全文获取类型
收费全文 | 8882篇 |
免费 | 64篇 |
国内免费 | 12篇 |
专业分类
航空 | 4171篇 |
航天技术 | 3052篇 |
综合类 | 31篇 |
航天 | 1704篇 |
出版年
2021年 | 88篇 |
2019年 | 60篇 |
2018年 | 183篇 |
2017年 | 123篇 |
2016年 | 128篇 |
2015年 | 63篇 |
2014年 | 213篇 |
2013年 | 257篇 |
2012年 | 253篇 |
2011年 | 373篇 |
2010年 | 275篇 |
2009年 | 401篇 |
2008年 | 452篇 |
2007年 | 266篇 |
2006年 | 199篇 |
2005年 | 237篇 |
2004年 | 229篇 |
2003年 | 278篇 |
2002年 | 189篇 |
2001年 | 300篇 |
2000年 | 172篇 |
1999年 | 211篇 |
1998年 | 246篇 |
1997年 | 151篇 |
1996年 | 229篇 |
1995年 | 274篇 |
1994年 | 260篇 |
1993年 | 150篇 |
1992年 | 209篇 |
1991年 | 75篇 |
1990年 | 79篇 |
1989年 | 195篇 |
1988年 | 86篇 |
1987年 | 77篇 |
1986年 | 89篇 |
1985年 | 246篇 |
1984年 | 196篇 |
1983年 | 164篇 |
1982年 | 176篇 |
1981年 | 263篇 |
1980年 | 71篇 |
1979年 | 64篇 |
1978年 | 69篇 |
1977年 | 60篇 |
1975年 | 80篇 |
1974年 | 58篇 |
1973年 | 46篇 |
1972年 | 61篇 |
1971年 | 51篇 |
1970年 | 50篇 |
排序方式: 共有8958条查询结果,搜索用时 15 毫秒
41.
T.E. Moore M.O. Chandler M.-C. Fok B.L. Giles D.C. Delcourt J.L. Horwitz C.J. Pollock 《Space Science Reviews》2001,95(1-2):555-568
The discovery of terrestrial O+ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres. 相似文献
42.
The motion of a spacecraft with small asymmetry relative to its center of mass is considered. The restoring aerodynamic moment of the spacecraft is described by the Fourier series in terms of the angle of attack with the two first sinusoidal and the first cosinusoidal terms. A solution for the angle of attack in the undisturbed rotational motion is found. The analytical expression is obtained for the integral of action taken along the separatrices that separate the rotational and oscillatory regions of the phase portrait of a system. The transition of the spacecraft's motion from planar rotational to oscillatory is investigated. This transition is caused by a slow variation of moment characteristic coefficients, as well as by the presence of small asymmetry and damping and slow variation of their coefficients. Analytical formulas are obtained for determining the times of transition from rotational to oscillatory motion, as well as for the critical angular velocity of beyond-the-atmosphere rotation. When this critical velocity is exceeded, body rotation proceeds for a long time interval (planar autorotation arises). 相似文献
43.
Energetic particle observations in the interplanetary medium provide fundamental information about the origin, development
and structure of coronal mass ejections. This paper reviews the status of our understanding of the ways in which particles
are energised at the Sun in association with CMEs. This understanding will remain incomplete until the relationship between
CMEs and flares is determined and we know the topology of the associated magnetic fields. The paper also discusses the characteristics
of interplanetary CMEs that may be probed using particle observations. 相似文献
44.
A numerical method is developed for the calculation of steady-state FM distortion in a linear passive network from the character of the FM signal input and the steady-state transfer characteristics of the network. The method of calculating FM distortion requires the solution of a finite set of linear equations which is accomplished readily by a digital computer. To illustrate the use of the method, the FM distortion introduced by a Chebyshev-response bandpass filter is calculated for a range of network and input signal parameters. 相似文献
45.
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
46.
Devajyoti Dutta Sanjay Sharma G.K. Sen B.A.M. Kannan S. Venketswarlu R.M. Gairola J. Das G. Viswanathan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
By using a Doppler Weather Radar (DWR) at Shriharikota (13.66°N & 80.23°E), an Artificial Neural Network (ANN) based technique is proposed to improve the accuracy of rain intensity estimation. Three spectral moments of a Doppler spectra are utilized as an input data to an ANN. Rain intensity, as measured by the tipping bucket rain gauges around the DWR station, are considered as a target values for the given inputs. Rain intensity as estimated by the developed ANN model is validated by the rain gauges measurements. With the help of a developed technique, reasonable improvement in the estimation of rain intensity is observed. By using the developed technique, root mean square error and bias are reduced in the range of 34–18% and 17–3% respectively, compared to Z–R approach. 相似文献
47.
S. Yu. Mikheev I. I. Shkarban Y. A. Ryzhov S. A. Khartov M. M. Gorshkov R. P. Markov 《Russian Aeronautics (Iz VUZ)》2012,55(1):108-112
A process of manufacture of porous membranes made of aluminum foil (Al 99.99%) by the method of electrochemical anodic oxidation followed by a continuous barrier layer removal with ion-plasma irradiation is described. The membranes having a 8 mm diameter made from AL2O3 with a thickness from 2 to 5 microns thick with a regular channel arrangement are obtained. A possibility to change the channel diameter in the range of 20?C100 nm by adjusting voltage and temperature during the oxidation is shown. We present the experimental dependencies of aluminum and aluminum oxide sputtering coefficients on the energy and incidence angle of xenon ions within the energy range of 100??400 eV. 相似文献
48.
This paper discusses and evaluates two innovative SETI education programs conducted at the University of Western Sydney, viz: the SETI Pathways Program and the Life in the Universe Curriculum Project. 相似文献
49.
M. Naim A. Ali Pacha C. Serief 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(7):2077-2103
With the rapid growth of the number of Earth observation satellite (EOS) supporting critical applications, it is required to improve the security techniques to protect the sensitive data and images during the transmission between the satellites and the ground stations. This paper introduces a new satellite image encryption algorithm based on the Linear Feedback Shift Register (LFSR) generator, SHA 512 hash function, hyperchaotic systems, and Josephus problem. LFSR generates a matrix that is used to construct the 512-bits value of the hash function. These bits are used to set the initial values and parameters of the proposed encryption algorithm. Firstly, the six dimensions (6-D) hyperchaotic system is divided into three parts, where every two equations are considered as one part. Secondly, the 1-D hyperchaotic logistic-tent system is considered as the controller to select one part. The selected part is used to generate a matrix that is XORed with the original image. Thirdly, the scrambling operation by Josephus sequences is applied to the output of the previous step by scrambling the rows and the columns according to the selected part to produce the pre-encrypted image. Finally, if the number of iterations is less than the required number which is considered as a parameter of the secret key, the previous operations will be repeated in the pre-encrypted image; otherwise, the pre-encrypted image is considered as the final cipher image. Experimental and analyses results show that the proposed algorithm has good performance in terms of high level of security, large enough key-space, tolerance to Single Event Upsets (SEU) as well as low time complexity. 相似文献
50.
Howard G. Levine A.D. Krikorian 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
We present results on the analysis of 100 mL medium samples extracted from sterilized foam (Smithers-Oasis, Kent OH) used to support the growth of a representative dicotyledon (Haplopappus gracilis) and a representative monocotyledon (Hemerocallis cv Autumn Blaze) in NASA’s Plant Growth Unit (PGU) during a 5-day Space Shuttle flight and ground experiments. At recovery, the media remaining within replicate (n = 5) foam blocks (for both the spaceflight and ground experiments) were extracted under vacuum, filtered and subjected to elemental analyses. A unique aspect of this experiment was that all plants were either aseptically-generated tissue culture propagated plantlets or aseptic seedling clones. The design of the PGU facilitated the maintenance of asepsis throughout the mission (confirmed by post-flight microbial sampling) and thus any possible impact of microorganisms on medium composition was eliminated. Concentration levels of some elements remained the same, while some decreased and others increased. There was a significant two-fold difference between the final concentrations of potassium when the Earth-based and microgravity experiments were contrasted. 相似文献