全文获取类型
收费全文 | 1346篇 |
免费 | 66篇 |
国内免费 | 64篇 |
专业分类
航空 | 649篇 |
航天技术 | 548篇 |
综合类 | 60篇 |
航天 | 219篇 |
出版年
2022年 | 10篇 |
2021年 | 22篇 |
2020年 | 22篇 |
2019年 | 13篇 |
2018年 | 15篇 |
2017年 | 7篇 |
2016年 | 20篇 |
2015年 | 20篇 |
2014年 | 41篇 |
2013年 | 41篇 |
2012年 | 51篇 |
2011年 | 65篇 |
2010年 | 62篇 |
2009年 | 82篇 |
2008年 | 114篇 |
2007年 | 45篇 |
2006年 | 46篇 |
2005年 | 57篇 |
2004年 | 47篇 |
2003年 | 38篇 |
2002年 | 36篇 |
2001年 | 60篇 |
2000年 | 29篇 |
1999年 | 46篇 |
1998年 | 43篇 |
1997年 | 22篇 |
1996年 | 36篇 |
1995年 | 24篇 |
1994年 | 35篇 |
1993年 | 24篇 |
1992年 | 34篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 34篇 |
1988年 | 14篇 |
1987年 | 7篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 27篇 |
1983年 | 22篇 |
1982年 | 20篇 |
1981年 | 24篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1970年 | 3篇 |
1969年 | 4篇 |
排序方式: 共有1476条查询结果,搜索用时 15 毫秒
991.
基于多目标管理方法,阐述了玉兔号巡视器定位多目标管理的概念。兼顾技术成熟度、定位效率、冗余验证和阶段化分层管理等原则,对玉兔号巡视器着陆点和导航点的定位方法进行建模及分析比较,给出对应的定位方案。最后,将基于此多目标管理方法制定的方案,应用于玉兔号巡视器的定位任务,定位精度优于亚像素级,相对定位精度优于4%。 相似文献
992.
An algorithm, combining velocity/height estimates, obtained from an airborne body fixed image shift estimator with auxiliary on-board measurements and sparsely stored terrain profile information constitutes an entirely passive autonomous navigation system suitable for moderate-g flight missions. Two versions are addressed. The "naive estimator," in which altitude estimates are multiplied by velocity/height estimaters, yields ground velocity. Position, obtained by integration, diverges with time. The "extended Kalman filter" (EKF) version, in which velocity and position are defined as state space components, locks on the stored terrain profile and does not diverge with time. It degenerates into the "naive estimator" if the terrain is completely flat. Numerical examples indicate excellent performance potential of the EKF estimator. 相似文献
993.
This paper addresses estimation of motion from the optical flow observed by an airborne down looking electro-optical sensor. The paper is in two parts. Part I addresses the development and analysis of a velocity-to-height ratio estimation algorithm and its principal error characteristics. In part II, it is shown how the information provided by the motion estimator can be integrated with additional on-board sensors to provide a complete autonomous navigation system. Part I as presented here is a summary version of the full length paper.? The algorithm implements recursive registration of successive images by using the gradient of a similarity function between them to control the tracking of their relative shift. The shift estimate provides velocity/height information. Substantial saving in memory and computation as compared to conventional full frame registration is achieved by using only a single line in the TV frame. Stochastic mathematical models for the image, terrain and vehicle velocity perturbations are used in the analysis. The choice of the most appropriate similarity function in the registration algorithm is addressed. Performance analysis indicates very small error variances, as illustrated by numerical examples. 相似文献
994.
Pierre Y. Bely Holland C. Ford Richard Burg Larry Petro Rick White John Bally 《Space Science Reviews》1995,74(1-2):101-112
The tropopause, typically at 16 to 18 km altitude at the lower latitudes, dips to 8 km in the polar regions. This makes the cold, dry and nonturbulent lower stratosphere accessible to tethered aerostats. Tethered aerostats can fly as high as 12 km and are extremely reliable, lasting for many years. In contrast to free-flying balloons, they can stay on station for weeks at a time, and payloads can be safely recovered for maintenance and adjustment and relaunched in a matter of hours. We propose to use such a platform, located first in the Arctic (near Fairbanks, Alaska) and, potentially, later in the Antarctic, to operate a new technology 6-meter, diluted aperture telescope with diffraction-limited performance in the near infrared. Thanks to the low ambient temperature (220 K), thermal emission from the optics is of the same order as that of the zodiacal light in the 2 to 3 micron band. Since this wavelength interval is the darkest part of the zodiacal light spectrum from optical wavelengths to 100 microns, the combination of high resolution images and a very dark sky make it the spectral region of choice for observing the redshifted light from galaxies and clusters of galaxies at moderate to high redshifts.Affiliated to the Astrophysics Division, Space Science Department, European Space Agency 相似文献
995.
996.
H Wu M Durante Y Furusawa K George T Kawata F A Cucinotta 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,31(6):1537-1542
Confluent human fibroblast cells were exposed to 6 Gy gamma-rays or 200 MeV/nucleon Fe ions at 0.7 or 3 Gy. The cells were allowed to repair for 24 hours after exposure and chromosomes were collected using a premature chromosome condensation technique with calyculin-A. Chromosome aberrations were analyzed using the multicolor FISH (mFISH) technique that allows identification of both complex and truly incomplete exchanges. Results showed that both doses of the Fe ions produced higher ratios of complex to simple exchanges and lower ratio of complete to incomplete exchanges than the 6 Gy gamma-exposure. The ratios of aberration yields were similar for the two doses of Fe ions. After 0.7 Gy of Fe ions, most complex aberrations were found to involve three or four chromosomes, indicating this is the maximum number of chromosome domains traversed by a single Fe ion track. 相似文献
997.
Y Wada H Suzuki S Watanabe 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(8):385-388
It gets into difficult to explain the mechanism of caloric nystagmus only by convection theory from results of microgravity experiments. One of the other theories is an occurrence of a relative volume change due to a temperature change. Since the volume change must lead to a pressure change after caloric stimulation, we tried to measure the ampulla pressure of the horizontal semicircular canal in pigeons (Columba livia) using an improved servo micropipette system. The main result was that the ampulla pressure increased by cooling and decreased by heating. The changes of the ampulla pressure depended on the temperature change but were not influenced by the pigeon's head position. 相似文献
998.
An alternate set of equations is given for the exact computation of the Kalman gains under the conditions of no maneuvering input noise and measurements in position and velocity. They are simpler than the standard recursive equations, and are useful in applications where implementation of the standard Kalman filter is not possible due to real-time restrictions. When there is maneuvering input noise, the same gains can still approximate the optimal gains with a very minor degradation in performance, even when some parameters, for example the measurement interval, change during a track. Simulation studies have indicated that there is negligible performance degradation with this method of gain approximation 相似文献
999.
Sinyak Y Grigoriev A Gaydadimov V Gurieva T Levinskih M Pokrovskii B 《Acta Astronautica》2003,52(7):575-580
Heavy water containing deuterium displays toxic property. It is stated that any quantity of a heavy isotope of hydrogen--deuterium--is undesirable to animals and plants. It was earlier shown by us that physical-chemical life support systems on board the "MIR" station fractionate (change) isotopes of hydrogen, oxygen and carbon. Therefore, the problem of regenerative systems in habitable space objects should include removal, from water, of a heavy stable isotope of hydrogen--deuterium. In this article we consider one method of obtaining deuterium-free water--decomposition of distillate water in an electrolyser to hydrogen and oxygen with subsequent synthesis in a catalytic or high-temperature reactor. The influence of deuterium-free water on the growth and development of Arabidopsis thaliana and Japanese quail is investigated. It is shown that with the help of the electrolysis method it is possible to fabricate water containing 80% less deuterium in comparison with SMOW. Experimentally, it is proved on a culture of Arabidopsis thaliana and Japanese quail that water with reduced contents of deuterium (80%) displays positive biological activity. 相似文献
1000.
V. N. Kuril'chik M. Y. Boudjada H. O. Rucker I. F. Kopaeva S. V. Mironov 《Cosmic Research》2005,43(6):413-422
Results of almost four years of continuous observations of the subauroral nonthermal radio emission (SANE) onboard the Interball-1 satellite are presented. The main features of SANE are described and discussed: the intensity and character of a signal, its appearance in time, beam directivity, propagation distance, and other observed parameters. 相似文献