全文获取类型
收费全文 | 8276篇 |
免费 | 1313篇 |
国内免费 | 1065篇 |
专业分类
测绘学 | 1718篇 |
大气科学 | 928篇 |
地球物理 | 2227篇 |
地质学 | 2191篇 |
海洋学 | 1095篇 |
天文学 | 467篇 |
综合类 | 743篇 |
自然地理 | 1285篇 |
出版年
2024年 | 20篇 |
2023年 | 82篇 |
2022年 | 247篇 |
2021年 | 266篇 |
2020年 | 311篇 |
2019年 | 357篇 |
2018年 | 255篇 |
2017年 | 356篇 |
2016年 | 350篇 |
2015年 | 373篇 |
2014年 | 427篇 |
2013年 | 488篇 |
2012年 | 571篇 |
2011年 | 522篇 |
2010年 | 445篇 |
2009年 | 492篇 |
2008年 | 472篇 |
2007年 | 534篇 |
2006年 | 515篇 |
2005年 | 436篇 |
2004年 | 386篇 |
2003年 | 359篇 |
2002年 | 277篇 |
2001年 | 275篇 |
2000年 | 240篇 |
1999年 | 235篇 |
1998年 | 264篇 |
1997年 | 144篇 |
1996年 | 153篇 |
1995年 | 142篇 |
1994年 | 134篇 |
1993年 | 138篇 |
1992年 | 85篇 |
1991年 | 63篇 |
1990年 | 43篇 |
1989年 | 62篇 |
1988年 | 35篇 |
1987年 | 32篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
QuickBird影像的正射校正研究 总被引:1,自引:0,他引:1
根据现有的数据特点和项目要求,在Erdas软件中,对Quickbird影像提出了可行的正射校正流程,重点分析有理函数模型的数学推导和算法实现,最终解决在校正过程中遇到的问题。 相似文献
992.
We inquire the phenomena of clustering of galaxies in an expanding universe from a theoretical point of view on the basis
of thermodynamics and correlation functions. The partial differential equation is developed both for the point mass and extended
mass structures of a two-point correlation function by using thermodynamic equations in combination with the equation of state
taking gravitational interaction between particles into consideration. The unique solution physically satisfies a set of boundary
conditions for correlated systems and provides a new insight into the gravitational clustering problem. 相似文献
993.
M. A. Vashkov’yak 《Astronomy Letters》2003,29(6):416-423
A modified method for averaging the perturbing function in Hill’s problem is suggested. The averaging is performed in the revolution period of the satellite over the mean anomaly of its motion with a full allowance for a variation in the position of the perturbing body. At its fixed position, the semimajor axis of the satellite orbit during the revolution of the satellite is constant in view of the evolution equations, while the remaining orbital elements undergo secular and long-period perturbations. Therefore, when the motion of the perturbing body is taken into account, the semimajor axis of the satellite orbit undergoes the strongest perturbations. The suggested approach generalizes the averaging method in which only the linear (in time) term is included in the perturbing function. This method requires no expansion in powers of time. The described method is illustrated by calculating the perturbations of the semimajor axes for two distant satellites of Saturn, S/2000 S 1 and S/2000 S5. An approximate analytic solution is compared with the results of numerical integration of the averaged system of equations of motion for these satellites. 相似文献
994.
针对遥感影像使用过程中,含水印影像经过仿射变换后水印与影像的同步性被破坏,导致水印无法正常检测的问题,本文提出了一种适用于遥感影像的抗仿射变换盲水印算法。首先通过ASIFT算法提取影像具有仿射不变性的特征点,根据特征尺度的大小与特征点间的欧式距离筛选适量特征点,构造对应的正方形特征区域;然后通过计算特征区域的仿射不变矩得出归一化变换函数的参数,对特征区域进行归一化处理,并以归一化特征区域的不变质心为中心提取子区域作为水印嵌入区域,对该区域进行二级离散小波变换,得到水印嵌入区域的低频信息;运用量化嵌入规则将水印嵌入到低频信息中,依次进行低频信息逆小波变换,水印嵌入前后特征区域差值图像反归一化,最后将反归一化差值图像叠加在原始影像特征区域上,完成水印的嵌入。实验选用了3000像素×3000像素的高分二号遥感影像作为载体影像,含版权信息的二值图像作为水印,实验表明:含水印的遥感影像经过旋转、平移、缩放在内的仿射变换后,仍能准确提取水印信息;算法可有效抵抗加噪、滤波、裁剪等常规水印攻击,攻击后影像提取的水印与原水印相关系数均高于0.9;水印具有良好不可感知性;在水印提取时无需原始遥感影像,属于盲水印算法。 相似文献
995.
һ���ɵ���ֱ�����굽��������ֱ��ת�� 总被引:2,自引:2,他引:2
?????????????????????????ι??????????????????????????????γ?????е?4?η???,???????????????γ??????????4?η?????????????????Ч?????????е???????????????????? 相似文献
996.
Peter Colwell 《Celestial Mechanics and Dynamical Astronomy》1991,52(2):203-204
Power series solutions of Kepler's Equation associated with Lagrange, Levi-Civita and Stumpff may be obtained by Newton's method as an algorithm for formal power series. 相似文献
997.
998.
Matthew R. Bate Ian A. Bonnell 《Monthly notices of the Royal Astronomical Society》2005,356(4):1201-1221
We investigate the dependence of stellar properties on the mean thermal Jeans mass in molecular clouds. We compare the results from the two largest hydrodynamical simulations of star formation to resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical except for the radii of the clouds, which are chosen so that the mean densities and mean thermal Jeans masses of the clouds differ by factors of 9 and 3, respectively.
We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system.
Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars. 相似文献
We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system.
Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars. 相似文献
999.
在获得中国区域1000个实测GPS台站速度场的基础上,利用多面函数法、滑动欧拉矢量法和滑动弹塑性模型法建立了中国大陆水平地壳运动模型,统计表明3种方法外部符合精度2~4mm/a,模型互差精度约为3mm/a。 相似文献
1000.
该文详细地介绍了相位校准单元的电路和数学原理以及测试结果。在电路原理部分主要介绍相位校准单元的工作原理、各个关键点的波形以及用隧道二极管产生梳状谱的典型电路。数学原理主要阐述了测量梳状谱的数学依据。最后给出了该相位校准单元的测试要求和测试结果。 相似文献