全文获取类型
收费全文 | 4813篇 |
免费 | 2篇 |
国内免费 | 11篇 |
专业分类
航空 | 2516篇 |
航天技术 | 1549篇 |
综合类 | 181篇 |
航天 | 580篇 |
出版年
2021年 | 29篇 |
2018年 | 68篇 |
2017年 | 36篇 |
2016年 | 40篇 |
2014年 | 72篇 |
2013年 | 99篇 |
2012年 | 106篇 |
2011年 | 158篇 |
2010年 | 101篇 |
2009年 | 182篇 |
2008年 | 191篇 |
2007年 | 114篇 |
2006年 | 87篇 |
2005年 | 84篇 |
2004年 | 114篇 |
2003年 | 139篇 |
2002年 | 179篇 |
2001年 | 198篇 |
2000年 | 93篇 |
1999年 | 123篇 |
1998年 | 153篇 |
1997年 | 99篇 |
1996年 | 138篇 |
1995年 | 168篇 |
1994年 | 146篇 |
1993年 | 95篇 |
1992年 | 122篇 |
1991年 | 57篇 |
1990年 | 63篇 |
1989年 | 125篇 |
1988年 | 58篇 |
1987年 | 60篇 |
1986年 | 51篇 |
1985年 | 152篇 |
1984年 | 120篇 |
1983年 | 108篇 |
1982年 | 117篇 |
1981年 | 153篇 |
1980年 | 56篇 |
1979年 | 46篇 |
1978年 | 57篇 |
1977年 | 37篇 |
1976年 | 40篇 |
1975年 | 48篇 |
1974年 | 39篇 |
1973年 | 25篇 |
1972年 | 49篇 |
1971年 | 47篇 |
1970年 | 30篇 |
1969年 | 33篇 |
排序方式: 共有4826条查询结果,搜索用时 125 毫秒
121.
A new current injected equivalent circuit approach (CIECA) to modeling switching dc-dc converter power stages is developed, which starts with the current injected approach and results in either a set of equations which completely describe input and out-put properties or an equivalent linear circuit model valid at small signal, low frequency levels. This approach to modeling switching dc-dc converter power stages has the merits but not the demerits of both the electronic equivalent circuit state space average approach and the current injected control type approach, namely, 1) the modeling is very clear and is simple whether the converter operates in continuous or discontinuous inductor conduction modes, 2) the modeling results in an equivalent circuit which is very close to the actual converter, and 3) the equivalent circuit can be used directly in the computer for theoretical predictions like SPICE, etc. 相似文献
122.
Large amounts of coronal material are propelled outward into interplanetary space by Coronal Mass Ejections (CMEs). Thus one might expect to find evidence for expanding flux ropes in the solar wind as well. To prove this assumption magnetic clouds were analyzed and correlated with H-observations of disappearing filaments. When clouds were found to be directly associated with a disappearing filament, the magnetic structure of the cloud was compared with that of the associated filament. Additionally the expansion of magnetic clouds was examined over a wide range of the heliosphere and compared with the expansion observed for erupting prominences. 相似文献
123.
R. S. Polidan 《Space Science Reviews》1989,50(1-2):85-94
In this paper we present new results from the Voyager ultraviolet spectrometers and the IUE spacecraft on V356 Sgr and β Lyr. The V356 Sgr observations cover, in detail, two eclipses and include one IUE high dispersion SWP image. During both eclipses the total strength of the UV emission lines were found to be invariant. Also, an uneclipsed UV continuum was detected at wavelengths shorter than 1600 Å. The IUE high dispersion SWP spectrum revealed that the emission lines are extremely broad, almost symmetrical lines with weak, slightly blue shifted absorption components. No evidence of carbon is seen in the emission or absorption spectrum of V356 Sgr in eclipse. A model for the origin of the circumstellar matter in this binary system is presented. The Voyager ultraviolet observations of β Lyr show a strong far-UV continuum that is detectable down to 912 Å The far-UV continuum flux level was variable on time scales shorter than the orbital period and displayed no obvious orbital modulation or eclipses. The spectral shape of the far-UV continuum closely resembles that of a UX UMa type cataclysmic variable. On 16 August 1985 an rapid brightening of the far-UV continuum was observed which was also reminiscent of cataclysmic variables. Analysis of the β Lyr data suggest that the central object must be small, with a radius on the order of 1 R⊙ or less. 相似文献
124.
A statistical technique for amplitude calibration of radar systems is presented. As distinct from the input-output crosscorrelation measurement of a linear system impulse response [1], this technique measures the amplitude transfer function of a memoryless, nonlinear system. A generalized theory is developed and calibration accuracy bounds are derived. Used in the calibration of a modern pulsed radar, the technique is compared with conventional reference pulse calibration. The relative merits of the deterministic and statistical approaches are compared. 相似文献
125.
Error Analysis of Space-Stable Inertial Navigation Systems 总被引:1,自引:0,他引:1
Nash R.A. Levine S.A. Roy K.J. 《IEEE transactions on aerospace and electronic systems》1971,(4):617-629
The error equations for a space-stable inertial navigation system are derived. This is done by directly perturbing the mechanization equations in the inertial frame and then transforming in open-loop fashion to the local-level frame. A rotating inertial platform and velocity and altitude damping are considered. The relations between errors in space-stable and local-level systems are noted. Numerical results are presented for certain random error sources. 相似文献
126.
An optimum filter for direct A/D conversion 总被引:1,自引:0,他引:1
The author describes a design procedure to find the optimum finite-impulse-response (FIR) filter coefficients when the characteristics of the bandpass filter preceding the analog-to-digital (A/D) converter are known. A simulation of direct A/D conversion performance shows the improvement over the filter coefficients designed to minimize the maximum error over a specified band of frequencies. DC cancellation may be added to the process with very little change in the coefficients of the optimum FIR filter or its performance.<> 相似文献
127.
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
128.
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
129.
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
130.
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