全文获取类型
| 收费全文 | 7805篇 |
| 免费 | 39篇 |
| 国内免费 | 18篇 |
专业分类
| 航空 | 3445篇 |
| 航天技术 | 2727篇 |
| 综合类 | 25篇 |
| 航天 | 1665篇 |
出版年
| 2021年 | 81篇 |
| 2019年 | 48篇 |
| 2018年 | 180篇 |
| 2017年 | 133篇 |
| 2016年 | 129篇 |
| 2015年 | 55篇 |
| 2014年 | 201篇 |
| 2013年 | 244篇 |
| 2012年 | 241篇 |
| 2011年 | 362篇 |
| 2010年 | 261篇 |
| 2009年 | 383篇 |
| 2008年 | 433篇 |
| 2007年 | 249篇 |
| 2006年 | 179篇 |
| 2005年 | 216篇 |
| 2004年 | 211篇 |
| 2003年 | 245篇 |
| 2002年 | 174篇 |
| 2001年 | 263篇 |
| 2000年 | 144篇 |
| 1999年 | 176篇 |
| 1998年 | 206篇 |
| 1997年 | 125篇 |
| 1996年 | 191篇 |
| 1995年 | 224篇 |
| 1994年 | 200篇 |
| 1993年 | 132篇 |
| 1992年 | 162篇 |
| 1991年 | 54篇 |
| 1990年 | 58篇 |
| 1989年 | 158篇 |
| 1988年 | 66篇 |
| 1987年 | 66篇 |
| 1986年 | 77篇 |
| 1985年 | 216篇 |
| 1984年 | 168篇 |
| 1983年 | 122篇 |
| 1982年 | 136篇 |
| 1981年 | 238篇 |
| 1980年 | 59篇 |
| 1979年 | 50篇 |
| 1978年 | 54篇 |
| 1977年 | 51篇 |
| 1975年 | 56篇 |
| 1974年 | 48篇 |
| 1973年 | 40篇 |
| 1972年 | 36篇 |
| 1971年 | 38篇 |
| 1970年 | 40篇 |
排序方式： 共有7862条查询结果，搜索用时 15 毫秒
61.
62.
The planetary radio astronomy experiment will measure radio spectra of planetary emissions in the range 1.2 kHz to 40.5 MHz. These emissions result from wave-particle-plasma interactions in the magnetospheres and ionospheres of the planets. At Jupiter, they are strongly modulated by the Galilean satellite Io.As the spacecraft leave the Earth's vicinity, we will observe terrestrial kilometric radiation, and for the first time, determine its polarization (RH and LH power separately). At the giant planets, the source of radio emission at low frequencies is not understood, but will be defined through comparison of the radio emission data with other particles and fields experiments aboard Voyager, as well as with optical data. Since, for Jupiter, as for the Earth, the radio data quite probably relate to particle precipitation, and to magnetic field strength and orientation in the polar ionosphere, we hope to be able to elucidate some characteristics of Jupiter auroras.Together with the plasma wave experiment, and possibly several optical experiments, our data can demonstrate the existence of lightning on the giant planets and on the satellite Titan, should it exist. Finally, the Voyager missions occur near maximum of the sunspot cycle. Solar outburst types can be identified through the radio measurements; when the spacecraft are on the opposite side of the Sun from the Earth we can identify solar flare-related events otherwise invisible on the Earth. 相似文献
63.
In order to stabilize the altitude calculation in an inertial navigation system, an altimeter is commonly used. In a conventional local-level mechanization, this is generally accomplished by correcting the vertical channel integrators with the difference between the inertial system and altimeter indication of vertical position. However, in a space-stable system the procedure is not as clear since a vertical channel is not physically present. Three altitude damping mechanizations for a space-stable inertial navigation system are proposed. The equivalent local-level mechanizations are then found by comparing error propagation equations in a common coordinate frame. 相似文献
64.
In radars that achieve a high subclutter visibility by coherent processing over several pulses, a serious problem appears in the form of blind Dopplers, or ?speeds,? at which target detection is impossible. Of the possible methods of eliminating these blind speeds, the most basic one that is employed when the performance requirements are high involves the use of several PRF's. These PRF's are chosen so that coverage is obtained at any Doppler with at least one PRF. The problem faced by the radar designer is to select the set of PRF's and the pulse numbers for each PRF so that the search frame time is minimized. This paper evolves a systematic method for the design of the blind-speed elimination scheme. A formalized approach is offered that shows the possible combinations of wavelength, PRF, and pulse number and the tradeoffs involved, without introducing the confusion ordinarily associated with multiparameter choices. 相似文献
65.
A technique is presented which simplifies the analysis of sampled-data control systems. The use of a finite-term approximation to the Poisson summation rule, in conjunction with digital computer evaluation, is presented as a good alternative to use of the Z transform whenever frequency-domain analysis is required. The Poisson summation rule is reviewed and its application to sampled-data system analysis is discussed. The form of the computer program used in the analysis is presented, along with a design criterion to evaluate the accuracy of the technique. A detailed example of a typical application is presented. 相似文献
66.
In satellite-to-helicopter communications, interference exists on the incoming signal when the receiving antenna is located below the rotor blades. A bound is established for the performance of a coherent fixed-tone ranging system operating at L band in this interference environment. The scalar diffracted field beneath the rotating blades, at L band and above, is found to satisfy the criterion of Fresnel diffraction, and is computed using the techniques of Fourier optics. The diffracted field is expressed in terms of a narrow-band signal. The amplitude and phase components are calculated from a Fourier Series expansion using the FFT algorithm. The significant harmonics of the phase component of the interference combine with the baseband of the narrow-band, phase-modulated ranging signal. This results in CW interference, and in rearrangement of the first-order, sideband, ranging-tone channel powers. The degradation in ranging accuracy is evaluated by computing the signal-to-interference (SIR) ratio for a set of ranging tones. The post-detection (SIR)PD at the output of the correlator is shown to be a function of the amplitude of the phase harmonics of the interference, the relative difference between the ranging tone and interference center frequencies (a function of rotor speed), the rangetone modulation indices, and the post-detection filter noise bandwidth. 相似文献
67.
An algorithm is described which finds optimum transmitter and receiver weights to maximize clutter suppression in a predetermined clutter region when using burst waveforms. It is assumed that the transmitter weights can only take on values from a finite set. This optimization problem is solved using a branch and bound algorithm. An example is given which shows the improvement in clutter suppression when this new design procedure is used as compared to a simpler nonoptimal procedure. 相似文献
68.
Paulikas G.A. Blake J.B. Imamoto S.S. 《IEEE transactions on aerospace and electronic systems》1975,(6):1138-1144
The Aerospace Corporation energetic electron-proton spectrometer operating on Applications Technology Satellite-6 (ATS-6) detects energetic electrons in four channels between 140 keV and greater than 32 MeV, and measures energetic protons in five energy channels between 2.3 and 80 MeV and energetic alpha particles in three channels between 9.4 and 94 MeV. After more than a year of operation in orbit, the experiment continues to return excellent data on the behavior of energetic magnetospheric electrons as well as information regarding the fluxes of solar protons and alpha particles. 相似文献
69.
A low frequency, small signal model, valid for all types of converters, both in the heavy and the light mode of conduction, is developed in block diagram form. The model provides a clear insight into the behavior of such converters and enhances the respective contributions of input and output filters. 相似文献
70.
R.C Miller S.G Martin W.R Hanson S.A Marino E.J Hall 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(12):1719-1723
The oncogenic potential of high-energy 56Fe particles (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at the Brookhaven National Laboratory was examined utilizing the mouse C3H 10T1/2 cell model. The dose-averaged LET for high-energy 56Fe is estimated to be 143 keV/μm with the exposure conditions used in this study. For 56Fe ions, the maximum relative biological effectiveness (RBEmax) values for cell survival and oncogenic transformation were 7.71 and 16.5 respectively. Compared to 150 keV/μm 4He nuclei, high-energy 56Fe nuclei were significantly less effective in cell killing and oncogenic induction. The prostaglandin E1 analog misoprostol, an effective oncoprotector of C3H 10T1/2 cells exposed to X rays, was evaluated for its potential as a radioprotector of oncogenic transformation with high-energy 56Fe. Exposure of cells to misoprostol did not alter 56Fe cytotoxicity or the rate of 56Fe-induced oncogenic transformation. 相似文献