the signal from a distressed vehicle is normally immersed in a background of other ELT signals (false alarms), interference, and noise, different methods of spectral estimation can provide advantages in estimating carrier frequency. A comparison between average spectrum and minimum spectrum for several real signals is provided here. 相似文献