全文获取类型
| 收费全文 | 8584篇 |
| 免费 | 36篇 |
| 国内免费 | 23篇 |
专业分类
| 航空 | 3942篇 |
| 航天技术 | 3054篇 |
| 综合类 | 30篇 |
| 航天 | 1617篇 |
出版年
| 2021年 | 81篇 |
| 2019年 | 57篇 |
| 2018年 | 172篇 |
| 2017年 | 115篇 |
| 2016年 | 117篇 |
| 2015年 | 53篇 |
| 2014年 | 205篇 |
| 2013年 | 256篇 |
| 2012年 | 239篇 |
| 2011年 | 366篇 |
| 2010年 | 263篇 |
| 2009年 | 392篇 |
| 2008年 | 451篇 |
| 2007年 | 247篇 |
| 2006年 | 191篇 |
| 2005年 | 219篇 |
| 2004年 | 222篇 |
| 2003年 | 262篇 |
| 2002年 | 183篇 |
| 2001年 | 265篇 |
| 2000年 | 161篇 |
| 1999年 | 208篇 |
| 1998年 | 240篇 |
| 1997年 | 138篇 |
| 1996年 | 214篇 |
| 1995年 | 246篇 |
| 1994年 | 247篇 |
| 1993年 | 152篇 |
| 1992年 | 187篇 |
| 1991年 | 70篇 |
| 1990年 | 72篇 |
| 1989年 | 195篇 |
| 1988年 | 83篇 |
| 1987年 | 81篇 |
| 1986年 | 85篇 |
| 1985年 | 254篇 |
| 1984年 | 189篇 |
| 1983年 | 154篇 |
| 1982年 | 159篇 |
| 1981年 | 291篇 |
| 1980年 | 79篇 |
| 1979年 | 67篇 |
| 1978年 | 66篇 |
| 1977年 | 58篇 |
| 1975年 | 76篇 |
| 1974年 | 53篇 |
| 1973年 | 49篇 |
| 1972年 | 49篇 |
| 1970年 | 55篇 |
| 1969年 | 48篇 |
排序方式： 共有8643条查询结果，搜索用时 15 毫秒
481.
Space Science Reviews - It is demonstrated by means of model calculations that while the general configuration of middle atmosphere dynamics (i.e., winter westerlies and summer easterlies) is... 相似文献
482.
Rijsdijk F.B. van der Spek G.A. 《IEEE transactions on aerospace and electronic systems》1978,(2):226-236
For a planar-array antenna with a monopulse feed horn, this study describes a simple algorithm for the determination of the direction of target echoes. Antenna pattern measurements of the array indicate that the direction sines of a received wavefront can be independently obtained with one simple relation between a normalized difference channel output and a direction sine. This paper determines the accuracy of the algorithm. 相似文献
483.
A track-while scan (TWS) algorithm is developed for targets in a clutter environment. The problem has been studied using only the position measurements [1, 5-8], but the simulation results have not been satisfactory. Modern processing techniques (FFT processor) ) in air traffic control and surveillance radar receivers provide both position and radial velocity. The radial velocity measurement may be conveniently used in the target-track correlation process, which will reduce the association ambiguity in the clutter environment. t. In the clear environment the algorithm using the position and radial velocity measurements has been treated in [3, 4]. A TWS algorithm, using both position and radial velocity measurements for targets in a clutter environment, is presented here. The algorithm obtained is nonlinear and adaptive. In order to evaluate the improvement due to radial velocity measurement a simulation has been performed on a digital computer. The algorithm was run with and without radial velocity measurements to compare its performances. An improvement was noted especially when the target path included an accelerated portion. 相似文献
484.
This paper reviews the present state of knowledge concerning the diurnal and semidiurnal variations in the galactic cosmic ray intensity. The analytical procedures that are required for extracting from the original data the desired information concerning the characteristics of the anisotropies outside the magnetosphere are described. These include corrections for atmospheric fluctuations, the determination of the amplitudes and phases of the daily variations, and their interpretation in terms of the free space anisotropies that give rise to them. The experimental results concerning the 24-h wave, including its long-term variations with periods of one and two solar cycles, and the characteristics of the total diurnal anisotropy are then discussed. The semidiurnal anisotropy is considered next. Transient anisotropies which manifest themselves as day-to-day variations, recurrence tendencies, cosmic ray storms, and diurnal variation trains, and which can introduce appreciable changes in the spectral parameters, are also of interest. The evolution of theoretical models developed to account for the diurnal and semidiurnal anisotropies, and fluctuations thereof, are then discussed in the light of the experimental results. Finally, the general features that are now well established, and the nature of the remaining problems, are summarized. 相似文献
485.
The paper presents a method by which the zeros of the polynomial representing a Huffman (impulse-equivalent) pulse sequence can be chosen so as to exert a degree of control on the form of the energy distribution of the signal in the time-frequency plane. This makes it possible to design Huffman pulse sequences which are suitable for use as radar or sonar signals in situations where significant target velocity occurs. 相似文献
486.
A position fix in a passive mode using satellites usually necessitates an expensive computer or lengthy hand calculation. This is the largest drawback of passive navigation and it would be more desirable if the user could find his position by a mere glance at a chart and table, as one uses Loran. The first step toward this goal is to use a synchronous satellite because it simplifies the problem. The next step is to find the position of the user by a Loran type of chart, which is universal, and correct this apparent position by looking at a special table which is made according to the amount of perturbation of both the satellite and the user's position. An example of the position fix along the route between Yokohama to Hawaii is shown. The concept can be extended to orbiting satellites due to the rules which govern the motion of satellites, if the fix accuracy is in the order of 2 to 5 miles. This method should be more accurate than the common sextant and more practical due to the fact that the satellite can be used at any time and in any weather. As a total system, it will be better than Omega because it could provide additional navigation information such as communication or traffic control by using satellites. 相似文献
487.
Corrections of pointing data provided by gimbal readout transducers can be used to compensate for the errors that arise because of gimbal misalignments and inaccuracies in transducer readout. This paper reviews the sources of pointing error in precision gimbaled pointing systems and describes the techniques for determining them. In particular, a method of calibration that is suitable for systems in which constraints on gimbal travel preclude the use of conventional calibration procedures is presented, and the pointing readout accuracy that can be achieved by this method is evaluated. 相似文献
488.
489.
490.
Approaches in the determination of plant nutrient uptake and distribution in space flight conditions. 总被引：2，自引：0，他引：2
A G Heyenga A Forsman L S Stodieck A Hoehn M Kliss 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,26(2):299-302
The effective growth and development of vascular plants rely on the adequate availability of water and nutrients. Inefficiency in either the initial absorption, transportation, or distribution of these elements are factors which impinge on plant structure and metabolic integrity. The potential effect of space flight and microgravity conditions on the efficiency of these processes is unclear. Limitations in the available quantity of space-grown plant material and the sensitivity of routine analytical techniques have made an evaluation of these processes impractical. However, the recent introduction of new plant cultivating methodologies supporting the application of radionuclide elements and subsequent autoradiography techniques provides a highly sensitive investigative approach amenable to space flight studies. Experiments involving the use of gel based 'nutrient packs' and the radionuclides calcium-45 and iron-59 were conducted on the Shuttle mission STS-94. Uptake rates of the radionuclides between ground and flight plant material appeared comparable. 相似文献