全文获取类型
收费全文 | 2277篇 |
免费 | 10篇 |
国内免费 | 10篇 |
专业分类
航空 | 1098篇 |
航天技术 | 828篇 |
综合类 | 10篇 |
航天 | 361篇 |
出版年
2021年 | 23篇 |
2018年 | 39篇 |
2017年 | 21篇 |
2016年 | 26篇 |
2014年 | 48篇 |
2013年 | 59篇 |
2012年 | 52篇 |
2011年 | 93篇 |
2010年 | 64篇 |
2009年 | 92篇 |
2008年 | 97篇 |
2007年 | 52篇 |
2006年 | 43篇 |
2005年 | 57篇 |
2004年 | 70篇 |
2003年 | 72篇 |
2002年 | 37篇 |
2001年 | 59篇 |
2000年 | 40篇 |
1999年 | 47篇 |
1998年 | 68篇 |
1997年 | 42篇 |
1996年 | 67篇 |
1995年 | 79篇 |
1994年 | 55篇 |
1993年 | 49篇 |
1992年 | 64篇 |
1991年 | 30篇 |
1990年 | 19篇 |
1989年 | 49篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 85篇 |
1984年 | 55篇 |
1983年 | 54篇 |
1982年 | 60篇 |
1981年 | 74篇 |
1980年 | 21篇 |
1979年 | 26篇 |
1978年 | 26篇 |
1977年 | 26篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1974年 | 20篇 |
1973年 | 11篇 |
1972年 | 14篇 |
1970年 | 18篇 |
1969年 | 19篇 |
1967年 | 10篇 |
排序方式: 共有2297条查询结果,搜索用时 359 毫秒
951.
Adaptive arrays utilizing an internally generated reference signal to drive least mean square (LMS) weight determining loops have experienced difficulty arising from phase shifts induced by the reference generating circuits. The phenomenon observed is that the expected value of the weights oscillate in the steady state modulating the incoming signal. A scheme is reported which avoids this problem. It differs from earlier methods in that the reference generator has no infinite limiter so that the amplitude of the reference is not constant and in that one element is left unweighted. Alternative schemes were considered wherein the reference signal is drawn from all the array elements or from the weighted elements only. Only the latter is fully reported here, and is found superior. It is shown that in the presence of a desired signal and independent element noise, the processing scheme proposed produces weights whose expected values converge to a constant nonoscillatory state provided certain mild constraints are satisfied. In particular, if a cos ? ? 1, a being the gain and ? the phase shift of the filter in the reference generator, the weights converge. In addition, the steady state signal-to-noise power ratio (SNR) is determined. It is found that with a cos ? close to unity the SNR is that of an (N-1) element array coherently combined, where N is the number of elements. The SNR falls off with departures of a and ? from 1 and 0, respectively, but not drastically. 相似文献
952.
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
953.
The problem of designing finite-pulse-train radar signals and receivers to maximize the detectability of targets masked by thermal noise and clutter returns is considered in this paper. A practical constraint is introduced: the amplitude of each subpulse in the transmit waveform is taken to be fixed. The need for such a constraint is dictated in most radar applications, because the transmitter is most efficiently utilized by saturating its amplifying tube. An algorithm for generating optimal waveforms subject to this new constraint is presented, and the performance of the resulting waveforms is compared with those obtained using existing optimization techniques. 相似文献
954.
Interplanetary streams and their interaction with the earth 总被引:1,自引:0,他引:1
L. F. Burlaga 《Space Science Reviews》1975,17(2-4):327-352
Plasma and magnetic field observations of interplanetary streams near 1 AU are summarized. Two types of streams have been identified — corotating streams and flare-associated, and other flow patterns are present due to interactions among streams. The theory of corotating streams, which attributes them to a high temperature region near the Sun, satisfactorily explains many of the effects observed at 1 AU. A correspondingly complete theory of flare-associated streams does not exist. Streams are a key link in the chain that connects solar and geomagnetic activity. The factors that most influence geomagnetic activity are probably related to streams and determined by the dynamics of streams. The evolution of streams on scales of 27 days and 11 years probably determines the corresponding variations of geomagnetic activity. 相似文献
955.
The likelihood functional for estimating parameter differences in coherent multiple-sensor receivers is developed assuming Gaussian statistics on both signal and noise. The development relies on a matrix formulation and a subsequent factorization of a parameter constraint matrix from the signal matrix. A two-antenna phase-difference radar example is presented for cases of uncorrelated and antenna-correlated noise. 相似文献
956.
F. Dahl G. Klein K. Proetel N. Römisch 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(4):131-134
The German Infrared Laboratory GIRL is a liquid helium-cooled telescope with four focal plane instruments dedicated to astronomical and aeronomical observations.Hardware tests were performed with a thermal model of the cryostat and other components as active phase separator, optical switches, main mirror, baffle etc.In the test phase the thermal behavior of the system was checked out in a step by step procedure. The timeline of the individual experiments and of two representative orbits were simulated by electrical heaters. Temperatures and helium flow rates for the different operation modes were measured.An outlook shows that the project phase in 1982 is dedicated to further development and tests of hardware and complete definition and specification of all GIRL systems. 相似文献
957.
O.A. Molchanov O.A. Maltseva E.E. Titova V.I. Di F. Jirˇi´cˇek P. Trˇi´ska 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):227-229
Signals of VLF transmitters of the Omega navigation system located in the auroral zone (66.4°N, 13.2°E, L= 5) were recorded by the VLF receiving equipment of the Interkosmos 19 satellite. Signals at frequencies between 10.2 and 13.6 kHz were received in a region above the transmitters, frequently with whistler-type echoes. An analysis of these echoes has shown their predominating occurrence in periods of low geomagnetic activity (Kp<2+). The occurrence region of these phenomena in the outer ionosphere has the dimension of about 1000 km and its position is betweenL= 2.5 and L= 4.4. The delay of echo-signals is practically the same during one satellite pass but its values for different satellite revolutions lie between 2.5 and 3.5 s. The frequency spectrum of these signals can be broadened up to 100 Hz. On the basis of calculations made, it can be shown that the experimental results are generally in accordance with the hypothesis of nonlinear ducting of VLF waves in the magnetosphere. 相似文献
958.
The effect of spectral bandwidth and spectral shape upon laser binary communication systems is investigated. The probability of error is plotted as a function of bandwidth for Lorentzian and Gaussian shaped spectra. The results show that even when the average noise power and the signal-to-noise ratio are held constant, the probability of error varies considerably as a function of spectral bandwidth. 相似文献
959.
960.
D. J. Lennon Paolo A. Mazzali F. Pasian P. Bonifacio V. Castellani 《Space Science Reviews》1993,66(1-4):169-172
Medium resolution (2A/px) but high s/n spectra of approximately twenty of the brightest blue stars in the young open cluster NGC 330 in the SMC have been analyzed in order to determine their atmospheric parameters and the evolutionary status. Stellar parameters are determined by comparison with LTE and NLTE model atmosphere calculations and an HR diagram constructed. Luminosities of the sample stars lie in the range 4.0L
*/L
)<5.0 and spectral types between O9 and late-B. The stars in our sample appear to define 4 groups: main-sequence B-stars (B2-B4), B-supergiants (B4) in a blue-loop phase of evolution, a small number of blue stragglers (O9-B0 near main-sequence stars) and a group of luminous giants (B1-B2) which reside in the so-called post main-sequence gap of the HR diagram. Furthermore, we have confirmed spectroscopically the very high incidence of Be stars in this cluster. Finally the almost complete absence of metal lines (at this resolution) is in keeping with the expected very low metallicity of the SMC. 相似文献