全文获取类型
收费全文 | 5678篇 |
免费 | 22篇 |
国内免费 | 15篇 |
专业分类
航空 | 2638篇 |
航天技术 | 2144篇 |
综合类 | 16篇 |
航天 | 917篇 |
出版年
2021年 | 40篇 |
2019年 | 31篇 |
2018年 | 87篇 |
2017年 | 72篇 |
2016年 | 61篇 |
2015年 | 31篇 |
2014年 | 100篇 |
2013年 | 146篇 |
2012年 | 126篇 |
2011年 | 213篇 |
2010年 | 149篇 |
2009年 | 230篇 |
2008年 | 293篇 |
2007年 | 146篇 |
2006年 | 110篇 |
2005年 | 162篇 |
2004年 | 165篇 |
2003年 | 202篇 |
2002年 | 113篇 |
2001年 | 201篇 |
2000年 | 101篇 |
1999年 | 161篇 |
1998年 | 165篇 |
1997年 | 126篇 |
1996年 | 123篇 |
1995年 | 176篇 |
1994年 | 198篇 |
1993年 | 90篇 |
1992年 | 120篇 |
1991年 | 52篇 |
1990年 | 66篇 |
1989年 | 119篇 |
1988年 | 47篇 |
1987年 | 58篇 |
1986年 | 57篇 |
1985年 | 173篇 |
1984年 | 142篇 |
1983年 | 127篇 |
1982年 | 125篇 |
1981年 | 206篇 |
1980年 | 55篇 |
1979年 | 56篇 |
1978年 | 52篇 |
1977年 | 46篇 |
1976年 | 41篇 |
1975年 | 49篇 |
1974年 | 39篇 |
1973年 | 36篇 |
1972年 | 58篇 |
1969年 | 31篇 |
排序方式: 共有5715条查询结果,搜索用时 15 毫秒
251.
C. J. Hansen M. A. Caplinger A. Ingersoll M. A. Ravine E. Jensen S. Bolton G. Orton 《Space Science Reviews》2017,213(1-4):475-506
Junocam is a wide-angle camera designed to capture the unique polar perspective of Jupiter offered by Juno’s polar orbit. Junocam’s four-color images include the best spatial resolution ever acquired of Jupiter’s cloudtops. Junocam will look for convective clouds and lightning in thunderstorms and derive the heights of the clouds. Junocam will support Juno’s radiometer experiment by identifying any unusual atmospheric conditions such as hotspots. Junocam is on the spacecraft explicitly to reach out to the public and share the excitement of space exploration. The public is an essential part of our virtual team: amateur astronomers will supply ground-based images for use in planning, the public will weigh in on which images to acquire, and the amateur image processing community will help process the data. 相似文献
252.
In August, 1981, the Westerbork Synthesis Radio Telescope was used for 4 h to search for narrowband pulsing radio beacons in the direction of the Galactic Center. By using both the spatial discrimination and temporal stability available to an interferometric measurement, weak intermittent signals can be detected even in the face of the strong, naturally caused radiation from this region. A radio beacon within our bandwidth, centered on the 21 cm neutral hydrogen line, would be recognizable if it had a repetition period between 40 sec and 1/2 h. The rms sensitivity to point sources was approximately 50 mJy/cycle, and the detection limit was 500 mJy/cycle. The limit degrades for pulse widths < 0.02s. No repetitive signals were found. For a swept, narrow-band radio beacon constrained to the Galactic Disk (beamwidth = 0.02 rad), our detection limit corresponds to a transmitter power of 10(11) MW at the Galactic Center. 相似文献
253.
Theoretical pressure balance arguments have implied that steady convection is hardly possible in the terrestrial magnetotail and that steady energy input necessarily generates a cyclic loading-unloading sequence, i.e., repetitive substorms. However, observations have revealed that enhanced solar wind energy input to the magnetospheric system may either lead to substorm activity or enhanced but steady convection. This topic is reviewed with emphasis on several recent case studies of the Steady Magnetospheric Convection (SMC) events. In these cases extensive data sets from both satellite and ground-based instruments from various magnetospheric and ionospheric regions were available.Accurate distinction of the spatial and temporal scales of the magnetospheric processes is vital for correct interpretation of the observations during SMC periods. We show that on the large scale, the magnetospheric configuration and plasma convection are stable during SMC events, but that both reveal considerable differences from their quiet-time assemblies. On a shorter time scale, there are numerous transient activations which are similar to those found during substorms, but which presumably originate from a more distant tail reconnection process, and map to the poleward boundary of the auroral oval. The available observations and the unresolved questions are summarized here.The tail magnetic field during SMC events resembles both substorm growth and recovery phases in the neartail and midtail, respectively, but this configuration may remain stable for up to ten hours. Based on observations and model results we discuss how the magnetospheric system avoids pressure balance problems when the plasma convects earthward.Finally, the importance of further coordinated studies of SMC events is emphasized. Such studies may shed more light on the substorm dynamics and help to verify quantitatively the theoretical models of the convecting magnetosphere. 相似文献
254.
255.
The Lunar Gravity Ranging System for the Gravity Recovery and Interior Laboratory (GRAIL) Mission 总被引:1,自引:0,他引:1
William M. Klipstein Bradford W. Arnold Daphna G. Enzer Alberto A. Ruiz Jeffrey Y. Tien Rabi T. Wang Charles E. Dunn 《Space Science Reviews》2013,178(1):57-76
The Lunar Gravity Ranging System (LGRS) flying on NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission measures fluctuations in the separation between the two GRAIL orbiters with sensitivity below 0.6 microns/Hz1/2. GRAIL adapts the mission design and instrumentation from the Gravity Recovery and Climate Experiment (GRACE) to a make a precise gravitational map of Earth’s Moon. Phase measurements of Ka-band carrier signals transmitted between spacecraft with line-of-sight separations between 50 km to 225 km provide the primary observable. Measurements of time offsets between the orbiters, frequency calibrations, and precise orbit determination provided by the Global Positioning System on GRACE are replaced by an S-band time-transfer cross link and Deep Space Network Doppler tracking of an X-band radioscience beacon and the spacecraft telecommunications link. Lack of an atmosphere at the Moon allows use of a single-frequency link and elimination of the accelerometer compared to the GRACE instrumentation. This paper describes the implementation, testing and performance of the instrument complement flown on the two GRAIL orbiters. 相似文献
256.
K. Yoshimura K. Abe H. Fuke S. Haino T. Hams M. Hasegawa A. Horikoshi K.C. Kim T. Kumazawa A. Kusumoto M.-H. Lee Y. Makida S. Matsuda Y. Matsukawa J.W. Mitchell A.A. Moiseev J. Nishimura M. Nozaki R. Orito J.F. Ormes K. Sakai M. Sasaki E.S. Seo Y. Shikaze R. Shinoda R.E. Streitmatter J. Suzuki K. Takeuchi N. Thakur K. Tanaka T. Yamagami A. Yamamoto T. Yoshida 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
257.
258.
This paper presents an analytical solution for static analysis of thick rectangular beams with different boundary conditions.Carrera's Unified Formulation (CUF) is used in order to consider shear deformation theories of arbitrary order.The novelty of the present work is that a boundary discontinuous Fourier approach is used to consider clamped boundary conditions in the analytical solution,unlike Navier-type solutions which are restricted to simply supported beams.Governing equations are obtained by employing the principle of virtual work.The numerical accuracy of results is ascertained by studying the convergence of the solution and comparing the results to those of a 3D finite element solution.Beams subjected to bending due to a uniform pressure load and subjected to torsion due to opposite linear forces are considered.Overall,accurate results close to those of 3D finite element solutions are obtained,which can be used to validate finite element results or other approximate methods. 相似文献
259.
260.
Multiposition alignment of strapdown inertial navigation system 总被引:3,自引:0,他引:3
Lee J.G. Park C.G. Park H.W. 《IEEE transactions on aerospace and electronic systems》1993,29(4):1323-1328
The authors demonstrate that the stationary alignment of strapdown inertial navigation system (SDINS) can be improved by employing the multiposition/technique. Using an observability analysis, it is shown that an optimal two-position alignment not only satisfies complete observability conditions but also minimizes alignment errors. This is done by analytic rank testing of the stripped observability matrix and numerical calculation of the error covariance. It is also shown that an optimal three-position alignment accelerates the convergence of the alignment error compared with two-position alignment 相似文献