全文获取类型
收费全文 | 6176篇 |
免费 | 512篇 |
国内免费 | 517篇 |
专业分类
测绘学 | 3499篇 |
大气科学 | 472篇 |
地球物理 | 635篇 |
地质学 | 936篇 |
海洋学 | 512篇 |
天文学 | 341篇 |
综合类 | 400篇 |
自然地理 | 410篇 |
出版年
2024年 | 13篇 |
2023年 | 40篇 |
2022年 | 166篇 |
2021年 | 212篇 |
2020年 | 203篇 |
2019年 | 278篇 |
2018年 | 144篇 |
2017年 | 306篇 |
2016年 | 296篇 |
2015年 | 311篇 |
2014年 | 305篇 |
2013年 | 369篇 |
2012年 | 380篇 |
2011年 | 340篇 |
2010年 | 288篇 |
2009年 | 297篇 |
2008年 | 372篇 |
2007年 | 397篇 |
2006年 | 358篇 |
2005年 | 349篇 |
2004年 | 283篇 |
2003年 | 233篇 |
2002年 | 179篇 |
2001年 | 186篇 |
2000年 | 154篇 |
1999年 | 102篇 |
1998年 | 132篇 |
1997年 | 101篇 |
1996年 | 90篇 |
1995年 | 65篇 |
1994年 | 56篇 |
1993年 | 53篇 |
1992年 | 39篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1984年 | 1篇 |
1982年 | 4篇 |
1978年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有7205条查询结果,搜索用时 31 毫秒
11.
Anastassios E. Petropoulos James M. Longuski 《Celestial Mechanics and Dynamical Astronomy》1994,60(4):431-454
An exact analysis of the coverage obtained by spacecraft using cross-track scanning and nadir-centered conical imaging, under imposed viewing obliqueness and resolution requirements, is presented. In addition to exact expressions for the area acquired and the area acquisition rate, envelope theory is introduced to obtain the boundary of the imaged area. These expressions are relatively compact, allowing rapid machine computation. The effects of the sun phase angle, and of imaging system limitations are also examined. The Galileo mission encounter with Callisto is used as a numerical example, from which certain general conclusions are drawn regarding optimal imaging trajectories. 相似文献
12.
We have analysed short-exposure high-resolution images obtained on the William Herschel Telescope (WHT), Big Telescope Azimuthal (BTA) and CanadaFranceHawaii Telescope (CFHT) in order to investigate mechanical vibrations that are capable of reducing the high angular resolution allowed by speckle-interferometric methods. After filtering the photon noise we have computed power spectra of the image centres of gravity. In these spectra we have found vibration features that differ from one telescope to another and that vary in particular with the zenith angle. In the case of the WHT we discuss these results in terms of possible causes and in terms of degradation of the transfer function. We present some means to improve the data. 相似文献
13.
We present the results of a three-year observational program of long-slit spectroscopy and UBVRI photometry of Triton. We find evidence for short-lived albedo variations at wavelengths less than 0.5 μm with the most notable reddening event measured in October 1997 and “normal” colors returning by May 1998. We report a possible variation in the relative 0.73/0.89 μm methane absorption band strengths, suggesting that the transport or denudation of this species may play a major role in the reddening events. 相似文献
14.
卫星电视授时系统及精度估计 总被引:1,自引:0,他引:1
我国已建成短波、长波授时台,进行毫秒级、微秒级的时间服务.随着现代科技和经济的发展,上述方法已不能满足国家及国际交流的需要.本文提出卫星电视授时系统,同步卫星定位精度小于110m,卫星授时精度为亚微秒。两原子钟利用卫星进行时刻同步,其精度与两原子钟的距离有关,相距3千多公里的两钟同步精度小于35ns,相距数百公里的两原子钟同步精度小于20ns 相似文献
15.
16.
MAO Duan-qian ZHANG Jie BAO Hao-sheng 《中国地理科学(英文版)》2005,15(3):275-280
1IN T R O D U C T IO NAn image isa setofbelief,s ideasand impressionsthatapersonholdsaboutan object(KOTLER,1991).Similar-lydefinitionistheimage of a destinationas thesum ofbelief,sideasand impressionsthatpeoplehave ofa placeor destination(CROMPTON,1979;KO… 相似文献
17.
18.
数字图像压缩方法在天文上的应用 总被引:1,自引:0,他引:1
概述了数字图像压缩技术在天文领域应用的必要性。针对天文观测的特点和研究的需要,经过研究、分析和比较,提出了天文图像压缩的可行方案。通过应用计算机编程及压缩实验并给出相应的结果。 相似文献
19.
《The Professional geographer》1987,39(1):85-94
Reviews of geographic software in this article: DEMO-GRAPHICS: WORLD POPULATIONS AND PROJECTIONS. ESP GAUSS. CEMODEL S. Damus LIMDEP. William H. Greene MICROSTAT 4.1 OTIS PCIPS. (Personal Computer Image Processing System) . H.J. Meyers and R. Bernstein. REGRESSION ANALYSIS OF TIME SERIES (RATS) SPSS/PC+ URBAN DATA MANAGEMENT SOFTWARE (UDMS) 相似文献
20.
Statistical analysis of sand grain/bed collision process recorded by high-speed digital camera 总被引:1,自引:0,他引:1
A high‐speed digital camera was employed to record the sand grain/bed collision process. With image processing and a statistical method, a series of parameters of the collision process were obtained. The results show that the collision process of a grain with rebounding can be represented by two parameters: the kinetic energy restitution coefficient and the collision angle. Both parameters satisfy a normal distribution, and they are dependent on one another. With an increase of the collision angle, the distribution of the kinetic energy restitution gradually reduces from a broad to a narrow range with low values. The percentage of vertical velocity restitution coefficients greater than 1 can reach 70% or more, which ensures that the settling time of the sand grains in the air increases and that they receive more energy from the air to progress the saltation movement. 相似文献