全文获取类型
收费全文 | 2440篇 |
免费 | 321篇 |
国内免费 | 382篇 |
专业分类
测绘学 | 209篇 |
大气科学 | 286篇 |
地球物理 | 700篇 |
地质学 | 1037篇 |
海洋学 | 343篇 |
天文学 | 126篇 |
综合类 | 162篇 |
自然地理 | 280篇 |
出版年
2024年 | 10篇 |
2023年 | 40篇 |
2022年 | 104篇 |
2021年 | 113篇 |
2020年 | 109篇 |
2019年 | 109篇 |
2018年 | 147篇 |
2017年 | 121篇 |
2016年 | 125篇 |
2015年 | 123篇 |
2014年 | 152篇 |
2013年 | 134篇 |
2012年 | 127篇 |
2011年 | 159篇 |
2010年 | 136篇 |
2009年 | 140篇 |
2008年 | 114篇 |
2007年 | 103篇 |
2006年 | 82篇 |
2005年 | 79篇 |
2004年 | 74篇 |
2003年 | 77篇 |
2002年 | 117篇 |
2001年 | 100篇 |
2000年 | 69篇 |
1999年 | 48篇 |
1998年 | 59篇 |
1997年 | 57篇 |
1996年 | 36篇 |
1995年 | 42篇 |
1994年 | 29篇 |
1993年 | 28篇 |
1992年 | 34篇 |
1991年 | 18篇 |
1990年 | 10篇 |
1989年 | 18篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1973年 | 3篇 |
排序方式: 共有3143条查询结果,搜索用时 31 毫秒
91.
Chung-Liang Chang 《GPS Solutions》2016,20(2):201-213
Pre-processing traditional navigation signals in global navigation satellite system (GNSS) receivers includes the conversion of an analog-to-digital sample and acquisition following the basic principle of Nyquist sampling theory. This condition inevitably increases the system computation time and cost of a modern wideband receiver. In recent years, the compressive sensing (CS) approach has been proven to effectively reduce the number of measurement samples required for digital signal acquisition systems. This method gives new potential to this modern design. In this study, a modified compressive sensing algorithm for the acquisition of a GNSS signal that is contaminated by an interfering signal is presented. The proposed method attempts to combine CS demodulation and the subspace projecting method to enhance GNSS signal acquisition performance with interference present. First, the received signal is sub-sampled and aliased from a compressive sampling process. This operation maintains the restricted isometry property (RIP) condition of the second sampling process using a Toeplitz-structured sensing matrix, which replaces a conventional random sensing matrix. The matrix ensures that distances between desired signals on the set of sparse space are not influenced by the sampling process. Next, the interference is eliminated through the orthogonal feature between the interference signal and the desired signal using the subspace projecting method. This also preserves the RIP of the projecting matrix to ensure that the original structure of the linear projection of the signal is preserved. After this, an iterative least-square method is utilized to recover the correlator output from the reception samples taken by the CS demodulator. In addition, the signal detection performance in the presence of co-channel interference using a CS demodulator is analyzed and evaluated. Finally, the relationships between signal detection probability, compressive factor and signal bandwidth are also illustrated. Several numerical results are presented to verify the theory. 相似文献
92.
The conventional saponification method could result in lower recoveries and artificial changes of long-chain fatty acids. The main reason is the error judgment of the intermediate layer suspended between the aqueous and organic layer during the liquid–liquid extraction process. This study shows that the intermediate layer consists of lots of medium- to long-chain carboxylic salts for their special physical and chemical properties. An improved saponification extraction method is also developed and the results show that the carboxylic salts distributed in the intermediate layer could be obtained completely, which greatly enhances the authenticity and accuracy of fatty acid analysis. Additionally, the possible reasons of formation of the intermediate layer are also discussed. 相似文献
93.
Chunying Chang Fangbai Li Chengshuai Liu Jianfeng Gao Hui Tong Manjia Chen 《中国地球化学学报》2016,35(4):329-339
Soil secondary minerals are important scavengers of rare earth elements (REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs (La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs (LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs (HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs. The substantial fractions of REEs in soils extracted by oxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile (10 %–30 %), which were similar to the mass fraction of Fe (10 %–20 %). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation. 相似文献
94.
95.
96.
97.
98.
99.
100.