全文获取类型
收费全文 | 8862篇 |
免费 | 1451篇 |
国内免费 | 1957篇 |
专业分类
测绘学 | 785篇 |
大气科学 | 1826篇 |
地球物理 | 2046篇 |
地质学 | 4143篇 |
海洋学 | 1102篇 |
天文学 | 907篇 |
综合类 | 604篇 |
自然地理 | 857篇 |
出版年
2024年 | 28篇 |
2023年 | 84篇 |
2022年 | 288篇 |
2021年 | 353篇 |
2020年 | 314篇 |
2019年 | 334篇 |
2018年 | 379篇 |
2017年 | 372篇 |
2016年 | 421篇 |
2015年 | 403篇 |
2014年 | 463篇 |
2013年 | 558篇 |
2012年 | 520篇 |
2011年 | 586篇 |
2010年 | 538篇 |
2009年 | 584篇 |
2008年 | 526篇 |
2007年 | 503篇 |
2006年 | 496篇 |
2005年 | 424篇 |
2004年 | 364篇 |
2003年 | 366篇 |
2002年 | 429篇 |
2001年 | 381篇 |
2000年 | 308篇 |
1999年 | 267篇 |
1998年 | 219篇 |
1997年 | 219篇 |
1996年 | 180篇 |
1995年 | 170篇 |
1994年 | 162篇 |
1993年 | 134篇 |
1992年 | 94篇 |
1991年 | 90篇 |
1990年 | 80篇 |
1989年 | 68篇 |
1988年 | 53篇 |
1987年 | 48篇 |
1986年 | 39篇 |
1985年 | 40篇 |
1984年 | 51篇 |
1983年 | 40篇 |
1982年 | 27篇 |
1981年 | 30篇 |
1980年 | 18篇 |
1978年 | 22篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1974年 | 26篇 |
1972年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
油田抽油机的抽油杆幌动幅度过大,是引起抽油杆断裂的一个重要因素,检测这种幌动幅度是防止抽油杆断裂的一种有效手段。设计了一种基于面阵CCD和普通半导体激光器(LD)测量这种幌动的悬点投影测量方法,通过数字卷积滤波,达到了范围为0-40mm,误差<0.2mm的技术检测指标。 相似文献
12.
本文依据稳恒磁场中的高斯定理和安培环路定理,用迭代法推导出对称轴上的磁感应强度与空间任意一点磁感应强度的关系式,从而可得出轴对称稳恒磁场的无电流分布空间的解式。 相似文献
13.
本文针对低纬度地区频率域化磁极存在的问题,在频谱分析的基础上,提出了一种新的滤波方法即振幅滤波法。该方法能将低纬度地区化极磁场的频谱恢复到垂直磁化磁场的频谱,大大提高了低纬度地区化磁极的精度。 相似文献
14.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
15.
V. Delouille J. De Patoul J. F. Hochedez L. Jacques J. P. Antoine 《Solar physics》2005,228(1-2):301-321
The extreme ultraviolet imaging telescope (EIT) of SOHO offers a unique record of the solar atmosphere for its sampling in
temperature, field of view, resolution, duration, and cadence. To investigate globally and locally its topology and evolution
during the solar cycle, we consider a multi-scale approach, and more precisely we use the wavelet spectrum.
We present three results among the applications of such a procedure. First, we estimate the typical dimension of the supergranules
as seen in the 30.4 nm passband, and we show that the evolution of the characteristic network scale is almost in phase with
the solar cycle. Second, we build pertinent time series that give the evolution of the signal energy present in the corona
at different scales. We propose a method that detects eruptions and post-flaring activity in EUV image sequences. Third, we
introduce a new way to extract active regions in EIT images, with perspectives in, e.g., long-term irradiance analysis. 相似文献
16.
17.
基于ArcSDE的地理数据入库技术研究 总被引:1,自引:0,他引:1
分析了GIS开发中空间数据的导入原理和技术,提出了一种基于ArcSDE的自建命令,实现了一种简单高效的空间数据入库方法。 相似文献
18.
J. M. De Buizer 《Monthly notices of the Royal Astronomical Society》2003,341(1):277-298
The results of a survey searching for outflows using near-infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide-field near-infrared images of the sites of linearly distributed methanol masers using a narrow-band 2.12-μm filter. This filter is centred on the H2 v = 1–0 S(1) line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H2 emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H2 emission in these fields is not distributed randomly, but instead the majority of sources are found to have H2 emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed. 相似文献
19.
在常规照相天体测量工作中 ,对暗天体的定位是采用逐级定标星过渡的方法实现的。由于星表系统差和局部差的存在 ,最终定位结果可能包含难以把握的误差积累。在CCD小视场观测情况下时常难以找到足够数量的定标星。鉴于此 ,推导了CCD观测联合平差方法的严格矢量表达式 ,以期通过共同星的过渡 ,实现大天区统一平差。原理上此方法可实现相对于河外天体的定位 ,以削弱星表系统差和局部差对定位结果的影响。讨论了CCD联合平差方法实现中的问题 ,包括底片常数模型选择、切点位置改正、共同星的较差改正和法方程解算方法等 ,并分析了具体的处理措施。用实例初步检验了CCD联合平差方法的效果 ,表明通过对多张CCD观测的联合平差 ,可以削弱单张CCD观测参考星数目过少、单张CCD观测参考星分布不均、个别参考星存在位置偏差和局部区域参考星存在系统性位置偏差等不利因素对归算结果的影响 ,进而达到扩大视场和提高归算精度的目的 相似文献
20.
1 INTRODUCTION Pit connection is an important feature in Rhodophyta and significant in algal phylogeny. Various documents have illustrated the existence of pit-connection in red alga previously (Migita, 1967; Ramus 1969a, b; Bourne et al., 1970; Lee and F… 相似文献