空中目标探测系统信号处理增益分析

针对利用全球导航卫星反射信号(GNSS-R)进行空中目标探测时,信号微弱无法可靠接收的问题,该文提出了一种计算信号处理增益的方法。通过合理设置信号处理算法参数,保证了对微弱信号的后续处理。首先对GNSS-R信号接收链路进行了分析,计算得到了当目标的雷达散射截面(RCS)变化时对应的目标回波功率范围;然后对空中目标上升、平飞和下降三种飞行状态双基地雷达系统几何模型进行了研究,给出了3种飞行状态下对应的有效探测时间,为接收机后续相干/非相干累加算法的处理时间提供阈值限制;最后在阈值范围内,选定合适相关时间,分析了经过相干/非相干累加处理后的反射信号输出信噪比范围。结果表明:在有效探测时间的限制下,相关处理时间取0.6s时,处理后的信噪比在10dB左右,满足一般码跟踪环所需的门限要求,可以保证接收机后续数据处理的顺利进行。     

Estimated travel time for walking trails in natural areas

An increase in walking or hiking activities in natural areas requires improvements in information and advice about trails, including their difficulty, available services and estimated travel time. Comparative studies show remarkable differences between measured and calculated travel times obtained by available predictive procedures (Naismith’s rule, Tobler’s hiking function or MIDE). A new procedure has been designed by combining pre-existing methods (Tobler’s and MIDE), and travel times have been calculated for 21 trails located in different protected natural areas of Spain. Times obtained are compared with travel times measured by individual users and uploaded into specialized walking-hiking websites (Wikiloc). Results show that the new procedure (Modified Tobler) reduces differences between calculated and measured travel times, which makes it suitable not only for trail managers to estimate travel times but also as a key part of pedestrian transport analysis in trail networks.     

International knowledge mobility and urban development in rapidly globalizing areas: building global hubs for talent in Dubai and Abu Dhabi

This paper examines the processes through which the United Arab Emirates’ (UAE) cities of Abu Dhabi and Dubai attract and integrate knowledge workers into their labor markets. It focuses on how the UAE has acquired the human capital to create post-oil economies, deploying its oil windfalls into massive urban development strategies in order to create global hubs for talent. More significantly, it analyzes how the UAE’s strategies and frameworks for attracting global knowledge flows ultimately determine the degree to which expatriate knowledge embeds locally. Presentation of results from a large-scale human capital survey of firms in Dubai and Abu Dhabi, as well as key-informant interviews with senior human resource administrators at these firms, demonstrate these processes.     

居民日常活动对扒窃警情时空格局的影响

扒窃现象与居民的日常活动有较强的关系,已有文献研究了不同活动场所对扒窃的影响,但鲜有考虑不同类型居民活动出行量及其场所对扒窃影响的时间差异。本文基于日常活动理论,以南方某大城市为例,以派出所为分析单元,利用居民入户调查问卷、兴趣点（Point of Interest, POI）、扒窃报警数据,检验不同类型活动出行量与扒窃的时间相关性,并划分时间段,建立多个空间滞后负二项回归模型分析不同时段居民日常活动对应场所与扒窃的关系。研究发现,扒窃警情存在明显的时空集聚性,其中,凌晨和早上的集聚性最强,白天和晚上相对较弱;不同类型的日常活动对扒窃时空格局的作用存在差异。买菜活动出行量与扒窃的时间变化存在正相关关系,买菜场所在凌晨和早上容易吸引扒窃者;购物活动与扒窃亦存在较强的时间正相关,白天和晚上在购物场所附近被盗的风险较高;外出吃饭的出行量与扒窃行为无直接关系,但在凌晨和早上餐馆周边容易吸引扒窃者;娱乐活动与扒窃同样存在较强的时间正相关,不同类型的娱乐场所对扒窃的影响存在差异,电影院、酒吧在大部分时间段均会吸引犯罪者,凌晨时段KTV附近区域的被盗风险较高。场所对扒窃的影响主要与犯罪机会吸引（犯罪目标的丰富性、暴露性）及场所自身属性有关（是否为犯罪者的集聚地）。文中最后对时间地理学和犯罪地理学结合的可能性、研究的应用意义等方面进行了探讨。     

黄土高原草地和农田生态系统碳滞留时间及固碳潜力

采用概率反演分析法估算黄土高原草地和农田生态系统各碳库滞留时间及固碳潜力。结果表明:除农田植物地上部分外,植物地上部分、地下部分、代谢性凋落物及土壤活性有机碳库滞留时间最短,为25~203d;惰性凋落物碳库滞留时间为2.4~3a;慢性有机碳库和惰性有机碳库滞留时间最长,分别为57.4~79.6a和593~598a。多年生草地的根系滞留时间显著高于农田;而农田代谢性凋落物碳库和慢性有机碳库的滞留时间显著高于草地。根据反演所得参数模拟,在当前气候和土壤条件下,农田和草地系统土壤有机碳在250a左右时达稳定状态,农田系统固碳潜力约为2 680g C·m^-2,草地约为3 130g C·m^-2。农田有机肥的输入能使土壤有机碳多固定4 000g C·m^-2。有机肥的输入及土壤有机碳周转缓慢使耕作农田比草地更有利于土壤有机碳积累。     

A Prospective on Zelinsky's Hypothesis of the Mobility Transition

Wilbur Zelinsky's 1971 paper in Geographical Review entitled the “Hypothesis of the Mobility Transition” was both forward‐looking and offered innovative ideas regarding human geographic mobility. One of the most interesting aspects of the paper was a set of predictions for mobility in a “future superadvanced society”. Many of these predictions have now come to pass, including a general decline in international and internal migration and residential change, the increasing regulation of migration ‐ especially internally, and the possibility that the widespread adoption of information and communication technologies has impacted human geographic mobility. Hence, this essay looks at the mobility transition not as an obsolete frame of reference but as a prescient, pliable, and adaptable framework which not only informs the study of human geographic mobility today but also, perhaps, even into the future.     

Immigrant groups and the local environment: socio-spatial differentiation in Czech metropolitan areas

The spatial distribution of immigrant groups is a more relevant topic now than ever before. While the study of this topic has a long tradition in the “traditional” immigration countries, it is rare in the very different context of Central and Eastern Europe, especially at the level of metropolitan areas. This study aims to address this gap by providing an analysis of the spatial distribution of various immigrant groups (defined by country of citizenship) in 17 Czech metropolitan areas, and its determinants. First, we characterize the specific situation of the Czech immigration system. Then we use cluster analysis to create a typology of neighbourhoods and compare the distribution of immigrant groups in each type of neighbourhood. Finally, we use regression analysis to examine which characteristics of the local environment are connected to concentrations of different immigrant groups. We show that the presence of foreigners remains associated with core urban areas. Some predominantly Western citizens act as gentrifiers, being associated with spacious pre-war apartments in prestigious inner-city areas, but there are also signs of suburbanization among more well-off immigrants to Czech metropolitan areas. We identify little evidence of ghettoization of immigrants into socially excluded areas.     

Analysis of the coupled relationship between grain yields and agricultural labor changes in China

In this paper we establish a model that expresses the coupled relationship between grain yield and agricultural labor changes in China, and present a preliminary discussion of the coupled processes involved in changes in these factors at the county level. Thus, we develop two coefficients on the basis of county-level statistical data for grain yield and agricultural labor for the years 1991, 2000, and 2010, namely, the grain-labor elasticity coefficient (GLEC) and the agricultural labor-transfer effect coefficient (ALTEC). The results of this study show that during the transformation process of agricultural development in China, different kinds of coupled relationships between grain yield and agricultural labor changes co-existed at the same time. For example, between 1991 and 2010, counties characterized by three different coupled modes (i.e., increasing grain yield and decreasing agricultural labor, increasing grain yield and agricultural labor, and decreasing grain yield and agricultural labor) account for 48.85%, 29.11%, and 19.74% of the total across the study area, respectively. Interestingly, a coupled relationship between increasing grain yield and decreasing agricultural labor is mainly concentrated in the traditional farming areas of China, while a coupled relationship between increasing grain yield and agricultural labor is primarily concentrated in pastoral areas and agro-pastoral ecotones in underdeveloped western China. At the same time, a coupled relationship between decreasing grain yield and agricultural labor is concentrated in areas that have experienced a rapid development transition in agriculture, especially the developed southeastern coast of China. The results of this study also show that between 1991 and 2010, 1961 counties experienced a decline in the proportion of agricultural labor; of these, 1452 are also characterized by increasing grain yield, 72.38% of the total. This coupled relationship between grain yield and changes in the proportion of agricultural labor shows a stepped fluctuation and has continually strengthened over time. Data show that mean values for the GLEC and ALTEC increased from–0.25 and–2.93 between 1991 and 2000 to–0.16 and–1.78 between 2000 and 2010, respectively. These changes in GLEC and ALTEC illustrate that the influence of agricultural labor changes on increasing grain yields has gradually diminished. Finally, the results of this study reveal that the ‘Hu Huanyong Line’ is a significant boundary sub-dividing this coupled relationship between grain yield and changes in agricultural labor.     

Spatial and temporal variation of vegetation phenology and its response to climate changes in Qaidam Basin from 2000 to 2015

Based on TIMESAT 3.2 platform, MODIS NDVI data (2000–2015) of Qaidam Basin are fitted, and three main phenological parameters are extracted with the method of dynamic threshold, including the start of growth season (SGS), the end of growth season (EGS) and the length of growth season (LGS). The spatial and temporal variation of vegetation phenology and its response to climate changes are analyzed respectively. The conclusions are as follows: (1) SGS is mainly delayed as a whole. Areas delayed are more than the advanced in EGS, and EGS is a little delayed as a whole. LGS is generally shortened. (2) With the altitude rising, SGS is delayed, EGS is advanced, and LGS is shortened and phenophase appears a big variation below 3000 m and above 5000 m. (3) From 2000 to 2015, the temperature appears a slight increase along with a big fluctuation, and the precipitation increases evidently. (4) Response of phenophase to precipitation is not obvious in the low elevation humid regions, where SGS arrives early and EGS delays; while, in the upper part of the mountain regions, SGS delays and EGS advances with temperature rising, SGS arrives early and EGS delays with precipitation increasing.     

南疆短时强降水概念模型及环境参数分析

利用南疆2010-2016年自动气象站及区域自动气象站逐小时降水量资料,NCEP/NCAR 1°×1°再分析资料以及探空资料,分析不同强度的短时强降水的时空分布,得出南疆短时强降水事件的天气型有明显的季节性特点和区域性特征。总结了典型短时强降水过程的环境背景场特征,建立了短时强降水的三种概念模型:中亚低槽(涡)型、西伯利亚低槽(涡)型和西风短波型。通过7个探空站的温湿廓线形态、地面露点温度、T_(850)-T_(500)、T_(700)-T_(500)、对流有效位能(CAPE)、对流抑制能量(CIN)、抬升凝结高度、0～6 km垂直风切变等分析了南疆短时强降水的环境背景:短时强降水Ⅰ型(整层湿)、短时强降水Ⅱ型(上湿下干)和短时强降水Ⅲ型(上干下湿)发生前大气水汽含量充沛、存在一定的CAPE和较明显的垂直风切变以及0℃层高度偏低、暖云层厚度偏厚等特征,而合适的CIN,有利于对流不稳定能量的积聚和爆发,促进短时强降水的发生;短时强降水Ⅳ型(干绝热型)存在大气层结较干和较大的T_(850)-T_(500)、T_(700)-T_(500);Ⅰ型和Ⅱ型是南疆短时强降水的主要类型,常出现在南疆中部、西部地区的盛夏和夏末,多为西伯利亚低值系统(低涡、低槽)型和中亚低值系统(低涡、低槽)型影响。