全文获取类型
收费全文 | 7494篇 |
免费 | 5篇 |
国内免费 | 19篇 |
专业分类
航空 | 3658篇 |
航天技术 | 2508篇 |
综合类 | 191篇 |
航天 | 1161篇 |
出版年
2021年 | 51篇 |
2018年 | 110篇 |
2017年 | 73篇 |
2016年 | 71篇 |
2014年 | 127篇 |
2013年 | 172篇 |
2012年 | 164篇 |
2011年 | 262篇 |
2010年 | 182篇 |
2009年 | 286篇 |
2008年 | 323篇 |
2007年 | 188篇 |
2006年 | 143篇 |
2005年 | 180篇 |
2004年 | 199篇 |
2003年 | 222篇 |
2002年 | 242篇 |
2001年 | 284篇 |
2000年 | 138篇 |
1999年 | 175篇 |
1998年 | 222篇 |
1997年 | 157篇 |
1996年 | 181篇 |
1995年 | 230篇 |
1994年 | 239篇 |
1993年 | 132篇 |
1992年 | 173篇 |
1991年 | 83篇 |
1990年 | 92篇 |
1989年 | 171篇 |
1988年 | 81篇 |
1987年 | 78篇 |
1986年 | 78篇 |
1985年 | 240篇 |
1984年 | 190篇 |
1983年 | 175篇 |
1982年 | 167篇 |
1981年 | 261篇 |
1980年 | 79篇 |
1979年 | 73篇 |
1978年 | 80篇 |
1977年 | 58篇 |
1976年 | 62篇 |
1975年 | 75篇 |
1974年 | 62篇 |
1973年 | 50篇 |
1972年 | 81篇 |
1971年 | 64篇 |
1970年 | 46篇 |
1969年 | 50篇 |
排序方式: 共有7518条查询结果,搜索用时 15 毫秒
941.
O A Kuznetsov C S Brown H G Levine W C Piastuch M M Sanwo-Lewandowski K H Hasenstein 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,28(4):651-658
The effect of spaceflight on starch development in soybean (Glycine max L., BRIC-03) and potato (Solanum tuberosum, Astroculture-05) was compared with ground controls by biophysical and biochemical measurements. Starch grains from plants from both flights were on average 20-50% smaller in diameter than ground controls. The ratio delta X/delta rho (delta X --difference of magnetic susceptibilities, delta rho--difference of densities between starch and water) of starch grains was ca. 15% and 4% higher for space-grown soybean cotyledons and potato tubers, respectively, than in corresponding ground controls. Since the densities of particles were similar for all samples (1.36 to 1.38 g/cm3), the observed difference in delta X/delta rho was due to different magnetic susceptibilities and indicates modified composition of starch grains. In starch preparations from soybean cotyledons (BRIC-03) subjected to controlled enzymatic degradation with alpha-amylase for 24 hours, 77 +/- 6% of the starch from the flight cotyledons was degraded compared to 58 +/- 12% in ground controls. The amylose content in starch was also higher in space-grown tissues. The good correlation between the amylose content and delta X/delta rho suggests, that the magnetic susceptibility of starch grains is related to their amylose content. Since the seedlings from the BRIC-03 experiment showed elevated post-flight ethylene levels, material from another flight experiment (GENEX) which had normal levels of ethylene was examined and showed no difference to ground controls in size distribution, density, delta X/delta rho and amylose content. Therefore the role of ethylene appears to be more important for changes in starch metabolism than microgravity. 相似文献
942.
The handling of containers in terminals requires, in particular in the application of “chaotic storekeeping”, optimal logistic organization-supported by technical and innovative methods. In this manner, it is possible to guarantee that the existing resources are used at maximal cost and time efficiency. The satellite supported DGPS location system represents a substantial contribution to this problem, which determines the container location during the pick-up and set-down phases in the decimeter range and transmits this information via data telemetry link to the central control office. This allows the realization of a fully automatic administration of the container storage facility, as well as integration into the company's data radio link and terminal control 相似文献
943.
M W Zimmermann K E Gartenbach A R Kranz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):47-51
This article highlights the first results of investigations on the general vitality and damage endpoints caused by cosmic ionizing radiation in dry, dormant plant seeds of the crucifer plant Arabidopsis thaliana (L.) Heynh. and the ascomycete Sordaria fimicola after 69 month stay in space. Wild-type and mutant gene marker lines were included in Free Flyer Biostack containers and exposed on earth and side tray of the LDEF-1 satellite. The damage in biological endpoints observed in the seeds increased in the side tray sample compared to the earth tray sample. For the ascospores we found different effects depending on the biological endpoints investigated for both expositions. 相似文献
944.
M L Griem A Robotewskyj R H Nagel 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):555-563
Cultured endothelial cells of blood vessels have a Do of 2 Gy for X-rays. A dose of 0.5 Gy of X-rays has an acute effect on vessel diameter. The vessels may show other acute effects such as change in permeability including a change in the blood brain barrier. Changes occurring from late effects of chronic exposure in vascular architecture include telangiectasia and decrease in vascular density. Changes in the perivascular connective tissue particularly collagen may play a role in these changes. After charged particle exposure of 15 and 30 Gy, radiation changes in the blood brain barrier and vascular changes are noted in the nervous system. These long term changes are recorded by PET, MRI, and CT imaging. Chronic exposure to alpha particles causes vascular damage in compact bone resulting in bone infarcts. Using tandem scanning confocal microscopy in-situ imaging of the capillaries and collagen of the papillary dermis provides a non-invasive method of serial recording of changes in irradiated microvasculature. 相似文献
945.
R Facius K Scherer G Reitz H Bucker L V Nevzgodina E N Maximova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):93-103
The potentially specific importance of the heavy ions of the galactic cosmic radiation for radiation protection in manned spaceflight continues to stimulate in situ, i.e., spaceflight experiments to investigate their radiobiological properties. Chromosome aberrations as an expression of a direct assault on the genome are of particular interest in view of cancerogenesis being the primary radiation risk for man in space. In such investigations the establishment of the geometrical correlation between heavy ions' trajectories and the location of radiation sensitive biological substructures is an essential task. The overall qualitative and quantitative precision achieved for the identification of particle trajectories in the order of approximately 10 micrometers as well as the contributing sources of uncertainties are discussed. We describe how this was achieved for seeds of Lactuca sativa as biological test organisms, whose location and orientation had to be derived from contact photographies displaying their outlines and those of the holder plates only. The incidence of chromosome aberrations in cells exposed during the COSMOS 1887 (Biosatellite 8) and the COSMOS 2044 (Biosatellite 9) mission was determined for seeds hit by cosmic heavy ions. In those seeds the incidence of both single and multiple chromosome aberrations was enhanced. The results of the Biosatellite 9 experiment, however, are confounded by spaceflight effects unrelated to the passage of heavy ions. 相似文献
946.
Visibility is important for the pilot controlling an aircraft in flight conditions close to the ground, particularly when landing. Therefore, poor visibility yields a great restriction for aircraft operations. Restrictions exist for landing sites which are equipped with facilities providing a landing approach aid like ILS since a minimum is required for visibility. For landing sites providing no approach aids, restrictions are much more severe. This holds even if aircraft are equipped with modern instrumentation and navigation devices. The natural view of the pilot is dependent on various meteorological conditions like darkness, dust, fog, rain etc. The degradation in view caused by these conditions can be compensated for partially or even completely by technical means providing artificial vision cues. Such technical means may be based on radar or optical sensor information. Concepts which employ these techniques are known as “Synthetic Visual Systems” or “Enhanced Visual Systems,” . The present paper is concerned with computer generated vision as a further technique providing visual cues for the pilot. Computer generated vision may be used in combination with the aforementioned sensor based techniques. Thus, it is possible to compensate for limitations which sensor based visual systems have in providing sufficient visibility range or in generating a normal looking image. In addition, computer generated imagery has the potential providing additional information to the pilot for controlling the flight path or for warning purposes. This potential can yield improved and/or more information as compared with the natural view when looking out of the cockpit window 相似文献
947.
Davis G.W. Hodges G.L. Madeka F.C. 《Aerospace and Electronic Systems Magazine, IEEE》1993,8(11):15-20
The design of a feasible hybrid electric vehicle for use in near-term applications is presented. The challenge involved cost effectiveness, acceleration, range, safety, and emissions, which were incorporated into the vehicle design. The relationship of the design goals was studied, and compromises were made to provide near-optimal system design. This process resulted in the selection and design of the major vehicle components. The design decisions and the actual vehicle components are reviewed 相似文献
948.
R D Evans 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(11):367-375
Controlling microorganisms in flowing nutrient solutions involves different techniques when targeting the nutrient solution, hardware surfaces in contact with the solution, or the active root zone. This review presents basic principles and applications of a number of treatment techniques, including disinfection by chemicals, ultrafiltration, ultrasonics, and heat treatment, with emphasis on UV irradiation and ozone treatment. Procedures for control of specific pathogens by nutrient solution conditioning also are reviewed. 相似文献
949.
Viswanath H.C. Jones R.A. 《IEEE transactions on aerospace and electronic systems》1993,29(4):1195-1203
The anomaly in the displacement estimates obtained from a low-level Kalman-based average displacement estimator is used to detect multiple targets in a forward-looking infrared (FLIR) scene. The displacement estimates originating from the targets could be associated with the detected multiple targets and an updated estimated position of the target could be obtained. This procedure could be repeated for all targets in the scene to obtain multitarget tracking. The behavior of the expected value of the displacement estimates as a function of the number of iterations is investigated. The behavior of the displacement error covariance matrix and the Kalman gain matrix are discussed as functions of the number of iterations 相似文献
950.
A Gaussian sum estimation algorithm has previously been developed to deal with noise processes that are non-Gaussian. Inherent in this algorithm is a serious growing memory problem that causes the number of terms in the Gaussian sum to increase exponentially at each iteration. A modified Gaussian sum estimation algorithm using an adaptive filter is developed that avoids the growing memory problem of the previous algorithm while providing effective state estimation. The adaptive filter is comprised of a fixed set of estimators operating in parallel with each individual estimate possessing its own corresponding weighting term. A simulation example illustrates the new non-Gaussian estimation technique 相似文献