全文获取类型
收费全文 | 7615篇 |
免费 | 34篇 |
国内免费 | 14篇 |
专业分类
航空 | 3759篇 |
航天技术 | 2450篇 |
综合类 | 27篇 |
航天 | 1427篇 |
出版年
2021年 | 62篇 |
2019年 | 42篇 |
2018年 | 221篇 |
2017年 | 167篇 |
2016年 | 113篇 |
2015年 | 65篇 |
2014年 | 169篇 |
2013年 | 182篇 |
2012年 | 212篇 |
2011年 | 348篇 |
2010年 | 269篇 |
2009年 | 402篇 |
2008年 | 452篇 |
2007年 | 271篇 |
2006年 | 158篇 |
2005年 | 234篇 |
2004年 | 181篇 |
2003年 | 243篇 |
2002年 | 138篇 |
2001年 | 248篇 |
2000年 | 126篇 |
1999年 | 160篇 |
1998年 | 189篇 |
1997年 | 131篇 |
1996年 | 156篇 |
1995年 | 223篇 |
1994年 | 202篇 |
1993年 | 114篇 |
1992年 | 163篇 |
1991年 | 79篇 |
1990年 | 68篇 |
1989年 | 160篇 |
1988年 | 73篇 |
1987年 | 76篇 |
1986年 | 69篇 |
1985年 | 181篇 |
1984年 | 157篇 |
1983年 | 143篇 |
1982年 | 153篇 |
1981年 | 190篇 |
1980年 | 64篇 |
1979年 | 60篇 |
1978年 | 56篇 |
1977年 | 45篇 |
1976年 | 44篇 |
1975年 | 55篇 |
1974年 | 45篇 |
1973年 | 40篇 |
1972年 | 52篇 |
1971年 | 38篇 |
排序方式: 共有7663条查询结果,搜索用时 437 毫秒
761.
An automatic data processing system for the evaluation of statistically occurring Doppler-difference bursts is described. In addition to the higher accuracy compared with the data acquisition procedures known so far, this technique allows the Doppler-difference method to be used as an operational instrument for investigations into turbulent flows, especially in the case of low particle density and extremely small turbulence degrees. 相似文献
762.
Aya Bamba Hiroya Yamaguchi Katsuji Koyama Junko S. Hiraga Steve Holt John P. Hughes Hideaki Katagiri Jun Kataoka Satoru Katsuda Shunji Kitamoto Motohide Kokubun Hironori Matsumoto Emi Miyata Koji Mori Hiroshi Nakajima Masanobu Ozaki Rob Petre Akiko Sekiguchi Tadayuki Takahashi Takaaki Tanaka Yukikatsu Terada Hiroshi Tomida Yohko Tsuboi Masahiro Tsujimoto Hiroshi Tsunemi Yasunobu Uchiyama Masaru Ueno Shin Watanabe 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(3):411-415
SN 1006 is one of the supernova remnants (SNRs) with relatively low-temperature electrons, considering the young age of just 1000 years. We carried out SN 1006 mapping observations with the X-ray Imaging Spectrometers (XIS) and the Hard X-ray Detector (HXD) onboard Suzaku, the fifth Japanese X-ray satellite. Thanks to the excellent spectral resolution of XIS in the soft X-ray band, H-like and He-like oxygen emission lines were clearly detected, and we could make a map of the line intensity, and as well as a flux and the photon index of nonthermal component. We found that these parameters have spatial dependences from region to region in the SNR; the north region is bright in nonthermal, while dim in thermal; the east region is bright in both nonthermal and thermal; the inner region shows dim nonthermal and bright thermal emission. The photon index is the smallest in the north region. 相似文献
763.
The FAA's Flight 2000 project is an innovative initiative to implement and validate selected operational improvements leading to Free Flight. It integrates new avionics, new ground systems, new procedures, avionics certification, and operational approval. Approximately 2000 aircraft operating in Alaska, Hawaii, and Oceanic airspace will be equipped with new capabilities enabling benefits such as improved situational awareness, increased flexibility, and efficiency gains. By developing and fielding these operational improvements, the actual benefits of new procedures and capabilities will be validated and risks associated with national implementation will be reduced 相似文献
764.
V.E. Zuev S.I. Kavkyanov G.M. Krekov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(6):81-85
A single-frequency lidar, using aerosol scattering as an informative component, is the simplest and reliable facility for remote sensing of the atmosphere. The information on vertical distribution of atmospheric aerosol which can be obtained using such a lidar is necessary for investigating the physics of atmospheric processes and forecast of optical state of the atmosphere. At the same time, the interpretation of data on single-frequency sounding is associated with some difficulties of fundamental character, mainly due to insufficient software of the experiment. Under such conditions the problems of optimal processing of lidar returns aiming at extracting the useful information on aerosol are of great importance, especially if one takes into account the hindering effects of atmospheric background and optical noises. This paper presents a statistical approach to this problem, and the possibilities of single-frequency sounding from space are analyzed. 相似文献
765.
G.J. Nagy D.N. Severov V.A. Pshennikov M. De los Santos J.J. Lagomarsino K. Sans E.G. Morozov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(11):1876-1881
This paper deals with the application of SeaWIFS images to characterize spatial and temporal variability of fronts in the Rio de la Plata estuarine system over the period 2000–2003. We aim to depict the relationship between river outflow and variability of fronts’ loci on monthly to ENSO-related timescales and the influence of the winds along Rio de la Plata (axial winds) on the abrupt changes in frontal dynamics over synoptic timescales. During the studied period both La Niña (July 1999–June 2000) and El Niño (April 2002–May 2003) events induced significant displacements of fronts. Three distinct fronts were analyzed between river, estuarine, coastal and marine waters of the Rio de la Plata: Main Turbidity Front, Main Marine Front, and Secondary Marine Front. Their number, location and separation seem to be mainly related to river outflow and second, to fresh (>8 m/s) axial winds. During low discharge periods (i.e. summer time and/or La Niña events) these winds induce abrupt changes in the location of fronts (100–200 km) and greater separation between them over synoptic timescale, whereas during high river discharge or ENSO years some of the variability of fronts location is explained by the river’s outflow fluctuations, especially by the high variability of the River Uruguay discharge. 相似文献
766.
Benchmark for radar allocation and tracking in ECM 总被引:4,自引:0,他引:4
767.
Estimates of drag characteristics of the space vehicles with orbit heights of 450–540 and 700–900 km before and after strong
(with a magnitude M ≥ 6.5) crust earthquakes of 2000–2006 are presented. The method of estimation of seismic orbital effects is presented using
as an example the small Mozhaets-4 spacecraft. Two weeks prior to earthquakes, variations in the drag of low-orbital spacecraft increase. 3–6 days prior to
strong crust earthquakes with epicenters on the land, the drag of low-orbit spacecraft in the upper atmosphere increases.
The effect of increased viscosity of the neutral component of the atmosphere at spacecraft heights 3–6 days prior to strong
crust earthquakes is consistent with the results of studies of disturbances in the ionization density variations in the ionospheric
F region prior to earthquakes. No anomalies are found in the day of the earthquake. In the future, it is proposed to use elements
of space debris for diagnostics of seismic orbital effects and disturbances of the upper atmosphere. 相似文献
768.
M.B. Moldwin D. Fiello E. Harter G. Holman N. Nagumo A. Pryharski C. Takunaga 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
An elementary science education professional development partnership between Culver City Unified School District teachers and UCLA has been formed. The project was designed to assist teachers to comfortably present introductory space science concepts, to support them in their efforts, and to aid them in encouraging their students to develop inquiry skills related to space sciences. The project encourages teacher use of observational science techniques in their classrooms, the use of NASA solar mission images and enhanced use of astronomical observation to facilitate discovery learning. The integrated approach of the project has fostered collegial learning activities among the participating teachers and offered them opportunities for continued renewal and professional development of teacher competencies in astronomy and space science. The activities used in the classroom were developed by others, classroom tested, and specifically address National Science Education and California Science Content Standards. These activities have been sustained through on-going collaboration between the scientist and the teachers, a summer Research Experience for Teachers program, and on-going, grade-specific, district-sponsored workshops. Assessment of the value of the program is done by the school district and is used to continuously improve each workshop and program component. Culver City (California) Unified School District is a small urban school district located on the Westside of Los Angeles. This paper describes the program and the plans for incorporating IHY-themed science into the classroom. 相似文献
769.
S. Guo X. LiuW. Ai Y. TangJ. Zhu X. WangM. Wei L. QinY. Yang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(5):736-741
Based on a formerly developed ground-based prototype of space plant-growing facility, the development of its improved prototype has been finished, so as to make its operating principle better adapt to the space microgravity environment. According to the developing experience of its first generation prototype and detailed demonstration and design of technique plan, its blueprint design and machining of related components, whole facility installment, debugging and trial operations were all done gradually. Its growing chamber contains a volume of about 0.5 m3 and a growing area of approximate 0.5 m2; the atmospheric environmental parameters in the growing chamber and water content in the growing media were controlled totally and effectively; lighting source is a combination of both red and blue light emitting diodes (LED). The following demonstrating results showed that the entire system design of the prototype is reasonable and its operating principle can nearly meet the requirements of space microgravity environment. Therefore, our plant-growing technique in space was advanced further, which laid an important foundation for next development of the space plant-growing facility and plant-cultivating experimental research in space microgravity condition. 相似文献
770.
S.P. Teng Y.K. Chen K.S. Cheng H.C. Lo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(11):1744-1754
Remote sensing images and technologies have been widely applied to environmental monitoring, in particular landuse/landcover classification and change detection. However, the uncertainties involved in such applications have not been fully addressed. In this paper two hypothesis-test-based change detection methods, namely the bivariate joint distribution method and the conditional distribution method, are proposed to tackle the uncertainties in change detection by making decisions based on the desired level of significance. Both methods require a data set of class-dependent no-change pixels to form the basis for class-dependent hypothesis test. Using an exemplar study area in central Taiwan, performance of the proposed methods are shown to be significantly superior to two other commonly applied methods (the post-classification comparison and the image differencing methods) in terms of the overall change detection accuracies. The conditional distribution method takes into consideration the correlation between digital numbers of the pre- and post-images and the effect of the known pre-image digital number on the range of the post-image digital number, and therefore yields the highest change detection accuracy. It is also demonstrated that the class-dependent change detection is crucial for accurate landuse/landcover change detection. 相似文献