首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6473篇
  免费   4篇
  国内免费   15篇
航空   3290篇
航天技术   2234篇
综合类   186篇
航天   782篇
  2021年   41篇
  2019年   36篇
  2018年   88篇
  2017年   57篇
  2016年   54篇
  2014年   105篇
  2013年   141篇
  2012年   141篇
  2011年   213篇
  2010年   143篇
  2009年   255篇
  2008年   272篇
  2007年   146篇
  2006年   118篇
  2005年   131篇
  2004年   160篇
  2003年   204篇
  2002年   217篇
  2001年   269篇
  2000年   128篇
  1999年   180篇
  1998年   209篇
  1997年   148篇
  1996年   174篇
  1995年   219篇
  1994年   206篇
  1993年   122篇
  1992年   155篇
  1991年   77篇
  1990年   84篇
  1989年   163篇
  1988年   75篇
  1987年   82篇
  1986年   65篇
  1985年   200篇
  1984年   149篇
  1983年   141篇
  1982年   155篇
  1981年   196篇
  1980年   77篇
  1979年   66篇
  1978年   74篇
  1977年   49篇
  1976年   50篇
  1975年   62篇
  1974年   50篇
  1972年   60篇
  1971年   51篇
  1970年   37篇
  1969年   43篇
排序方式: 共有6492条查询结果,搜索用时 15 毫秒
951.
The shape of electron density profile in the International Reference Ionosphere could be improved significantly if the height hg and electron density Ng of the F region sub-peak inflexion point were entered in the set of the profile standard parameters. To study variations of these important parameters, the N(h) analysis of the statistically-summarized ionograms at the latitudes of 40–80°N of the Eastern hemisphere has been carried out for the two-hours intervals of local time, three seasons (winter, equinox and summer) and two levels of solar activities characterized by Covington indicesF10.7 = 100 and 200. It is shown that the parameters of the inflexion point can be expressed in most cases via the peak parameters of the F2 layer ashg= 0.8 hmF2 and Ng= 0.5 NmF2.  相似文献   
952.
953.
The IKS infrared spectro-photometer will fly on board the VEGA platforms. It is designed to characterize the size, temperature and emissivity of the Comet Halley nucleus, to identify the major gaseous components of the inner coma and to detect the emission of the cometary grains. This paper presents the “calibration” experiments required to reduce the raw data: (i) absolute wavelength calibration of the filter wheels; (ii) modeling of the internal signal, as a function of the temperature of the different sub-systems; (iii) absolute and spectral responsivities of each of the spectrometric and photometric channels, as a function of the wavelength and position of the source in the field of view. Finally, we shall indicate the expected S/N ratios.  相似文献   
954.
The bandwidth of adaptive arrays with tapped delay lines behind the elements is examined. Such processing offers improved bandw over that attainable with quadrature hybrid processing. The performance of a two-element array with four types of processing (equarature hybrids, single delay lines, 3-tap delay lines, and 5-tap delay lines) is compared. It is shown that with half-wavelength element spacing, a quadrature hybrid and single delay-line processor are inadequate at 10-percent bandwidth. A 3-tap processor is adeq however, up to 40-percent bandwidth.  相似文献   
955.
High energy, high-Z (HZE) particles are present in high-altitude and high-inclination satellite orbits. Most of the HZE dose above LET = 200 keV/micrometer is due to Fe nuclei. Individual HZE particles can damage several cells adjacent to one another along the particle track in tissue. The outcome has been described as a "microlesion" by D. Grahn. The present study attempts to define conditions for microlesions in specific tissues, to seek biological evidence that microlesions are produced, and to evaluate the microlesion as a potentially useful unit of dose in assessing hazards to spaceworkers. Microlesions in individuals cells and hair follicles have been described. Microbial studies have provided some evidence for independent secondary electron action. Whether or not a few hundred microlesions would be damaging to the whole organism depends upon the nature of damage to critical tissues. For example, cancer may occur if microlesions kill several cells in a straight line and mutate other cells alongside the particle track. Fe particle irradiation of the mouse Harderian gland (Fry et al., this issue) produces tumors efficiently. Microlesions in the lens, cornea, and retina need to be considered. Further dialogue is required before a final decision can be made concerning the most appropriate way to assess the HZE hazard.  相似文献   
956.
Very Large Array (V.L.A.) measurements at 20 cm wavelength map emission from coronal loops with second-of-arc angular resolution at time intervals as short as 3.3 seconds. The total intensity of the 20 cm emission describes the evolution and structure of the hot plasma that is detected by satellite X-ray observations of coronal loops. The circular polarization of the 20 cm emission describes the evolution, strength and structure of the coronal magnetic field. Preburst heating and magnetic changes that precede burst emission on time scales of between 1 and 30 minutes are discussed. Simultaneous 20 cm and soft X-ray observations indicate an electron temperature Te 2.5 × 107 K and electron density Ne 1010 cm?3 during preburst heating in a coronal loop that was also associated with twisting of the entire loop in space. We also discuss the successive triggering of bursts from adjacent coronal loops; highly polarized emission from the legs of loops with large intensity changes over a 32 MHz change in observing frequency; and apparent motions of hot plasma within coronal loops at velocities V > 2,000 kilometerspersecond.  相似文献   
957.
Pre-germinated pine seedlings and germinating oat and mung bean seeds were flown on the STS-3 Space Shuttle mission. Overall, the seedlings grew and developed well in space. Some oat and mung bean roots, however, grew upward. Lignin content was slightly lower in flight tissues and protein content was higher.  相似文献   
958.
A computationally efficient scheme for estimating the digital instantaneous frequencies of narrowband inputs is introduced. The frequency estimates are obtained by searching for minima of the inverse input power spectrum. This spectrum is estimated at each input sample from the weights of an adaptive linear predictor which uses the LMS (least mean square) algorithm to update its weights. The related minima are sought via an iterative search algorithm, referred to as the iterative frequency estimator. This algorithm is computationally more efficient than available methods, and also provides a higher resolution. Simulation results are included; these include tracking of random message sequences in FM signals, and the formant frequency estimation of speech.  相似文献   
959.
Three scientist astronauts on the D1 spacelab mission participated in a series of orientation experiments before and after exposure to orbital weightlessness. Each subject was tilted about a roll axis at 15 deg intervals up to +/-90 deg. At each angle the subject set a luminous line to what he perceived to be the vertical. The results of these tests are presented.  相似文献   
960.
Heart rate, systolic time intervals (pre-ejection period, left ventricular ejection time , ejection friction, stroke volume and QT interval of two cosmonauts (Leonid Popov--L.P. and Dumitru Prunariu--D.P.) were studied before, during, and after an ergometric bicycle exercise test performed before and after the seven-day Soviet-Romanian orbital flight on the Soyuz 40-Salyut 6 Complex in May 1981. For this purpose one precordial electrocardiogram (ecg) and the ear photodensitopram (den) were recorded stimulaneously. The method used permitted recording even during exercise, Ecg and den signals were stored on magnetic tape, processed in an analogue device and in a digital computer. The data obtained after landing suggest a slight cardiac deconditioning in L.P., demonstrated especially by augmentation of the pre-ejection period, which was unchanged in D.P. corresponding to a sympathoadrenergic hypertonia. The seven-day orbital flight has not produced important cardiovascular changes.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号