排序方式: 共有16条查询结果,搜索用时 15 毫秒
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洮河林区是国家天然林资源保护工程重点实施单位,也是长江上游与黄河上中游重要的水源涵养林区和我国西北地区重要的绿色屏障,更是甘肃这样一个深处内陆、干旱少林省份不可多得的绿色宝库。林区内生物多样性丰富,拥有森林、灌丛、草甸、草原、荒漠、湿地等地球陆地生态系统,以及洮河流域生态体系。如何有效地保护好利用好洮河林区森林生物多样性,是各级林业主管部门及林业工作者面临的重大课题。 相似文献
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洪涝灾害评估研究综述 总被引:2,自引:0,他引:2
洪涝灾害在世界范围内发生频繁,灾情严重,带来损失巨大.对洪涝灾害的评估是认识和管理洪涝灾害的前提和要求.洪涝灾害评估可以分为灾前评估、灾中评估和灾后评估三个阶段.文章聚焦于灾前评估和灾后评估,在界定了相关概念的基础上,从致灾因子、孕灾环境的危险性和承灾体的脆弱性等角度概括了洪涝灾害风险评估的内容,从指标确定、损失计量和等级划分等角度介绍了洪涝灾害损失评估的内容,并列出了主流研究方法.最后,对洪涝灾害评估的未来研究方向作了展望. 相似文献
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为了克服传统无线传真接收机硬件依赖性强、功能单一、信号适应能力和可升级能力差等缺点,本文提出了一种基于软件无线电技术的无线传真接收机设计方案.该方案采用数字化器件和少量射频前端电路进行设计,将射频信号模数转换后,利用数字下变频器HSP50110和数字锁相环HSP50210对数字射频信号进行解调处理;由通用数字处理器将解调后的数据存储、打印或扩展接口外传其它设备;低功耗单片机承担人机交互任务,并将控制指令传达至通用数字处理器,由通用数字处理器控制整个接收机的运行.该设计方案使无线传真接收机具有更好的灵活性和可扩展性. 相似文献
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A new scaffold has been developed,which made from poly(ε-caprolactone)( PCL) membrane with porous structure,and reinforcement of PCL scaffold was achieved by embedding polyethylene terephthalate(PET) weft-knit tubular fabric. The aim of this paper is to study the variation tendency of the morphology and the mechanical properties of the sample with the changing of molecular weight. Weighing method was used to analyze the porosity of the sample,and scanning electron microscopy( SEM) images were taken to observe porous structure. The tensile and compressive strengths of the samples were tested by the universal mechanical tester and radial compression apparatus, respectively. And the results showed that the porosity and compressive strength were improved when increasing the molecular weight,and the elastic recovery rate was also improved slightly. However, molecular weight has little impact on the tensile strength properties,because the PET tubular fabric provides most of the strength support rather than PCL membrane. 相似文献
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在日趋复杂的电子对抗中,如何提高雷达辐射源信号(radar emitter signal, RES)识别率和抗噪性能是亟待解决的问题。为此提出了一种变分模态分解(variational mode decomposition, VMD)和特征融合相结合的RES识别方法。首先利用VMD算法对各雷达信号进行分解得到3个本征模态函数(intrinsic mode function, IMF);然后,对这3个IMF分量提取排列熵(permutation entropy, PE)和样本熵(sample entropy, SE)特征进行特征融合,构成六维特征向量;最后利用支持向量机对辐射源信号进行识别。利用6种不同的辐射源信号对该方法进行了验证,仿真实验结果表明,该方法在低信噪比(signal to noise ratio, SNR)下能取得较好的识别率,当SNR不低于0 dB时,六维特征向量的识别率达到100%,具有较强的抗噪性能。 相似文献
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A new entire biodegradable scaffold has been developed which does not require precelluiarization before transplantation. This new kind of vascular scaffold prototype made from porous poly- e-caprolactone (PCL) membrane to provide three-dimensional environment for cell growth, and embedded with weft-knitted polylactic acid (PLA) fabric to support mechanics. The aim of this paper is to study the variation tendency of mechanical properties with the fabric spacing changing. The basic geometrical parameters were measured to characterize properties of the samples. The tensile and compressive elastic recovery of the samples were tested by the universal mechanical tester and radial compression apparatus, respectively. Both tensile and compressive properties enhanced when reducing the fabric spacing of the composite vascular scaffold. 相似文献