全文获取类型
收费全文 | 2482篇 |
免费 | 7篇 |
国内免费 | 6篇 |
专业分类
航空 | 1220篇 |
航天技术 | 997篇 |
综合类 | 5篇 |
航天 | 273篇 |
出版年
2021年 | 13篇 |
2019年 | 14篇 |
2018年 | 41篇 |
2017年 | 34篇 |
2016年 | 25篇 |
2015年 | 11篇 |
2014年 | 50篇 |
2013年 | 62篇 |
2012年 | 50篇 |
2011年 | 87篇 |
2010年 | 62篇 |
2009年 | 105篇 |
2008年 | 149篇 |
2007年 | 62篇 |
2006年 | 46篇 |
2005年 | 62篇 |
2004年 | 71篇 |
2003年 | 91篇 |
2002年 | 46篇 |
2001年 | 101篇 |
2000年 | 45篇 |
1999年 | 86篇 |
1998年 | 85篇 |
1997年 | 62篇 |
1996年 | 63篇 |
1995年 | 88篇 |
1994年 | 97篇 |
1993年 | 37篇 |
1992年 | 59篇 |
1991年 | 22篇 |
1990年 | 28篇 |
1989年 | 56篇 |
1988年 | 20篇 |
1987年 | 35篇 |
1986年 | 21篇 |
1985年 | 74篇 |
1984年 | 42篇 |
1983年 | 47篇 |
1982年 | 55篇 |
1981年 | 77篇 |
1980年 | 26篇 |
1979年 | 24篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1976年 | 15篇 |
1975年 | 18篇 |
1974年 | 12篇 |
1972年 | 17篇 |
1969年 | 13篇 |
1966年 | 8篇 |
排序方式: 共有2495条查询结果,搜索用时 15 毫秒
541.
T. J. Ponman 《Space Science Reviews》1981,30(1-4):353-360
Evidence for periodicities of several days in the three bright galactic bulge sources GX349+2, GX17+2 and Ser X-1 has been found in Ariel V RMC data collected over 4 years. 相似文献
542.
Data relay satellites are being developed to provide real-time data links between research satellites in low earth orbits and central data acquisition and processing facilities. Frequency assignments for data relay satellite links will be made in bands allocated internationally to the space research service. One of the bands which will be used lies between 14.5 and 15.35 GHz, where the space research service has had a frequency allocation as a secondary service since 1971. During the General World Administrative Radio Conference of the International Telecommunication Union, held in Geneva in 1979, a primary frequency allocation was made in the band 14.5-14.8 GHz to the fixed-satellite service, specifically for use by earth-to-space links of the broadcasting satellite service. The feasibility of shared band operation is evaluated between data relay satellite uplinks and broadcasting-satellite feeder links in the band 14.5-14.8 GHz. Relationships for predicting interference power levels are formulated, as functions of satellite separation and of earth station separation. Tradeoffs between satellite separation angle and earth station separation are explored, and conclusions are drawn regarding the feasibility of band sharing. Co-channel operation is demonstrated to be technically feasible for typical systems, provided appropriate separations are maintained. 相似文献
543.
Hernandez M.L. Kirubarajan T. Bar-Shalom Y. 《IEEE transactions on aerospace and electronic systems》2004,40(2):399-416
The development of a general framework for the systematic management of multiple sensors in target tracking in the presence of clutter is described. The basis of the technique is to quantify, and subsequently control, the accuracy of target state estimation. The posterior Cramer-Rao lower bound (PCRLB) provides the means of achieving this aim by enabling us to determine a bound on the performance of all unbiased estimators of the unknown target state. The general approach is then to use optimization techniques to control the measurement process in order to achieve accurate target state estimation. We are concerned primarily with the deployment and utilization of limited sensor resources. We also allow for measurement origin uncertainty, with sensor measurements either target-generated or false alarms. An example in which the aim is to track a submarine by deploying a series of constant false-alarm rate passive sonobuoys is presented. We show that by making some standard assumptions, the effect of the measurement origin uncertainty can be expressed as a state-dependent information reduction factor which can be calculated off-line. This enables the Fisher information matrix (FIM) to be calculated quickly, allowing Cramer-Rao bounds to be utilized for real-time, dynamic sensor management. The sensor management framework is shown to determine deployment strategies that enable the target to be accurately localized, and at the same time efficiently utilize the limited sensor resources. 相似文献
544.
A technique of evaluating the micromechanical characteristics (namely, microhardness, microdurability, microbrittleness and
static crack resistance coefficient) using microindentation as applied to plasma zirconia-based thermal barrier coatings is
presented. 相似文献
545.
A functional scheme, primary informative signals, a method of electrode system structural parameters synthesis and evaluation of a systematic error of the ion-marking sensor for aerodynamic angle and airspeed measurement are the focus of this paper. 相似文献
546.
D Leugner T Streibel H Rocher G Reitz W Heinrich 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(4):511-515
Stacks of CR-39 plastic nuclear track detectors were mounted inside the MIR-station during the EUROMIR-94-mission. We present LET-spectra determined separately for long range cosmic ray heavy ions and for short range target fragments produced in nuclear interactions of cosmic rays and measured charge distributions for relativistic and stopping particles. 相似文献
547.
An accurate thermodiffusion model is of paramount importance to the petroleum industry for the prediction of the compositional variation in hydrocarbon reservoirs. Several theoretical models have been developed. Kempers and Firoozabadi models are the latest two models, which are not only applicable for binary mixtures but also for multi-component mixtures. In this paper, we applied the Firoozabadi model to a ternary hydrocarbon mixture of n-Dodecane, n-Butane and Methane with different mass fraction. It reveals that the accuracy of the thermal diffusion coefficients for a specific mixture of interest relies on the accuracy of the thermodynamic properties from equations of state, corresponding Fick's diffusion coefficients, and the thermal diffusion modeling. 相似文献
548.
T. Z. Gimadieva 《Russian Aeronautics (Iz VUZ)》2009,52(1):37-40
A mathematical model of motion for a string of liferafts being water dropped is presented under the assumption of a plane system motion. An example of calculations is given. 相似文献
549.
The unsteady aerodynamic loads (pressure increment and generalized forces) acting on a deformable thin airfoil section are determined for its arbitrary motion in the incompressible flow with transverse gusts. In this case, the exact Küssner solution in series is used for harmonic oscillation of an airfoil, in which the Theodorsen function is approximated by a sum of fractional functions with poles. In the time domain, these functions are replaced by the unknown functions that satisfy the one-type ordinary first order differential equations. The equations obtained are combined with the differential equations of the airfoil motion in the generalized coordinates and are used for analyzing the aeroelastic system stability and calculating its dynamic response to gust loads. 相似文献
550.
O. T. Demkiv E. L. Kordyum O. R. Kardash O. Ya. Khorkavtsiv 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(12):1999-2004
The gravitropism of protonemata of Pohlia nutans is described and compared with that of other mosses. In darkness, protonemata showed negative gravitropism. Under uniform illumination they grew radially over the substrate surface, whereas unilateral illumination induced positive phototropic growth. Gravitropism was coupled with starch synthesis and amyloplast formation. Protonematal gravitropic growth is more variable than the strict negative gravitropism of Ceratodon chloronema. 相似文献