全文获取类型
收费全文 | 18279篇 |
免费 | 53篇 |
国内免费 | 131篇 |
专业分类
航空 | 9959篇 |
航天技术 | 5481篇 |
综合类 | 240篇 |
航天 | 2783篇 |
出版年
2021年 | 160篇 |
2018年 | 220篇 |
2016年 | 158篇 |
2014年 | 436篇 |
2013年 | 515篇 |
2012年 | 422篇 |
2011年 | 595篇 |
2010年 | 412篇 |
2009年 | 768篇 |
2008年 | 804篇 |
2007年 | 379篇 |
2006年 | 426篇 |
2005年 | 388篇 |
2004年 | 441篇 |
2003年 | 527篇 |
2002年 | 481篇 |
2001年 | 575篇 |
2000年 | 365篇 |
1999年 | 462篇 |
1998年 | 440篇 |
1997年 | 317篇 |
1996年 | 392篇 |
1995年 | 451篇 |
1994年 | 447篇 |
1993年 | 360篇 |
1992年 | 327篇 |
1991年 | 250篇 |
1990年 | 243篇 |
1989年 | 417篇 |
1988年 | 208篇 |
1987年 | 246篇 |
1986年 | 236篇 |
1985年 | 641篇 |
1984年 | 523篇 |
1983年 | 415篇 |
1982年 | 487篇 |
1981年 | 613篇 |
1980年 | 244篇 |
1979年 | 184篇 |
1978年 | 190篇 |
1977年 | 146篇 |
1976年 | 155篇 |
1975年 | 189篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 189篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
A multiple model adaptive estimator (MMAE) has been formulated to estimate the state of a dynamic system modeled by a linear stochastic differential equation, from which measurements, described as a noise-corrupted space-time point process functionally related to that state, are extracted. Assumed certainty equivalence is used to combine such an estimator with the LQ full-state feedback controller to synthesize a practical, implementable controller. Performance of the estimator and resultant controller characteristics are investigated via simulation as a function of approximation method used to limit the full-scale estimator to finite dimensionality and also as a function of important parameters defining the dynamics and observation processes. 相似文献
232.
Nazar R. Ikhsanov Chul-Sung Choi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2901-2902
The state of subsonic propeller is intermediate between the states of supersonic propeller and accretor in the evolutionary tracks of magnetized compact stars. The rotational rate of a star at this stage decelerates due to the interaction between its magnetosphere and the surrounding hot, quasi-static plasma envelope. The magnetospheric radius is smaller than the corotation radius and the boundary of the magnetosphere is stable with respect to interchange instabilities. The rate of the mass flux from the inner radius of the envelope to the stellar surface is limited by the rate of plasma diffusion into the magnetic field of the star. As a result, the subsonic propeller would appear as a low-luminosity accretion-powered pulsar with a soft X-ray spectrum. 相似文献
233.
Michel D. Ingham Robert D. Rasmussen Matthew B. Bennett Alex C. Moncada 《Acta Astronautica》2006,58(12):648-661
It has become clear that spacecraft system complexity is reaching a threshold where customary methods of control are no longer affordable or sufficiently reliable. At the heart of this problem are the conventional approaches to systems and software engineering based on subsystem-level functional decomposition, which fail to scale in the tangled web of interactions typically encountered in complex spacecraft designs. Furthermore, there is a fundamental gap between the requirements on software specified by systems engineers and the implementation of these requirements by software engineers. Software engineers must perform the translation of requirements into software code, hoping to accurately capture the systems engineer's understanding of the system behavior, which is not always explicitly specified. This gap opens up the possibility for misinterpretation of the systems engineer's intent, potentially leading to software errors. This problem is addressed by a systems engineering methodology called State Analysis, which provides a process for capturing system and software requirements in the form of explicit models. This paper describes how requirements for complex aerospace systems can be developed using State Analysis, using representative spacecraft examples. 相似文献
234.
Atmospheric tides 总被引:3,自引:0,他引:3
235.
Many digital techniques are available for providing a desired shift in frequency of a given carrier signal, but the resulting output usually contains spurious harmonic content due to the digital means employed. This correspondence discusses a digital device which provides a variable shift in frequency of a given carrier signal, with the feature that no spurious harmonic content exists in the output. 相似文献
236.
A coherent CW superheterodyne radar system operating at frequencies of 9, 17, 35, and 70 GHz is described. The radars are installed on a free-flight range to study backscattering from wakes of hypersonic-velocity projectiles. Each radar is equipped with a focused-lens antenna oriented at an angle of approximately 45° to the flight axis. Amplitude and phase of the received signal are recorded separately. Some typical results are given to demonstrate the capabilities of the equipment. 相似文献
237.
Luh J. Y. S. Shafran Joel S. Harvey C. Arthur 《IEEE transactions on aerospace and electronic systems》1968,(2):218-233
This paper considers the problem arising from the design of an autopilot for a large booster. The motion-controlling actuators of the booster have both position and rate limits. The problem is formulated as a bounded phase-coordinate problem and analyzed by the ``backing out of the target' procedure. A method of constructing the optimal control is presented. An example of an oscillatory system with two control inputs is given, and the optimal control is expressed as an explicit time function. 相似文献
238.
Various methods are presented for estimating the flight time for vehicles which fly an optimum trajectory. A realistic example is considered in order to display the inherent accuracy of each of these methods. Numerical integration methods are found to be the most accurate. Simple formulas are derived for the case where a less accurate estimate is acceptable. All of these methods require less computation than would be required in order to solve the overall optimization problem. Thus any one of these methods can be applied to an onboard guidance scheme or control system which requires a real time estimate of the time-to-go. 相似文献
239.
The analysis and comments presented in this paper are meant to establish the general communication parameters associated with Martian flyby probes and with lander and manned vehicles. Fundamental data transfer problems are reviewed to define comparisons and trends of tradeoffs for future studies. Selected focal points are based upon the long propagation path length, with inherent time delays, and the high noise produced by the sun. These problems are magnified because large quantities of data must be obtained to satisfy the needs of the scientific community and the curiosity of an interested public. A comparison of two communication systems is provided: the microwave spectrum and the optical spectrum, as represented by the microwaves at 2.3 GHz and the laser at 6328 ?. A method of cost effectiveness or value received from space missions (a criterion of power input for data quantity received) is also presented. 相似文献
240.
On-board/in-flight checkout of future aerospace systems will necessitate a fundamental departure from today's design and checkout activities. The interrelationship of checkout, incorporated into prime vehicular functions, man as a subsystem, and standardization Of hardware/software must be considered as a functional and integral entity if the efforts of today's long-range planning are to become tomorrow's reality. This paper will describe future systems checkout, maintenance and support considerations, and actions and tentative measures necessary for implementation. The realization of the implementation of these concepts into functional elements will depend, accurately and economically, upon the degree of government perceptiveness and the extent of industrial support. 相似文献