全文获取类型
收费全文 | 3746篇 |
免费 | 2篇 |
国内免费 | 19篇 |
专业分类
航空 | 1964篇 |
航天技术 | 1054篇 |
综合类 | 10篇 |
航天 | 739篇 |
出版年
2019年 | 18篇 |
2018年 | 167篇 |
2017年 | 150篇 |
2016年 | 53篇 |
2015年 | 29篇 |
2014年 | 49篇 |
2013年 | 60篇 |
2012年 | 92篇 |
2011年 | 215篇 |
2010年 | 186篇 |
2009年 | 234篇 |
2008年 | 250篇 |
2007年 | 202篇 |
2006年 | 67篇 |
2005年 | 122篇 |
2004年 | 94篇 |
2003年 | 80篇 |
2002年 | 51篇 |
2001年 | 79篇 |
2000年 | 49篇 |
1999年 | 63篇 |
1998年 | 80篇 |
1997年 | 49篇 |
1996年 | 61篇 |
1995年 | 78篇 |
1994年 | 76篇 |
1993年 | 53篇 |
1992年 | 61篇 |
1991年 | 31篇 |
1990年 | 30篇 |
1989年 | 72篇 |
1988年 | 26篇 |
1987年 | 29篇 |
1986年 | 30篇 |
1985年 | 119篇 |
1984年 | 68篇 |
1983年 | 57篇 |
1982年 | 58篇 |
1981年 | 103篇 |
1980年 | 34篇 |
1979年 | 26篇 |
1978年 | 24篇 |
1977年 | 28篇 |
1976年 | 18篇 |
1975年 | 31篇 |
1974年 | 19篇 |
1973年 | 25篇 |
1972年 | 20篇 |
1970年 | 24篇 |
1969年 | 26篇 |
排序方式: 共有3767条查询结果,搜索用时 93 毫秒
331.
332.
Education and public outreach are viewed by NASA as significant undertakings for all of its space missions. The IMAGE satellite is one of the first missions to explicitly include `E&PO in its original proposal to NASA in 1996. We will discuss what IMAGE has accomplished in this area to date, and what new activities it will conduct following a successful launch. 相似文献
333.
The goal of the Kinelite Project is to develop a space qualified motion analysis system to be used in space by the scientific community, mainly to support neuroscience protocols. The measurement principle of the Kinelite is to determine, by triangulation mean, the 3D position of small, lightweight, reflective markers positioned at the different points of interest. The scene is illuminated by Infra Red flashes and the reflected light is acquired by up to 8 precalibrated and synchronized CCD cameras. The main characteristics of the system are: Camera field of view: 45 degrees; Number of cameras: 2 to 8; Acquisition frequency: 25, 50, 100, or 200 Hz; CCD format: 256 x 256; Number of markers: up to 64; 3D accuracy: 2mm; Main dimensions: 45 cm x 45 cm x 30 cm; Mass: 23 kg; Power consumption: less than 200 W. The Kinelite will first fly aboard the NASA Spacelab; it will be used, during the NEUROLAB mission (4/98), to support the "Frames of References and Internal Models" (Principal Investigator: Pr. A. Berthoz, Co Investigators: J. McIntyre, F. Lacquaniti). 相似文献
334.
Jie Jiang Robert H. Cameron Dieter Schmitt Manfred Schüssler 《Space Science Reviews》2013,176(1-4):289-298
To reproduce the weak magnetic field on the polar caps of the Sun observed during the declining phase of cycle 23 poses a challenge to surface flux transport models since this cycle has not been particularly weak. We use a well-calibrated model to evaluate the parameter changes required to obtain simulated polar fields and open flux that are consistent with the observations. We find that the low polar field of cycle 23 could be reproduced by an increase of the meridional flow by 55% in the last cycle. Alternatively, a decrease of the mean tilt angle of sunspot groups by 28% would also lead to a similarly low polar field, but cause a delay of the polar field reversals by 1.5 years in comparison to the observations. 相似文献
335.
336.
The Monitor-E spacecraft executed uncontrolled flight due to emergency situation, no telemetry information on parameters of the spacecraft’s
attitude motion being available. So, the problem arose to determine the spacecraft’s rotational motion from the accessible
indirect information—the electric current provided by solar batteries. In this paper the integrated statistical technique
is described, that allows one to solve this problem. The values of current, obtained over the time interval some tens of minutes
long, have been processed simultaneously by the least squares method using the integration of the equations of spacecraft’s
rotational motion. As a result of processing, the initial conditions of motion were estimated, and the spacecraft’s moments
of inertia were updated, as well as the angles, specifying solar batteries position in the spacecraft-fixed coordinate system.
The results of processing of 12 data sets are presented, which allowed us to reconstruct the actual rotational motion of the
spacecraft. 相似文献
337.
338.
A robust satellite tracking antenna is designed to cope with the sensor imprecision and the highly noisy sea environment. Fuzzy logic is utilized for the controller imprecision and the highly noisy sea environment. Fuzzy logic is utilized for the controller design as well as inaccurate data interpretation. The fuzzy-rule based controller eliminates the need to model the nonlinear and noisy ship-mounted antenna system. With Global Positioning System and the tracking controller the antenna can be brought to a neighborhood of the desired orientation. Spiral search with signal power feedback can then servo the antenna to the true orientation. Computer simulations and antenna experiments verify our design is indeed robust and effective 相似文献
339.
An expression is derived for the probability of error for a conventional binary, noncoherent, frequency-shift-key (NCFSK) communications system under the influence of bandpass Gaussian noise and a linear frequency-modulation jamming waveform. The resulting integral is expressed in terms of the well-known Q function, which depends upon average signal, noise, and jamming powers. The analytical procedures used can be applied to the analysis of the effects of other types of jamming. 相似文献
340.
A method is presented for the selection of a communication system based upon optimization of both frequency-dependent and frequency independent parameters. The indicator of quality used is the effective radiated power per cycle per second of information bandwidth. The method involves consideration of the frequency-dependent qualities such as system temperature and propagation losses independently from the frequency-independent quantities such as modulation loss, signal-to-noise ratio, performance margin, etc. Except for temperature and propagation loss effects, the method is made frequency-independent by considering the spacecraft antenna in terms of area while the ground terminal antenna is treated in terms of gain, thus removing wavelength from the transmission equation. An example is presented of the application of the method to the selection of a system for transmission of wide-band video signals over a satellite to earth link. The example illustrates the exchanges that can be made between the frequency-dependent and -independent quantities in order to arrive at an optimum system. 相似文献