全文获取类型
收费全文 | 2506篇 |
免费 | 30篇 |
国内免费 | 15篇 |
专业分类
航空 | 1009篇 |
航天技术 | 923篇 |
综合类 | 12篇 |
航天 | 607篇 |
出版年
2022年 | 13篇 |
2021年 | 27篇 |
2019年 | 24篇 |
2018年 | 75篇 |
2017年 | 65篇 |
2016年 | 50篇 |
2015年 | 20篇 |
2014年 | 78篇 |
2013年 | 91篇 |
2012年 | 83篇 |
2011年 | 146篇 |
2010年 | 102篇 |
2009年 | 145篇 |
2008年 | 148篇 |
2007年 | 77篇 |
2006年 | 64篇 |
2005年 | 78篇 |
2004年 | 80篇 |
2003年 | 83篇 |
2002年 | 56篇 |
2001年 | 75篇 |
2000年 | 39篇 |
1999年 | 47篇 |
1998年 | 52篇 |
1997年 | 50篇 |
1996年 | 44篇 |
1995年 | 56篇 |
1994年 | 35篇 |
1993年 | 38篇 |
1992年 | 56篇 |
1991年 | 13篇 |
1990年 | 16篇 |
1989年 | 41篇 |
1988年 | 19篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 60篇 |
1984年 | 51篇 |
1983年 | 41篇 |
1982年 | 41篇 |
1981年 | 57篇 |
1980年 | 34篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 9篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 12篇 |
1971年 | 7篇 |
1969年 | 7篇 |
排序方式: 共有2551条查询结果,搜索用时 68 毫秒
811.
812.
813.
Biswas B.N. Ray S.K. Bhattacharya A.K. Sarkar B.C. Banerjee P. 《IEEE transactions on aerospace and electronic systems》1980,(2):150-158
Transfer characteristics of sinusoidal, triangular, and sawtooth-type phase detectors in response to noisy and noisy fading signals have been studied in detail. A new analytical model of the swatooth-type phase detector has been suggested. Detailed experimental results have been supplemented in support of the theoretical findings. Both the theoretical findings and the experimental results clearly indicate that the superiority of a sawtooth-type phase detector over the other two varieties is completely lost in noisy and noisy fading environments. 相似文献
814.
Urkowitz H. Geisler J.D. Ricciardi N.A. 《IEEE transactions on aerospace and electronic systems》1973,(4):486-494
The Doppler sidelobes of a received pulse burst may be partially controlled by varying the amplitudes of the pulses in the burst or sequence upon transmission and/or reception. When there is a peak power limitation, weighting the amplitudes produces a loss in signal-to-noise ratio. A general expression is derived for the loss factor under the peak power limitation, and loss factor formulas are given for the following cases: Case A: Full nominal weights upon reception. Uniform weights on transmission. Case B: Square roots of the nominal weights on both transmission and reception. Case C: Full nominal weights on both transmission and reception. These cases are listed in order of increasing loss. Numerical results are tabulated for regular spacings and for the following nominal weights: 1) Dolph-Tchebycheff, 2) Taylor, 3) Hamming, and 4) Hann. 相似文献
815.
Morinaga N. Kawamura K. Namekawa T. 《IEEE transactions on aerospace and electronic systems》1973,(3):412-420
Signal interference in the half-wave linear detector has been studied only for amplitude modulation. In this paper, we treat signal interference for both an amplitude-and an angle-modulation communication system. The input to the half-wave linear detector is assumed to be composed of an amplitude-modulated wave, an angle-modulated wave, and narrow-band Gaussian noise. In particular, when strongweak relations exist in the input processes, a detailed analysis for the output component is presented to clarify some useful output properties. 相似文献
816.
The present knowledge of the structure of low-mass X-ray binary systems is reviewed. We examine the orbital period distribution of these sources and discuss how the orbital periods are measured. There is substantial observational evidence that the accretion disks in low-mass X-ray binaries are thick and structured. In a number of highly inclined systems, the compact X-ray emitting star is hidden from direct view by the disk and X-radiation is observed from these only because photons are scattered into the line of sight by material above and below the disk plane. In such systems the X-ray emission can appear extended with respect to the companion star, which can lead to partial X-ray eclipses. There are substantial variations in the thickness of the disk rim with azimuth. These give rise to the phenomenon of irregular dips in the X-ray flux which recur with the orbital period, or to an overall binary modulation of the X-ray flux if the source is extended. The X-ray spectra of low-mass X-ray binaries can be used to probe the innermost emission regions surrounding the compact star. The spectra of the bright Sco X-1 variables can be fitted with two components which are provisionally identified as originating in the inner disk and the boundary layer between the disk and the neutron star respectively. The characteristic energy dependent flaring of the Sco X-1 sub-class may be a geometric effect triggered by an increase in the thickness of the inner disk or boundary layer. The X-ray spectra of the lower luminosity systems, including the bursters, are less complex, and in many cases can be represented by a single power law with, in some sources, a high energy cut-off. Iron line emission is a characteristic of most low-mass X-ray binaries, irrespective of luminosity. 相似文献
817.
A.M. Galper V.G. Kirillov-Ugryumov Yu.D. Kotov L.V. Kurnosova A.V. Kurochkin N.G. Leikov V.I. Logachev L.A. Razorenov Yu.V. Smirnov M.I. Fradkin S.V. Damle P.K. Kunte B.V. Sreekantan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(11):101-106
The report presents some results on the cosmic radiation intensity research carried out with the high-altitude balloons for the period of 1977–1979. The intensity of gamma-radiation with the energy above 40 MeV was measured in two balloon flights at an altitude of 4–7 g/cm2 of residual atmosphere in the vicinity of the geomagnetic equator. A temporal analysis of the intensity to discover fluctuations with periods in the range of 4–60 min was made. Quasi-periodic fluctuations of gamma-radiation intensity with 5 min periods, amplitude ~20% and duration of several hours were discovered. Possible mechanisms of such fluctuation appearance are discussed.The report gives the results of measuring downward, upward and horizontal electron fluxes in the vicinity of the equator. The obtained data and the data provided by satellites are compared. The report discusses the prospects of further joint Soviet-Indian research of cosmic gamma-radiation. 相似文献
818.
The Walsh spectrum of a sinusoid of frequency f = p/2q with p odd consists entirely of lines at orders s that are odd multiples of 2/2q, and the Fourier spectrum of a Walsh function of order 2p/2q consists entirely of lines at frequencies that are odd multiples of 1/2q. For all other frequencies or orders, the spectrum contains no lines, but the power spectral density takes all values in the range [0,oo] in every interval, however short, while being almost everywhere zero. For detecting the presence of a sinusoid by means of Walsh analysis, the time scale should therefore be chosen so that the order of the Walsh function is a power of 2 and the Walsh function is a Rademacher function, i.e., a hard-limited sinusoid of the frequency being sought. 相似文献
819.
Experimental data of Prognoz, Prognoz-2 and HEOS-2 on interplanetary plasma and magnetosheath plasma in the period of unusual solar activity in August 1972 are considered. All measurements showed the extremal plasma properties with strong variations. Attempt is made to identify the structure according to near-Earth observations. The positions of the magnetopause and bow shock suffered significant variations during this time interval. 相似文献
820.
It has been suggested that under hypokinesia (reduced number of steps/day) and intensive physical exercise, the intensification of fluid excretion in men is apparently caused as a result of the inability of the body to retain optimum amounts of water. Thus, to evaluate this hypothesis, studies were performed with the use of fluid and sodium chloride (NaCl) supplements on 12 highly trained physically healthy male volunteers aged 19-24 years under 364 days of hypokinesis (HK) and a set of intensive physical exercises (PE). They were divided into two groups with 6 volunteers per group. The first group of subjects were submitted to HK and took daily fluid and salt supplements in very small doses and the second group of volunteers were subjected to intensive PE and fluid-salt supplements. For the simulation of the hypokinetic effect, both groups of subjects were kept under an average of 4000 steps/day. During the prehypokinetic period of 60 days and under the hypokinetic period of 364 days water consumed and eliminated in urine by the men, water content in blood, plasma volume, rate of glomerular filtration, renal blood flow, osmotic concentration of urine and blood were measured. Under HK, the rate of renal excretion of water increased considerably in both groups. The additional fluid and salt intake failed to normalize water balance adequately under HK and PE. It was concluded that negative water balance evidently resulted not from shortage of water in the diet but from the inability of the body to retain optimum amounts of fluid under HK and a set of intensive PEs. 相似文献