Analysis of spatial clustering optimization

Spatial clustering is widely used in many fields such as WSN (Wireless Sensor Networks), web clustering, remote sensing and so on for discovery groups and to identify interesting distributions in the underlying database. By discussing the relationships between the optimal clustering and the initial seeds, a clustering validity index and the principle of seeking initial seeds were proposed, and on this principle we recommend an initial seed-seeking strategy: SSPG (Single-Shortest-Path Graph). With SSPG strategy used in clustering algorithms, we find that the result of clustering is optimized with more probability. At the end of the paper, according to the combinational theory of optimization, a method is proposed to obtain optimal reference k value of cluster number, and is proven to be efficient.     

天津市地震前兆台网的运行监控与维护管理

介绍天津市地震前兆台网的基本概况以及前兆台网部的职责任务;阐述仪器设备运行监控、观测数据质量监控的方式与方法;总结在仪器设备、技术系统维护过程中解决的一些关键技术问题;最后提出了关于仪器和软件系统的一些问题与建议。     

玉米叶面积指数的CHRIS/PROBA数据反演分析

叶面积指数(LAI)是衡量植被生态状况和估算作物产量的一个重要指标。LAI的反演是定量遥感研究的重要内容。传统的经验统计反演方法基于单一观测角度的遥感数据进行,忽略了地物反射率的方向性。若在反演中加入多观测角度的信息,则有可能提升LAI反演的精度。以2008年甘肃省张掖市玉米实验区为研究区,利用欧空局的CHRIS/PROBA多角度高光谱数据对比分析了传统植被指数NDVI、RVI、EVI的变化规律及其反演玉米叶面积指数LAI的精度,并根据NDVI随观测角度的变化规律,构造出新型多角度归一化植被指数MNDVI,分别对实测叶面积指数进行线性回归并利用实测数据对估算LAI进行精度验证,结果表明：新型MNDVI指数相比于传统NDVI、RVI、EVI对LAI的反演精度有了显著提升,估算模型决定系数R²达到0.716,精度验证均方根误差为0.127,平均减小了33.3%。     

Bimodal distribution of area-weighted latitude of sunspots and solar North-South asymmetry

We study the latitudinal distribution of sunspots observed from 1874 to 2009 using the center-of-latitude (COL). We calculate COL by taking the area-weighted mean latitude of sunspots for each calendar month. We then form the latitudinal distribution of COL for the sunspots appearing in the northern and southern hemispheres separately, and in both hemispheres with unsigned and signed latitudes, respectively. We repeat the analysis with subsets which are divided based on the criterion of which hemisphere is dominant for a given solar cycle. Our primary findings are as follows: (1) COL is not monotonically decreasing with time in each cycle. Small humps can be seen (or short plateaus) around every solar maxima. (2) The distribution of COL resulting from each hemisphere is bimodal, which can well be represented by the double Gaussian function. (3) As far as the primary component of the double Gaussian function is concerned, for a given data subset, the distributions due to the sunspots appearing in two different hemispheres are alike. Regardless of which hemisphere is magnetically dominant, the primary component of the double Gaussian function seems relatively unchanged. (4) When the northern (southern) hemisphere is dominant the width of the secondary component of the double Gaussian function in the northern (southern) hemisphere case is about twice as wide as that in the southern (northern) hemisphere. (5) For the distribution of the COL averaged with signed latitude, whose distribution is basically described by a single Gaussian function, it is shifted to the positive (negative) side when the northern (southern) hemisphere is dominant. Finally, we conclude by briefly discussing the implications of these findings on the variations in the solar activity.     

A possible active galactic nucleus in M82?

MERLIN and VLA observations of the galaxy M82 have detected a jet-like feature emanating from one of the compact sources, 44.01+59.6. The proximity of this source to the dynamical centre of M82 led us to suspect that it could be a weak active nucleus rather than an SNR. We imaged this source using the EVN at 15 mas and although it shows a compact shell-like structure which could be consistent with an SNR, we note that the EVN image shows a bright region within the source which is elongated along the jet direction. There are distinct similarities between this source and the Sgr A complex at the centre of our own Galaxy which contains the compact AGN, Sgr A^*.     

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北斗三号组网卫星数据质量分析及单系统定轨精度初步评估

选取2018-01-23起10 d内16个iGMAS测站观测数据,对北斗三号卫星的观测数据质量及BDS单系统精密定轨精度进行评估。初步结果表明,老信号B1I、B3I北斗三号卫星的信噪比略强于二号卫星,噪声与多路径基本相当,均在0.1 m量级,新卫星不存在星内多路径偏差。新信号B1C/L1/E1频点GPS信噪比最强,Galileo和BDS卫星相当,B2a/L5/E5a和B2b/E5b各系统基本相当;噪声及多路径方面,B1C/L1/E1频点GPS优于BDS、Galileo卫星0.1 m量级,B2a/L5/E5a和B2b/E5b各系统基本相当,均在0.1 m量级,新信号中北斗三号卫星星内多路径偏差基本消失。单系统精密定轨试验中,分别进行有/无GEO卫星策略、太阳光压模型ECOM 五/九参数策略的比较,并使用卫星激光测距数据进行独立检核。初步结果表明,有GEO卫星、ECOM五参数光压模型的定轨精度最好,C19号卫星7个重叠弧段的平均定轨精度在沿迹向、法向、径向的精度分别为32 cm、16 cm、8 cm,与试验卫星的定轨精度基本相当。     

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