全文获取类型
收费全文 | 17209篇 |
免费 | 9206篇 |
国内免费 | 1496篇 |
专业分类
航空 | 21092篇 |
航天技术 | 3696篇 |
综合类 | 229篇 |
航天 | 2894篇 |
出版年
2024年 | 96篇 |
2023年 | 187篇 |
2022年 | 281篇 |
2021年 | 366篇 |
2020年 | 429篇 |
2019年 | 1182篇 |
2018年 | 1463篇 |
2017年 | 1326篇 |
2016年 | 1364篇 |
2015年 | 1226篇 |
2014年 | 1195篇 |
2013年 | 1227篇 |
2012年 | 1165篇 |
2011年 | 1202篇 |
2010年 | 1118篇 |
2009年 | 1292篇 |
2008年 | 1178篇 |
2007年 | 884篇 |
2006年 | 681篇 |
2005年 | 532篇 |
2004年 | 532篇 |
2003年 | 540篇 |
2002年 | 524篇 |
2001年 | 546篇 |
2000年 | 389篇 |
1999年 | 478篇 |
1998年 | 474篇 |
1997年 | 381篇 |
1996年 | 386篇 |
1995年 | 411篇 |
1994年 | 370篇 |
1993年 | 323篇 |
1992年 | 318篇 |
1991年 | 399篇 |
1990年 | 354篇 |
1989年 | 468篇 |
1988年 | 289篇 |
1987年 | 168篇 |
1986年 | 115篇 |
1985年 | 262篇 |
1984年 | 219篇 |
1983年 | 176篇 |
1982年 | 202篇 |
1981年 | 265篇 |
1980年 | 102篇 |
1979年 | 67篇 |
1978年 | 80篇 |
1975年 | 72篇 |
1974年 | 67篇 |
1972年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 906 毫秒
401.
Lee M.H. Kolodziej W.J. Mohler R.R. 《IEEE transactions on aerospace and electronic systems》1985,(5):594-600
The control of a linear system with random coefficients is studied here. The cost function is of a quadratic form and the random coefficients are assumed to be partially observable by the controller. By means of the stochastic Bellman equation, the optimal control of stochastic dynamic models with partially observable coefficients is derived. The optimal control is shown to be a linear function of the observable states and a nonlinear function of random parameters. The theory is applied to an optimal control design of an aircraft landing in wind gust. 相似文献
402.
Rubertus D.P. Hunter L.D. Cecere G.J. 《IEEE transactions on aerospace and electronic systems》1984,(3):243-249
Electromechanical actuation is a critical element that must be developed and verified to make the all-electric aircraft a viable concept. For several years the Flight Control Division of the Air Force Wright Aeronautical Laboratories has sponsored activities to demonstrate the credibility of electromechanical actuation systems (EMAS) for primary flight control actuation functions. The foundation for these EMAS activities and several electromechanical actuation development programs are described here. One involves the design, fabrication, and laboratory test of a rotary, hingeline electromechanical actuator. Another involves the development and flight test demonstration of a linear electromechanical actuator for controlling an aileron of a C-141 aircraft. A third involves the design and development of a linear electromechanical actuator for missiles having severe performance, temperature, and volumetric requirements. In addition, a brief summary of the results from two aircraft actuation trade studies compare the baseline (conventional) hydraulic flight control system with an all-electric airplane concept including quantitative comparisons of weight, reliability and maintainability, and life cycle costs. 相似文献
403.
A growing memory discrete dynamic model for performing temporal extrapolations along a predetermined path in a random field is presented. This dynamic model is used to drive a linear system that is itself driven by discrete white noise. The coupled system is used to derive a state estimation scheme that recursively processes noisy measurements of the system. In addition, using the aforementioned dynamic model as a reference (truth) model, the authors develop a covariance analysis to measure the estimation errors that occur when the dynamics along the path through the field are modeled as a Markov linear model and state estimation is performed using discrete Kalman filtering. The performance evaluation of an inertial navigation system influenced by the Earth's gravity field aboard a maneuvering ship is provided as a specific illustrative example. 相似文献
404.
405.
Lee C.Q. Liu R. Batarseh I. 《IEEE transactions on aerospace and electronic systems》1989,25(6):844-847
It is shown that by using a proper transformation of state variables, the third-order system of the parallel resonant converter (PRC) with LLC -type commutation can be analyzed by means of a two-dimensional state-plane diagram. A set of characteristic curves which can be used for the converter design is derived from the analysis. It is shown from these curves that the converter possesses more desirable features than the conventional PRC 相似文献
406.
Lee F.C. Carter R.A. Fang Z.D. 《IEEE transactions on aerospace and electronic systems》1983,(2):274-287
The stability and dynamic performances of a buck/boost regulator employing a current-injected control are examined. Small-signal models for the power state, the multiloop error processor, and the duty-cycle pulse modulator are developed. The error-processor model which incorporates the current-injected loop, the dc loop, and the compensation network permits evaluation of the effects of each individual control loop and their combined efforts toward shaping the performance characteristics of the closed-loop system. Comparisons are made between this modeling approach and earlier approaches. Some important yet subtle dissimilarities are discussed. This model predicts the constant-frequency 50-percent duty-cycle instability which is inherent to the current-injected control. 相似文献
407.
基于光滑粒子流体动力学方法(Smoothed Particle Hydrodynamics,SPH),开展了SPH新算法在蒸发燃烧领域的研究。建立了适用于SPH方法的蒸发数值模型,推导了基于傅立叶热传导公式和菲克扩散定律的SPH离散方程;借鉴VOF方法(Volume of Fluid)的思想,提出了SPH粒子的液相质量分数的概念,以有效表征蒸发过程中的相变问题。采用SPH方法对高温环境中单个液滴的蒸发过程进行数值模拟,结果符合D 2 ![]()
![]()
定律,与理论模型相一致;在强迫对流环境中,液滴的蒸发过程受到对流作用及表面张力的影响,蒸发速率加快;进一步对双液滴在静止、对流环境中的蒸发过程进行数值模拟研究。结果表明,液滴的间距、滴径对多个液滴的蒸发过程影响至关重要,液滴间距至少在两倍的液滴直径以上,相互之间的影响才可以近似忽略。通过本文研究,拓宽了SPH方法在蒸发相变领域的应用范围,研究结果也能够为进一步的燃烧问题研究奠定基础。 相似文献
408.
Multipath-adaptive GPS/INS receiver 总被引:2,自引:0,他引:2
Erickson J.W. Maybeck P.S. Raquet J.F. 《IEEE transactions on aerospace and electronic systems》2005,41(2):645-657
Multipath interference is one of the contributing sources of errors in precise global positioning system (GPS) position determination. This paper identifies key parameters of a multipath signal, focusing on estimating them accurately in order to mitigate multipath effects. Multiple model adaptive estimation (MMAE) techniques are applied to an inertial navigation system (INS)-coupled GPS receiver, based on a federated (distributed) Kalman filter design, to estimate the desired multipath parameters. The system configuration is one in which a GPS receiver and an INS are integrated together at the level of the in-phase and quadrature phase (I and Q) signals, rather than at the level of pseudo-range signals or navigation solutions. The system model of the MMAE is presented and the elemental Kalman filter design is examined. Different parameter search spaces are examined for accurate multipath parameter identification. The resulting GPS/INS receiver designs are validated through computer simulation of a user receiving signals from GPS satellites with multipath signal interference present The designed adaptive receiver provides pseudo-range estimates that are corrected for the effects of multipath interference, resulting in an integrated system that performs well with or without multipath interference present. 相似文献
409.
Space Science Reviews - The two isotopes of nitrogen, 14N and 15N, have relative abundances extremely variable among solar system reservoirs such as planets and their atmospheres, primitive and... 相似文献
410.
How to Climb the Gravity Wall 总被引:2,自引:0,他引:2
Space Science Reviews - What type of gravity satellite mission is required for the time after GRACE and GOCE? Essentially, the variables at our disposal are experiment altitude, compensation of... 相似文献