全文获取类型
收费全文 | 4773篇 |
免费 | 8篇 |
国内免费 | 10篇 |
专业分类
航空 | 1927篇 |
航天技术 | 1496篇 |
综合类 | 20篇 |
航天 | 1348篇 |
出版年
2022年 | 16篇 |
2021年 | 48篇 |
2019年 | 22篇 |
2018年 | 212篇 |
2017年 | 167篇 |
2016年 | 138篇 |
2015年 | 53篇 |
2014年 | 145篇 |
2013年 | 164篇 |
2012年 | 155篇 |
2011年 | 257篇 |
2010年 | 215篇 |
2009年 | 298篇 |
2008年 | 307篇 |
2007年 | 217篇 |
2006年 | 101篇 |
2005年 | 144篇 |
2004年 | 138篇 |
2003年 | 148篇 |
2002年 | 110篇 |
2001年 | 165篇 |
2000年 | 62篇 |
1999年 | 79篇 |
1998年 | 84篇 |
1997年 | 63篇 |
1996年 | 77篇 |
1995年 | 104篇 |
1994年 | 85篇 |
1993年 | 52篇 |
1992年 | 70篇 |
1991年 | 18篇 |
1990年 | 30篇 |
1989年 | 66篇 |
1988年 | 21篇 |
1987年 | 31篇 |
1986年 | 32篇 |
1985年 | 118篇 |
1984年 | 79篇 |
1983年 | 66篇 |
1982年 | 64篇 |
1981年 | 119篇 |
1980年 | 33篇 |
1979年 | 21篇 |
1978年 | 19篇 |
1977年 | 25篇 |
1976年 | 20篇 |
1975年 | 24篇 |
1974年 | 23篇 |
1972年 | 15篇 |
1971年 | 14篇 |
排序方式: 共有4791条查询结果,搜索用时 375 毫秒
771.
772.
Cantrell B.H. Gordon W.B. Trunk G.V. 《IEEE transactions on aerospace and electronic systems》1981,(2):213-221
The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface. 相似文献
773.
Space radiation dosimetry with active detections for the scientific program of the second Bulgarian cosmonaut on board the Mir space station. 总被引:1,自引:0,他引:1
J V Dachev TsPMatviichuk YuN Semkova R T Koleva B Boichev P Baynov N A Kanchev P Lakov P T Ivanov YaJTomo V M Petrov V I Redko V I Kojarinov R Tykva 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(10):247-251
A dosimetry-radiometry system has been developed at the Space Research Institute of the Bulgarian Academy of Science to measure the fluxes and dose rates on the flight of the second Bulgarian cosmonaut. The dosimetry system is designed for monitoring the different space radiations, such as solar cosmic rays, galactic cosmic rays and trapped particles in the earth radiation belts. The system consists of a battery operated small size detector unit and a "read-write" and telemetry microcomputer unit. The sensitivity of the instrument (3.67 x 10(-8) rad/pulse) permits high resolution measurements of the flux and dose rate along the track of the Mir space station. We report our initial results for the period of the flight between the 7th and 17th June 1988. 相似文献
774.
The BL Lac object Mkn 421 was observed by EXOSAT four times over a period of six days in February 1984. Significant X-ray variability was apparent on a timescale of less than a day, but with no accompanying spectral change. The source exhibited a very soft power law X-ray spectrum with an extremely low intrinsic column density (NH1020 cm–2). There was no evidence for an additional hard component attributable to synchrotron self-Compton emission. The observations when combined with other published data imply that significant changes occur in the form of the broad-band UV/X-ray continuum of this source. 相似文献
775.
F. Fárník S. Fischer L. Křivský B. Valníček O.B. Likin V.N. Lutsenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(13):243-246
During the observation of solar cosmic rays on the Prognoz 6 and Helios 1 and 2 spacecrafts, several events with anomalous composition of accelerated particles (higher abundance of 3-He or Fe nuclei) occurred. We found seven such events from the period September to December 1977 for which data from the Prognoz 6 solar X-ray photometer are available. This material together with published optical and radio data from terrestrial observatories enabled us to identify more reliably the source flares and describe their characteristics. It turned out that the character of X-ray emission accompanying the emission of accelerated particles with anomalous composition shows no pronounced difference from other flares. No correlation has been found among the ratio 3-He/4-He and the angular distance between the field lines connected with the source flare and the place of observation. If a solar flare with anomalous ratio 3-He/4-He appears in a given active region, this region will probably produce other anomalous events. 相似文献
776.
Results on radar cross section (RCS) measurements and inverse synthetic aperture radar images of a Mooney 231 aircraft using a ground-to-air measurement system (GTAMS) and a KC-135 airplane using an airborne radar are presented. The Mooney 231 flew in a controlled path in both clockwise and counterclockwise orbits, and successively with the gear down, flaps in the take-off position and with the speed brakes up. The data indicates that RCS pattern measurements from both ground-based and airborne radar of flying aircraft are useful and that the inverse synthetic aperture radar (ISAR) images obtained are valuable for signature diagnostics 相似文献
777.
Several forms of sequential hypothesis testing algorithms are described and their performance as classification algorithms for automatic target recognition is evaluated and compared. Several forms of parameteric algorithms, as well as a sequential form of a useful nonparametric algorithm are considered. The primary focus is the design of algorithms for automatic target recognition that produce maximally reliable decisions while requiring, on the average, a minimum number of backscatter measurements. The tradeoffs between the average number of required measurements and the error performance of the resulting algorithms are compared by means of Monte-Carlo simulation studies 相似文献
778.
Z Masinovsky G I Lozovaya A A Sivash 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(4):199-205
The early evolution of a photocatalytic system of the porphyrin type, able to efficiently collect and utilize solar energy for primary electron transfer is discussed. Experimental results concerning some spectral and photochemical properties of the porphyrins, biosynthetic precursors of chlorophyll and their complexes with polymeric templates are reviewed. Protoporphyrin IX associated with pigmented proteinoid is demonstrated to be a favourable candidate for a role of a photosensitizer of the first photosynthetic reaction centers. The origin and early evolution of the photosynthetic electron transfer chain and of the phosphorylating mechanism are discussed with emphasis on the energetic mechanisms of archaebacteria. 相似文献
779.
P S Haskins J E McKisson A G Weisenberger D W Ely T A Ballard C S Dyer P R Truscott R B Piercey A V Ramayya D C Camp 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(2-3):461-464
The Shuttle Activation Monitor (SAM) experiment was flown on the Space Shuttle Columbia (STS-28) from 8-13 August, 1989 in a 57 degrees, 300 km orbit. One objective of the SAM experiment was to determine the relative effect of different amounts of shielding on the gamma-ray backgrounds measured with similarly configured sodium iodide (NaI) and bismuth germante (BGO) detectors. To achieve this objective twenty-four hours of data were taken with each detector in the middeck of the Shuttle on the ceiling of the airlock (a high-shielding location) as well as on the sleep station wall (a low-shielding location). For the cosmic-ray induced background the results indicate an increased overall count rate in the 0.2 to 10 MeV energy range at the more highly shielded location, while in regions of trapped radiation the low shielding configuration gives higher rates at the low energy end of the spectrum. 相似文献
780.
M P R Waligórski M Hollmark B Lind I Gudowska 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(6):1378-1382
Based on irradiation with 45 MeV/u N and B ions and with Co-60 gamma rays, cellular parameters of Katz's track structure model have been fitted for the survival of V79-379A Chinese hamster lung fibroblasts. Cellular parameters representing neoplastic transformations in C3H10T/1/2 cells after their irradiation with heavy ion beams, taken from earlier work, were also used to model the radiation hazard in deep space, following the system for evaluating, summing and reporting occupational exposures proposed in 1967 by a subcommittee of NCRP. We have performed model calculations of the number of transformations in surviving cells, after a given fluence of heavy charged particles of initial energy 500 MeV/u, penetrating thick layers of cells. We take the product of cell transformation and survival probabilities, calculated along the path lengths of charged particles using cellular survival and transformation parameters, to represent a quantity proportional to the "radiation risk factor" discussed in the NCRP document. The "synergistic" effect of simultaneous charged particle transfers is accounted for by the "track overlap" mode inherent in the model of Katz. 相似文献