全文获取类型
收费全文 | 611篇 |
免费 | 80篇 |
国内免费 | 40篇 |
专业分类
测绘学 | 288篇 |
大气科学 | 14篇 |
地球物理 | 119篇 |
地质学 | 103篇 |
海洋学 | 35篇 |
天文学 | 57篇 |
综合类 | 110篇 |
自然地理 | 5篇 |
出版年
2024年 | 4篇 |
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 27篇 |
2020年 | 32篇 |
2019年 | 28篇 |
2018年 | 23篇 |
2017年 | 43篇 |
2016年 | 43篇 |
2015年 | 41篇 |
2014年 | 38篇 |
2013年 | 40篇 |
2012年 | 54篇 |
2011年 | 48篇 |
2010年 | 39篇 |
2009年 | 41篇 |
2008年 | 30篇 |
2007年 | 40篇 |
2006年 | 22篇 |
2005年 | 15篇 |
2004年 | 19篇 |
2003年 | 10篇 |
2002年 | 17篇 |
2001年 | 2篇 |
2000年 | 12篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1979年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有731条查询结果,搜索用时 14 毫秒
21.
Meredith T. Niles Hannah Waterhouse Robert Parkhurst Eileen L. McLellan Sara Kroopf 《Climate Policy》2019,19(7):893-907
The majority of emissions of nitrous oxide – a potent greenhouse gas (GHG) – are from agricultural sources, particularly nitrogen fertilizer applications. A growing focus on these emission sources has led to the development in the United States of GHG offset protocols that could enable payment to farmers for reducing fertilizer use or implementing other nitrogen management strategies. Despite the development of several protocols, the current regional scope is narrow, adoption by farmers is low, and policy implementation of protocols has a significant time lag. Here we utilize existing research and policy structures to propose an ‘umbrella’ approach for nitrogen management GHG emissions protocols that has the potential to streamline the policy implementation and acceptance of such protocols. We suggest that the umbrella protocol could set forth standard definitions common across multiple protocol options, and then modules could be further developed as scientific evidence advances. Modules could be developed for specific crops, regions, and practices. We identify a policy process that could facilitate this development in concert with emerging scientific research and conclude by acknowledging potential benefits and limitations of the approach.
Key policy insights
Agricultural greenhouse gas market options are growing, but are still underutilized
Streamlining protocol development through an umbrella process could enable quicker development of protocols across new crops, regions, and practices
Effective protocol development must not compromise best available science and should follow a rigorous pathway to ensure appropriate implementation
22.
与其他卫星导航系统不同,北斗卫星导航系统采用星地双向时间比对技术,直接测量卫星钟相对于地面保持的系统时间的钟差,并用于广播电文钟差参数的建模。讨论了电离层延迟误差、卫星相位中心误差等不同误差源对不同类型卫星双向时间同步卫星钟差精度的影响。实测数据分析结果表明,星地双向卫星钟差内符合精度(RMS)优于0.15 ns。利用双向卫星钟差序列,对广播星历钟差参数预报精度进行了分析,统计结果显示广播电文钟差参数预报1 h,精度在2 ns以内,移动卫星刚入境时,钟差参数预报6 h误差可达10 ns。 相似文献
23.
在这篇文章中, 提出了一种基于改进的指数平滑和Vondrak_Cepek联合平滑的氢铯综合时间尺度产生方法. 以最小误差方法为理论基础, 动态估计氢原子钟频率漂移参数, 提升氢原子钟钟差预测准确度; 基于改进的二次指数平滑产生氢原子钟组时间尺度、加权平均方法产生铯原子钟时间尺度, 同时设计Vondrak_Cepek滤波器以结合两类时间尺度长短期稳定度优势, 提升综合时间尺度性能. 实验结果表明, 所提方法产生的氢铯综合时间尺度时稳可达1.60x10-15,天稳可达3x10-15,优于ALGOS、AT1和Kalman滤波3种经典方法产生的时间
尺度性能. 相似文献
24.
分析现阶段伽利略空间信号的精度,并评估伽利略广播星历及其对单点定位的影响。统计30 d健康卫星广播星历,结果表明,FOC和IOV两种类型的广播星历卫星轨道误差优于0.8 m,并且径向误差最小,法向误差次之,切向误差最大;伽利略卫星的钟差误差精度达到1.0 ns;从空间信号测距误差对伽利略卫星进行整体分析,广播星历精度优于1.0 m,FOC与IOV卫星精度相当;用广播星历进行事后单点定位的精度可以达到dm级。 相似文献
25.
提出基于单观测值的Kalman滤波快速计算方法,并引入共享存储并行编程(OpenMP)技术实现协方差快速更新,从而实现非差GPS卫星钟差的快速实时计算。均匀选取55个IGS参考站,计算2017-03-20~03-30采样率为60 s的卫星钟差。与IGS事后30 s钟差相比,两者具有很好的一致性,RMS互差优于0.5 ns。选取未参与钟差解算的10个IGS参考站进行精密单点定位,结果表明,实时静态PPP水平方向精度优于2 cm,高程方向精度为2~4 cm;实时动态PPP水平方向精度为2~4 cm,高程方向精度为4~6 cm,能够满足实时PPP的精度要求。该方法在主频1.2 GHz服务器上8线程并行模式下单历元耗时4 s,相比串行模式效率提升1/3。 相似文献
26.
We describe the microfabrication of 85Rb vapour cells using a glass-silicon anodic bonding technique and in situ chemical reaction between rubidium chloride and barium azide to produce Rb. Under controlled conditions, the pure metallic Rb drops and buffer gases were obtained in the cells with a few mm3 internal volumes during the cell sealing process. At an ambient temperature of 90 ℃ the optical absorption resonance of 85Rb D1 transition with proper broadening and the corresponding coherent population trapping (CPT) resonance, with a signal contrast of 1.5% and linewidth of about 1.7 kHz, have been detected. The sealing quality and the stability of the cells have also been demonstrated experimentally by using the helium leaking detection and the after-9-month optoelectronics measurement which shows a similar CPT signal as its original status. In addition, the physics package of chip-scale atomic clock (CSAC) based on the cell was realized. The measured frequency stability of the physics package can reach to 2.1×10-10 at one second when the cell was heated to 100 ℃ which proved that the cell has the quality to be used in portable and battery-operated devices. 相似文献
27.
GPS�����Ӳ�Ĺ�����Ԥ���о� 总被引:3,自引:1,他引:2
???GPS???????????????????ù??????????????????????????GPS?????????????????????????????????????????????????????????????????????????????????ó?Block IIR??M Rb???????????????Block IIR Rb?????Block IIR Rb???????????????Block IIA Rb???Cs?????????????????????Rb???2~3????????????????????????????????????????????????Rb?????1????????????1.9ns????????????С??????????? 相似文献
28.
29.
I. Lerche 《Pure and Applied Geophysics》1994,143(4):537-562
Equations have been presented previously which predict that reflector curvature can affect significantly seismic reflection amplitudes at both zero and nonzero source-receiver offsets. Here the fact that faults are generally concave-upward is used to examine the curvature effect for compaction-driven faults which the sediments have both exponential and logarithmic porosity decreases with increasing depth.The curvature effect is generally larger over upwardly concave reflectors where amplitudes can either increase with offset (exposed focus) or decrease with offset (buried focus). The magnitude of the effect depends on the ratio between the depth to the structure and the radius of curvature of the structure. A phase change of 90° also occurs at a critical offset in the case of an exposed focus concave-upward reflector, with decreasing amplitude at offsets larger than the critical value. Dip move-out (DMO) dominantly removes the amplitude variation with offset due to curvature. These results suggest that when looking for amplitude variations with offset in a fault prospect, DMO should be applied as a preprocessing step. Compaction-driven faults have an exposed focus and, for an exponential variation of porosity with depth, there is a maximum curvature effect at a depth roughly the same as the scaling depth for the porosity. Logarithmic porosity with depth variations suggest a continued increase in the curvature effect of faults with increasing depth, to a maximum amplitude increase of about 35% relative to a plane interface. 相似文献
30.
温纳偏置电测深法压制表层电性不均匀体影响的研究 总被引:4,自引:1,他引:4
巷道底板浮煤等电性不均匀体及厚度变化是造成煤矿井下电测深曲线畸变的主要因素。通过模型实验研究了温纳偏置电测保法压制浮煤等表层电性不均匀体影响的能力。用感度分析法讨论了表层电性不均匀体产生地电干扰的机制。实测结果表明,该方法改善了电测深曲线质量,有效地提高了电测深法地质应用效果。 相似文献