全文获取类型
收费全文 | 2277篇 |
免费 | 7篇 |
国内免费 | 3篇 |
专业分类
航空 | 1227篇 |
航天技术 | 831篇 |
综合类 | 13篇 |
航天 | 216篇 |
出版年
2018年 | 25篇 |
2017年 | 23篇 |
2014年 | 24篇 |
2013年 | 48篇 |
2012年 | 28篇 |
2011年 | 59篇 |
2010年 | 47篇 |
2009年 | 57篇 |
2008年 | 126篇 |
2007年 | 38篇 |
2006年 | 32篇 |
2005年 | 46篇 |
2004年 | 69篇 |
2003年 | 72篇 |
2002年 | 37篇 |
2001年 | 55篇 |
2000年 | 49篇 |
1999年 | 27篇 |
1998年 | 75篇 |
1997年 | 53篇 |
1996年 | 62篇 |
1995年 | 69篇 |
1994年 | 85篇 |
1993年 | 51篇 |
1992年 | 69篇 |
1991年 | 29篇 |
1990年 | 33篇 |
1989年 | 70篇 |
1988年 | 24篇 |
1987年 | 29篇 |
1986年 | 51篇 |
1985年 | 87篇 |
1984年 | 54篇 |
1983年 | 60篇 |
1982年 | 57篇 |
1981年 | 68篇 |
1980年 | 34篇 |
1979年 | 27篇 |
1978年 | 25篇 |
1977年 | 24篇 |
1975年 | 24篇 |
1974年 | 24篇 |
1973年 | 24篇 |
1972年 | 21篇 |
1971年 | 31篇 |
1970年 | 17篇 |
1969年 | 25篇 |
1968年 | 18篇 |
1967年 | 20篇 |
1966年 | 19篇 |
排序方式: 共有2287条查询结果,搜索用时 0 毫秒
911.
McCormick W.S. Miller D.E. Tsui J.B.Y. 《IEEE transactions on aerospace and electronic systems》1995,31(1):2-8
An algorithm is proposed to resolve a fundamental 2π ambiguity problem occurring in multiple frequency spectral estimation. Given M frequencies fm, and I separate frequency estimators with unambiguous bandwidths Fi, the ambiguity problem can be stated as solving for the fm, given the estimator outputs, αmi, (1⩽m⩽M;1⩽i⩽I) where fm=αmi+KmiFi and Kmi is some integer. The proposed algorithm exhaustively resolves all possible αmi groupings into single frequency values using a noise insensitive technique that exchanges system bandwidth for noise protection. The correct multiple frequencies are then defined as the single frequencies that repeat a specified number of times. A complete analysis is included 相似文献
912.
Thompson M.W. Ho-Shin Chang 《IEEE transactions on aerospace and electronic systems》1994,30(2):452-461
The discrete-time detection of a time-varying, additive signal in independent Laplace noise is considered. Previous efforts in this area have been restricted to the constant signal, and identically distributed noise case. Theoretical (closed form) expressions for the false alarm and detection probabilities are developed for both the Neyman-Pearson optimal detector and the classical matched filter detector. Comparisons between the two detectors are made which illustrate the effects of signal-to-noise power ratio and sample size for certain false alarm and detection probability constraints. In view of the fact that the optimal Laplace detector is not UMP, we also investigate the effect of signal amplitude mismatch 相似文献
913.
S. W. Falk 《Space Science Reviews》1980,27(3-4):545-553
914.
R.A. West C.W. Hord K.E. Simmons H. Hart L.W. Esposito A.L. Lane R.B. Pomphrey R.B. Morris M. Sato D. Coffeen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(3):45-48
The Voyager 2 photopolarimeter experiment observed the intensity and polarization of scattered sunlight from the atmospheres of Saturn and Titan in the near-UV at 2640 Å and in the near-IR at 7500 Å. Measurements of Saturn's limb brightening and polarization at several phase angles up to 70° indicate that a significant optical depth of UV absorbers are present in the top 100 mbar of Saturn's atmosphere in the Equatorial Zone and north polar region, and possibly at other latitudes as well. UV absorbers are prominent in polar regions, suggesting that charged particle precipitation from the magnetosphere may be important in their formation.The whole-body polarization of Titan is strongly positive in both the UV and near IR. If spherical particles are responsible for the polarization, no single size distribution or refractive index can account for the polarization at both wavelengths. The model atmosphere proposed by Tomasko and Smith [1], characterized by a gradient in particle size with altitude, seems capable of explaining the Voyager observations. If non-spherical particles predominate, the Voyager observations place important constraints on their scattering properties. 相似文献
915.
A.W. Wolfendale 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):23-29
An examination of the evidence favouring the detection of gamma rays from the irradiation of giant molecular clouds by cosmic rays is given. Data relevant to specific nearby clouds are studied in some detail and the evidence favouring some of the so-called gamma ray sources being due to the irradiation of these and more distant clouds is critically assessed. The need for enhancement of cosmic ray intensity in some clouds is discussed. The relevance of this work to interpretation of current and planned X-ray measurements as well as future gamma-ray space missions is discussed. 相似文献
916.
Resolution limits and corresponding optimum linear apertures are determined in the presence of phase errors. Let ?(t) be the phase aberration at position t across the aperture; it is assumed that the random process ? has a power law structure function, E{[(?(t)-?(?)]2}= c|t-?|n. Beam tilting caused by the phase error is "removed" (for each sample of ?), then resolution formulas are developed. An approximate analysis is obtained in closed form and yields an optimum resolution proportional to c1/n for O < n < 2. The exact analysis is given for Gaussian ?, and again the optimum resolution is proportional to c1/n. In applications n= 5/3 is of interest, and in the Gaussian case the best obtainable equivalent rectangle resolution is ? ?)/2? (0.975)c3/5 radians with a corresponding optimum linear aperture of 14c-3/5. When long exposures are considered, imaging without removing beam tilting is of interest, and resolution is degraded by a factor of about 2.5 for a linear aperture. Alternatively, in some applications optimum focus as well as beam tilt should be considered, in this case resolution is improved by a factor of about 1.4 (again for n= 5/3). Finally, joint (tilt corrected) optimization over aperture length and taper is treated; however, as one might expect, the use of taper offers negligible resolution improvement. 相似文献
917.
Albersheim's detection formula is extremely accurate over the usual range of parameters. It is much more convenient to use than lengthy computer programs, and it provides direct insight into tradeoffs in design. Quantitative information on the accuracy bounds of Albersheim's formula is given here. Other contributions made by Albersheim are discussed and some biographical information is provided. 相似文献
918.
N. R. Sheeley Jr. R. A. Howard M. J. Koomen D. J. Michels K. L. Harvey J. W. Harvey 《Space Science Reviews》1982,33(1-2):219-231
This paper presents some of the results that have been obtained from the Kitt Peak observations of coronal holes and the NRL observations of coronal transients during the recent years near sunspot maximum (1979–1981). On the average, low-latitude coronal holes of comparable size contained 3 times more flux near sunspot maximum than near the previous minimum. In the outer corona, transients occurred at the observed rate of at least 2 per day, and quiet conditions persisted during less than 15 % of the observed days. We describe a sample of the more than 800 events that we have observed so far, including the observation of a comet apparently colliding with the Sun.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.Visiting Astronomer, KPNO.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
919.
Detection performance of four candidate receiver structures for the signal known except for direction (SKED) array problem is investigated. Included are the Bayes optimal detector, two estimate and plug structures, and a fixed estimate structure. Estimators considered are the maximum likelihood (ML) and maximum a posteriori (MAP). Performance degradation from optimal for the estimate and plug structures considered is shown to be significantly more severe the larger the array size. 相似文献
920.
R.W.L. Thomas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(5):157-166
A Monte Carlo simulation program has been used to compute the signal contribution of surface elements outside the field of view of a satellite borne sensor in both the visible and infrared spectra for realistic atmospheric models. For azimuthally symmetric reflection the effect of an arbitrary surface albedo pattern can be represented by two spread functions, namely, 1) the ground-satellite spread function, and 2) the ground-ground spread function. Examples of computed spread functions are shown together with their Fourier correspondents, the modulation transfer functions. It has been shown that the central portions of the line spread functions behave as the inverse distance while the point spread functions always possess a region behaving as the inverse square of the radial distance. The median distance is shown to relate to the weighted molecular and aerosol scale height, and for large distances the spread functions become independent of the aerosol loading. 相似文献