全文获取类型
收费全文 | 5607篇 |
免费 | 12篇 |
国内免费 | 13篇 |
专业分类
航空 | 2426篇 |
航天技术 | 1970篇 |
综合类 | 32篇 |
航天 | 1204篇 |
出版年
2021年 | 57篇 |
2019年 | 42篇 |
2018年 | 215篇 |
2017年 | 142篇 |
2016年 | 131篇 |
2015年 | 53篇 |
2014年 | 157篇 |
2013年 | 186篇 |
2012年 | 172篇 |
2011年 | 240篇 |
2010年 | 181篇 |
2009年 | 280篇 |
2008年 | 331篇 |
2007年 | 196篇 |
2006年 | 127篇 |
2005年 | 157篇 |
2004年 | 154篇 |
2003年 | 186篇 |
2002年 | 132篇 |
2001年 | 191篇 |
2000年 | 84篇 |
1999年 | 106篇 |
1998年 | 129篇 |
1997年 | 87篇 |
1996年 | 97篇 |
1995年 | 151篇 |
1994年 | 138篇 |
1993年 | 79篇 |
1992年 | 111篇 |
1991年 | 44篇 |
1990年 | 48篇 |
1989年 | 101篇 |
1988年 | 35篇 |
1987年 | 42篇 |
1986年 | 40篇 |
1985年 | 148篇 |
1984年 | 113篇 |
1983年 | 98篇 |
1982年 | 112篇 |
1981年 | 152篇 |
1980年 | 49篇 |
1979年 | 49篇 |
1978年 | 36篇 |
1977年 | 33篇 |
1976年 | 24篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 26篇 |
1971年 | 21篇 |
排序方式: 共有5632条查询结果,搜索用时 937 毫秒
481.
Cullers DK 《Acta Astronautica》1986,13(1):31-37
Highly monochromatic signals, such as TV carriers, can be detected sensitively by using a narrow filter (b < or = 1 Hz) and performing power accumulation on its output. If instead a low-duty-cycle pulsed signal of the same total energy is present, the sensitivity of a square law device, followed by a thresholding operation (to eliminate most samples containing only noise), followed by the algorithm to be described, is greater by about 7 dB in typical cases. This is particularly interesting to SETI because such a pulsed signal is exactly what is sent by a rotating beacon with a directional antenna. Such a pulsed signal is, therefore, a good candidate for an extraterrestrial beacon. Software for detecting this signal type is now ready for field testing with the NASA Multichannel Spectrum Analyzer (MCSA). 相似文献
482.
The detection of X-rays from Nova Muscae 1983 (discovered on January 18, 1983) constitutes the first detection of X-rays from a classical nova during outburst. X-ray observations were carried out on 1984 April 20 and July 14 when Nova Mus had entered the nebular stage. During both observations no significant flux was observed with the medium energy detectors (2–50 keV). The source was detected with the low energy detector (.04–2 keV) using 3000 Å Lexan and Parlene- N-Aluminium filters; counting rates of (3.4 ± 1.2) × 103 and (3.7 ± 1.2 × 10-3 counts/sec were measured with the respective filters. The source was detected again on July 14 with about the same intensity. Either a shocked shell of circumstellar gas emitting 107 thermal bremsstrahlung at 1035 erg/sec intensity or a white dwarf remnant emitting 3.5 × 105 blackbody radiation at 1037 erg/sec luminosity are compatible with the measurements.Spectra taken in the visual spectral range show strong forbidden coronal emission lines of [FeVII] 6085, [FeX] 6374, and as never observed before in such a strength, [FeXIV] 5303 requiring excitation temperatures of 2 × 106 °K. 相似文献
483.
A.V. Lukyanov 《Acta Astronautica》1981,8(4):349-360
There is a general possibility of creation in space of large controlled mirror reflectors for solar and electromagnetic radiation with specific mass order of 1 g m?2 or less. Such reflectors may be used in space energetics for concentration of solar energy for its further conversion into microwave beam and transmission to the Earth. They can also be used to illuminate the Earth surface in a dark period with reflected sunlight, to control the weather, for research work and some other purposes. Such reflector is a good solar sail. The control of its orientation and position in space is performed using solar energy and light pressure without spending fuel delivered from the Earth. Its form is maintained by centrifugal forces and light pressure. The film strength permits concentrators with radii of several kilometres and nearly flat reflectors for lighting applications with radii of hundreds of metres. Large series of identical reflectors can be built in space using superthin film tape at assembly station. For a year more than a hundred reflectors with a diameter of 600 m can be assembled at such a station. The assembly station can be placed at the height of 1000 km. The reflectors transfer to synchronous or other orbit is performed using their sail-likeness. For realization of such reflectors one should solve a very difficult problem of superthin film mass production as well as assembly technology problems. Careful study and experimental checks of their lifetime should be also made. 相似文献
484.
Analytical and experimental considerations of the large-signal transient responses of the buck-type switching regulator are described. The dynamic behavior under the large-signal condition is different from the small-signal operation because of the saturation characteristics of the pulsewidth modulator feedback controller. The effect of this nonlinearity is analyzed by dividing its operation into three modes. As a result the peak values of the inrush current and output voltage are obtained analytically both for the start-up and for the step change of the load condition. 相似文献
485.
M.B. Vasilyev A.S. Vyshlov M.A. Kolosov A.P. Mesterton N.A. Savich V.A. Samovol L.N. Samoznaev A.I. Sidorenko 《Acta Astronautica》1980,7(3):335-340
The experiment with two-frequency radio occultation measurements of the Venusian nightside ionosphere using the Venera-9 and 10 orbiters is described. The experiment is the first of a kind in which the radio link to one of the spacecraft is used to probe the ionosphere, while the other is measuring the variations in the integral electron number density along the path of radio ray propagation. The experiment has been designed to achieve the maximum possible accuracies in determining the parameters of the probed ionosphere using the bistatic two-frequency radio occultation method. 相似文献
486.
The dynamics of an aircraft following a fixed course line using Loran-C for position fixing are shown to interact with Loran-C receiver dynamics to result in cross-track aircraft positioning errors that are smaller than cross-track Loran receiver errors. In a particular case considered, this error reduction is on the order of 50 percent. 相似文献
487.
488.
The possibility of an ocean within the icy shell of Jupiter's moon Europa has established that world as a primary candidate in the search for extraterrestrial life within our Solar System. This paper evaluates the potential to detect evidence for microbial life by comparing laboratory studies of terrestrial microorganisms with measurements from the Galileo Near Infrared Imaging Spectrometer (NIMS). If the interior of Europa at one time harbored life, some evidence may remain in the surface materials. Examination of laboratory spectra of terrestrial extremophiles measured at cryogenic temperatures reveals distorted, asymmetric nearinfrared absorption features due to water of hydration. The band centers, widths, and shapes of these features closely match those observed in the Europa spectra. These features are strongest in reddish-brown, disrupted terrains such as linea and chaos regions. Narrow spectral features due to amide bonds in the microbe proteins provide a means of constraining the abundances of such materials using the NIMS data. The NIMS data of disrupted terrains exhibit distorted, asymmetric near-infrared absorption features consistent with the presence of water ice, sulfuric acid octahydrate, hydrated salts, and possibly as much as 0.2 mg cm(-3) of carbonaceous material that could be of biological origin. However, inherent noise in the observations and limitations of spectral sampling must be taken into account when discussing these findings. 相似文献
489.
Induced abnormality in Mir- and Earth grown Super Dwarf wheat. 总被引:4,自引:0,他引:4
D L Bubenheim J Stieber W F Campbell F B Salisbury M Levinski V Sytchev I Podolsky L Chernova I Pdolsky 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,31(1):229-234
Super-dwarf wheat grown on the Mir space station using the Svet "Greenhouse" exhibited morphological, metabolic and reproductive abnormalities compared with Earth-grown wheat. Of prominent importance were the abnormalities associated with reproductive ontogeny and the total absence of seed formation on Mir. Changes in the apical meristem associated with transition from the vegetative phase to floral initiation and development of the reproductive spike were all typical of 'Super-Dwarf' wheat up to the point of anthesis. Observation of ruptured anthers from the Mir-grown plants revealed what appeared to be normally developed pollen. These pollen gains, however, contained only one nuclei, while normal viable pollen is tri-nucleate. A potentially important difference in the flight experiment, compared with ground reference studies, was the presence of a high level of atmospheric ethylene (1,200 ppb). Ground studies conducted by exposing 'Super-Dwarf' wheat to ethylene just prior to anthesis resulted in manifestation of the same abnormalities observed in the space flight samples. 相似文献
490.
A new procedure for restoration of the plasma inhomogeneities with improved resolution is suggested. The procedure deals with the double weighted Fourier transform (DWFT) of the observed wavefield in coordinates of both receivers = (x, y) and sources
0 = (x
0, y
0) [1]. Phase increments between the sources and receivers, being found from DWFT representation, can be used for extracting information on small perturbations of the dielectric constant ~(, z) in a way similar to traditional radio tomography. The resulting resolution of the method is close to the diffraction limit = h/D in the horizontal direction and z = (h/D)2 in the vertical direction, where h is the height of inhomogeneities and D is the length of the ground-based receiving system. 相似文献