全文获取类型
收费全文 | 347篇 |
免费 | 11篇 |
国内免费 | 52篇 |
专业分类
林业 | 25篇 |
农学 | 27篇 |
基础科学 | 35篇 |
69篇 | |
综合类 | 202篇 |
农作物 | 2篇 |
水产渔业 | 16篇 |
畜牧兽医 | 11篇 |
植物保护 | 23篇 |
出版年
2024年 | 4篇 |
2023年 | 12篇 |
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 14篇 |
2019年 | 17篇 |
2018年 | 10篇 |
2017年 | 25篇 |
2016年 | 30篇 |
2015年 | 28篇 |
2014年 | 24篇 |
2013年 | 22篇 |
2012年 | 33篇 |
2011年 | 35篇 |
2010年 | 34篇 |
2009年 | 28篇 |
2008年 | 18篇 |
2007年 | 7篇 |
2006年 | 10篇 |
2005年 | 9篇 |
2004年 | 7篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1992年 | 1篇 |
排序方式: 共有410条查询结果,搜索用时 15 毫秒
81.
On the basis of the accuracy evaluation's demand of single impulse radar and other ground measure equipment, model of the inherent accuracy of ballistic camera as Evaluation's comparison criterion on fairway of a Rocket-Flight are worked out. Three respects analysis follows:1) Will the criterion be able to be used? 2) In case of two laser ballistic camera having effective data ,where does the one ballistic camera be as criterion? Where does the two ballistic camera be as criterion? 3) How can the speed be evaluated? The formula ,calculation method's simplification as well as the curve figure of the error transfer of the criterion are given and analysed. The model have been successfully used in single impulse radar's accuracy evaluation. It is a typical case of the accuracy analysis. 相似文献
82.
为提高林果园移动机器人导航系统的精确性与鲁棒性,提出一种基于激光雷达三维点云的果园行间高低频双源信息融合实时导航方法。首先,喷雾机器人搭载三维激光雷达采集两侧果树点云信息,对原始点云数据进行直通滤波、降采样和统计滤波等预处理,保留感兴趣区域内果树冠层点云;然后,将分别基于高频更新的牛顿插值算法和低频更新的非线性支持向量机(Non-linear support vector machine, NSVM)算法拟合的行间导航线进行互补融合;最后,在导航线切换时,对融合后导航线的稳定性进行优化,并使用三次B样条算法使导航线平滑。实验结果表明:融合优化后的导航线最大曲率为0.048 m-1,平均曲率为0.018 m-1;分别以0.5 m/s和1.0 m/s的行驶速度对融合优化后的导航线进行跟踪,绝对横向偏差最大值分别为0.104 m和0.130 m,平均值分别为0.053 m和0.049 m,说明该导航方法能够满足作业装备在果园行间自主导航作业的需求,为喷雾机器人在果园环境中的自主导航提供技术参考。 相似文献
83.
To improve the level of meteorological service for the Oilfield region in the Taklimakan Desert,the Urumqi Institute of Desert Meteorology of the China Meteorological Administration(CMA) conducted a detection experiment by means of wind profiling radar(WPR) in Tazhong Oilfield region of Xinjiang,China in July 2010.By using the wind profiler data obtained during the rainfall process on 27 July,this paper analyzed the wind field features and some related scientific issues of this weather event.The results indicated that:(1) wind profiler data had high temporal resolution and vertical spatial resolution,and could be used to analyze detailed vertical structures of rainfall processes and the characteristics of meso-scale systems.Before and after the rain event on 27 July,the wind field showed multi-layer vertical structures,having an obvious meso-scale wind shear line and three airflows from different directions,speeding up the motion of updraft convergence in the lower atmosphere.Besides,the wind directions before and after the rainfall changed inversely with increasing height.Before the rain,the winds blew clockwise,but after the onset of the rain,the wind directions became counterclockwise mainly;(2) the temperature advection derived from wind profiler data can reproduce the characteristics of low-level thermodynamic evolution in the process of rainfall,which is capable to reflect the variation trend of hydrostatic stability in the atmosphere.In the early stage of the precipitation on 27 July,the lower atmosphere was mainly affected by warm advection which had accumulated unstable energy for the rainfall event and was beneficial for the occurrence of updraft motion and precipitation;(3) the "large-value zone" of the radar reflectivity factor Z was virtually consistent with the onset and end of the rainfall,the height for the formation of rain cloud particles,and precipitation intensity.The reflectivity factor Z during this event varied approximately in the range of 18-38 dBZ and the rain droplets formed mainly at the layer of 3,800-4,500 m. 相似文献
84.
利用常规实况场、物理量场、雷达产品资料,对2007年7月30日河南省东北部出现的雷雨大风天气进行诊断分析。结果表明,造成这次强对流天气过程的原因是副高西北侧、槽前深厚的西南气流内产生的飑线发展东北移造成的;持续5 d的深厚低压槽前较强的西南急流,一方面把大量热量、水汽和动量向北输送,使当地具有大量的不稳定能量存储,为对流的发生提供了热力和动力条件,另一方面,高低空深厚一致的西南气流决定了雷雨大风的移动方向。雷达图上,2条回波带的合并形成的"人"字型回波使对流加强,雷雨大风最大风速出现在"人"字型回波顶端及其移动路径上;根据多普勒雷达风廓线上风场的突然上传,可以提前35 min预报雷雨大风天气。 相似文献
85.
[目的]分析2011年2月26日发生在承德市入冬以来最大的一次降雪天气过程的多普勒雷达回波演变特征。[方法]利用河北省承德市C波段多普勒雷达的反射率因子、径向速度、VWP产品,结合大尺度环流形势,对2011年2月26日发生在承德市入冬以来最大的一次降雪过程进行了分析,初步探讨了强降雪天气过程的多普勒雷达回波演变特征。[结果]降雪回波强度较弱,若有一定范围的30 dBz回波,且稳定少动,就有可能产生强降雪;暖平流加辐合、暖湿空气在低层浅薄的冷垫上爬升是这次降水形成的主要原因;"牛眼"结构的出现、低空急流的存在,为强降水的产生提供了源源不断的水汽;当垂直方向的风向发生改变且出现风速性辐散时,预示降水逐渐减弱并停止;暖平流与切变层的存在,是降水维持的重要原因。中层"ND"区域的出现、任意高度出现西北风预示着降水即将结束。[结论]该研究为强降雪的预报提供理论依据。 相似文献
86.
利用常规气象观测资料、美国NCEP再分析资料以及多普勒雷达、FY-2E气象卫星、加密自动站等非常规气象观测资料,对2011年8月28 ~30日本溪区域性暴雨过程的形成机理进行分析.结果表明,西风槽和副热带高压结合是此次过程的主导环流系统,副高减弱南撤是此次降水预报的关注重点,850 hPa切变线移动和冷暖性质是预报过程中的难点.此次暴雨天气过程动力条件、热力条件较好,水汽条件不足,低空急流未形成,中尺度分析更加清晰地阐述了暴雨过程发展演变情况,为暴雨成因分析提供了更好地理论依据. 相似文献
87.
利用安阳市2004—2009年的714雷达观测资料和实况资料,运用统计和对比的方法,对安阳市冰雹天气雷达回波特征进行综合分析。结果表明:源于山西涉县到林州西北部一带生成的对流单体,合并加强后沿涉县与平顺交界南段入侵林州市,强冰雹云移动路径基本为西北—东南方向,基本沿太行山东麓呈西北—东南贯穿林州市,在RHI上依据回波顶高度h与45 dBz回波顶高h45的差距识别冰雹云,当h-h45≤1.5 km时,为强冰雹云,当1.5 km3.0 km,为雷雨云。 相似文献
88.
新一代多普勒天气雷达运行质量的监测信息主要包括到报率、可用性和可靠性3个指标,这些性能指标反映了多普勒天气雷达的运行状况。在对雷达运行质量的监测信息进行全面分析的基础上,理解性能指标的物理意义,明确指标涉及的关键环节和影响因素,采取有效的技术措施和管理措施,可以大幅度提高新一代多普勒天气雷达的运行质量。通过对提高雷达运行质量的性能指标的技术方法研究,找出在实际雷达业务运行中影响性能指标的因素,并提出提高雷达运行质量的改进措施。 相似文献
89.
Kristin A. MacDonald Mark D. Kittleson Tracy Reed Richard Larson Philip Kass Erik R. Wisner 《Veterinary radiology & ultrasound》2005,46(3):192-199
The hypotheses were that cardiac magnetic resonance imaging (cMRI) would accurately determine LV mass in domestic cats and would do so more accurately than echocardiography (ECHO). ECHO was performed on seven sedated cats. LV mass was calculated using the truncated ellipse formula from a right parasternal long-axis view. T1 weighted gradient echo cMRI was acquired from anesthetized cats during multiple phases of the cardiac cycle. Short-axis images were obtained by acquiring 3 mm thick contiguous slices perpendicular to the cardiac long axis. LV mass was determined using Simpson's rule. Endocardial and epicardial borders were traced on each slice at end-systole, end-diastole, and mid-cycle and the difference in areas was myocardial area. Myocardial area was multiplied by slice thickness to calculate myocardial volume. Total (summated) myocardial volume was multiplied by myocardial density (1.05) to obtain LV mass at three measured phases of the cardiac cycle. Cats were euthanized and the LV was dissected and weighed to determine true mass. CMRI at end-systole most accurately quantified LV mass and was more accurate than echocardiography (P = 0.0078). Actual LV mass ranged from 6.5 to 10.5 g (mean = 8.5 g, SD = 1.6 g) compared with MRI LV mass at end-systole, which ranged from 6.7 to 11.1 g (mean = 8.7 g, SD = 1.7 g) and echocardiographic LV mass at enddiastole, which ranged from 5.2 to 9.1 g (mean= 7.1 g, SD = 1.8 g). Inter- and intraobserver variability for cMRI was 2%. CMRI obtained at end-systole accurately and reliably quantifies LV mass in domestic cats. It is more accurate than the echocardiographic method used in this study. 相似文献
90.
准确掌握黑土层厚度分布信息对于黑土资源评价和保护具有重要意义。然而,传统土层厚度测定方法包括土壤剖面法、插钎法和钻孔法等对于大范围的土壤厚度测定效率较低且连续性较差。本研究利用探地雷达探测了东北黑土区直型、凸型和凹型3种坡型坡面的黑土层厚度。室内模拟试验对黑土及黄土母质土壤分别设置不同的容重和含水量,探究土壤含水量和容重对土壤介电常数的影响以及探地雷达测定黑土厚度的可行性。野外试验通过开挖剖面和预埋标识物,验证了探地雷达测量黑土厚度的准确性。结果表明:土壤介电常数随容重的增大而增大,随土壤含水量的增加而减小;黑土和黄土母质层土壤含水量、容重和介电常数之间的关系可以用两个对数方程来描述,其精确度为95.26%~99.66%。探地雷达测量黑土厚度与剖面实测厚度相比,精确度为87.05%~95.58%。3个坡面的黑土厚度空间分布不同,且坡脚发生沉积处的黑土厚度较大,坡肩和坡背土壤侵蚀较严重处的黑土厚度较薄。本研究可为进一步探明和保护黑土资源提供一种高效、准确的土壤厚度调查方法。 相似文献