全文获取类型
收费全文 | 2759篇 |
免费 | 6篇 |
国内免费 | 8篇 |
专业分类
航空 | 1277篇 |
航天技术 | 965篇 |
综合类 | 9篇 |
航天 | 522篇 |
出版年
2021年 | 21篇 |
2019年 | 17篇 |
2018年 | 65篇 |
2017年 | 37篇 |
2016年 | 39篇 |
2015年 | 16篇 |
2014年 | 65篇 |
2013年 | 106篇 |
2012年 | 67篇 |
2011年 | 111篇 |
2010年 | 85篇 |
2009年 | 135篇 |
2008年 | 156篇 |
2007年 | 68篇 |
2006年 | 72篇 |
2005年 | 80篇 |
2004年 | 71篇 |
2003年 | 95篇 |
2002年 | 47篇 |
2001年 | 98篇 |
2000年 | 49篇 |
1999年 | 68篇 |
1998年 | 71篇 |
1997年 | 68篇 |
1996年 | 66篇 |
1995年 | 88篇 |
1994年 | 87篇 |
1993年 | 41篇 |
1992年 | 53篇 |
1991年 | 17篇 |
1990年 | 24篇 |
1989年 | 52篇 |
1988年 | 35篇 |
1987年 | 24篇 |
1986年 | 33篇 |
1985年 | 76篇 |
1984年 | 70篇 |
1983年 | 44篇 |
1982年 | 69篇 |
1981年 | 68篇 |
1980年 | 20篇 |
1979年 | 11篇 |
1978年 | 31篇 |
1977年 | 12篇 |
1975年 | 25篇 |
1974年 | 16篇 |
1973年 | 17篇 |
1972年 | 16篇 |
1970年 | 9篇 |
1969年 | 12篇 |
排序方式: 共有2773条查询结果,搜索用时 15 毫秒
631.
Sanderson R.B. Tsui J.B.Y. Freese N.A. 《IEEE transactions on aerospace and electronic systems》1992,28(4):950-956
A method for digital determination of a single frequency from a sine wave sampled at less than the Nyquist rate is discussed. The method makes use of phase shift information to eliminate the aliasing ambiguity. Multiple sampled data are required to eliminate ambiguity completely. Computer modeling using simulated data corrupted by uncorrelated Gaussian noise was used to verify the approach. The results demonstrate that the frequency of an input sine wave can be correctly identified. This approach can be applied to electronic warfare (EW) receivers to increase their input bandwidth 相似文献
632.
633.
DARN/SuperDARN 总被引:1,自引:0,他引:1
634.
The quality of a delivered product or system is ultimately limited by the quality of its design and manufacturing process. Defects that escape the design, manufacturing, and test processes are delivered to customers. Process Potential Index (Cp) and Process Capability Index (Cpk) are now widely used to express design and manufacturing quality. Design quality may be estimated from product models, if they exist, or measured using parameter test data. The difficulty in using test data is not so much in the calculations, but in the recording, storing, and selection of the data to be used in the calculations. This paper defines the metrics of quality estimation and describes an implementation of a system for estimating defect levels and process capabilities. The implementation includes the definition of a standardized parametric data exchange language for test data and an analytical system to standardize product quality measurement 相似文献
635.
K.B. Serafimov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(1):139-142
Methods were suggested in /1,2,3/ to find the maximum electron density NmF (or NmF2) and the) corresponding height hmF (hmF2) in the ionosphere through a combination of measurements on two airglow lines: the UV oxygen line at λ135.6 nm and the red oxygen line near λ630 nm. 相似文献
636.
B.J. Carr J. McDowell H. Sato 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,3(10-12)
We examine whether the IR background reported by Matsumoto (1983) could have been generated by stars or black holes in the pregalactic era, z=10100. 相似文献
637.
M.K. Goel B.C.N. Rao 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(1):111-117
In the present analysis, the mass spectrometer data from the ISS-b satellite, available in the form of contour plots at an average height of 1100 km for every alternate hour, is used. This analysis showed some interesting results in terms of the diurnal variation of the transition heights; at times dominance of He+ over the other ions, and the seasonal variations of different constituents. In the development of ion composition models, it is suggested that this type of result should be taken into consideration. 相似文献
638.
The depth and range of underwater source can be estimated from measurements of propagation delay differences along multiple propagation paths. The accuracy of depth and range estimation using the Cramer-Rao lower bound is studied. The formulas derived can be used in conjunction with a propagation medium (nonconstant velocity profile). Explicit formulas for the bounds are derived for the case of homogeneous medium (constant velocity profile). Numerical examples are presented to illustrate the behavior of these bounds 相似文献
639.
Controlled-root formulation for digital phase-locked loops 总被引:4,自引:0,他引:4
In a new formulation for digital phase-locked loops, loop-filter constants are determined from loop roots that can each be selectively placed in the s-plane on the basis of a new set of parameters, each with single and direct physical meaning in terms of loop noise band, width root-specific decay rate, or root-specific damping. Loops of first to fourth order are treated in the continuous-update approximation (BL T→0) and in a discrete-update formulation with arbitrary B LT. Deficiencies of the continuous-update approximation in large-BLT applications are avoided in the new discrete-update formulation. A new method for direct, transient-free acquisition with third- and fourth-order loops can improve the versatility and reliability of acquisition with such loops 相似文献
640.
V.V Benghin V.M Petrov Yu.V Ivanov V.A Shurshakov S.B Kozlova V.D Nguyen J.F Bottollier M Siegrist 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(12):167-170
The estimation of radiation quality factor in space flights is a serious problem of space dosimetry. The solution of this problem is based on LET-spectra measurements. The “NAUSICAA”-device installed on the “MIR” station has a detector with a large geometric factor, that gives the possibility of measuring LET-spectra with sufficient statistic in relatively short time interval and hence the corresponding Q. LET-spectra are accumulated through 24 hour periods and equivalent dose (H), absorbed dose (D), quality factor and flux (F) are measured every 10 minutes. The obtained data permit the estimate of the diurnal Q and Q in South Atlantic Anomaly. These values vary in the range of 1.4 – 2.2. The analysis of these values including their comparison with the calculated results is given in this paper. 相似文献