基于DDS技术的Loran—C信号源的杂散信号抑制的分析与实现

采用直接数字频率合成（DDS）技术设计的Loran—C信号源,具有输出杂散多且难以预测的缺点。基于对DDS基本原理的研究分析,针对DDS输出信号存在的相位舍位杂散问题,对其关键部位的相位累加模块进行优化设计,并基于FPGA技术,在QuartusII环境下完成了对Loran．C信号源的实现与仿真验证。结果表明,通过优化的设计算法能够产生失真小,稳定度好的输出波形,从而验证了该方法抑制杂散的有效性与可行性。     

基于不同轨道高度iHCO通信卫星的CAPS星座优化研究

基于通信卫星的导航系统可以利用比地球静止轨道（Geostationary Earth Orbit,GEO）高约200 km的倾斜高圆轨道（inclined Highly Circular Orbit,iHCO）通信卫星组成导航星座.结合两种轨道高度的倾斜高圆轨道通信卫星,仿真分析了利用倾斜高圆轨道卫星组成的中国区域定位系统（Chinese Area Positioning System,CAPS）的导航性能,并讨论了利用倾斜高圆轨道卫星组成的中国区域定位系统实现中国区域覆盖的最佳星座布局.     

接收机滤波器群时延对CAPS伪距测量影响的优化研究

在中国区域定位系统（Chinese Area Positioning System,CAPS）中,带通滤波器是用户接收机的重要组成部分,对于宽频测距码来说,其群时延特性是测距的重要误差来源之一.分析了接收机码跟踪环路,给出相关函数估算测距偏差,对带通滤波器群时延开展仿真分析,并研究了改进接收机设计以优化群时延特性的方法.研究对GNSS接收机的射频和环路设计有一定借鉴意义.     

Getting SMARTS on Novae: Highlights of the Early Evolution of Nova V475 Sct

There is a disconcerting global trend of retiring telescopes of modest aperture, supplanting them instead with fewer expensive telescopes of quite large aperture. As a consequence, the available time and feasibility of following transient objects in astrophysics is diminishing. We show the utility of having a suite of small to moderate aperture telescopes capable of conducting imaging and spectroscopic observations in a service queue mode. The example we provide is the high-cadence early observations of the classical nova V475 Scuti (2003) carried out with the SMARTS suite of telescopes located at CTIO.     

利用地基GPS数据研究2006年12月电离层磁暴现象

利用中国大陆全球定位GPS台网数据和中国周边地区国际IGS站数据，研究了2006年12月中旬的 磁暴对中国大陆上空电离层的影响。结果显示,该磁暴对中国大陆上空电离层的影响并不显著。     

Mass loss due to sputtering and thermal processes in meteoroid ablation

Conventional meteoroid theory assumes that the dominant mode of ablation (which we will refer to as thermal ablation) is by evaporation following intense heating during atmospheric flight. Light production results from excitation of ablated meteoroid atoms following collisions with atmospheric constituents. In this paper, we consider the question of whether sputtering may provide an alternative disintegration process of some importance. For meteoroids in the mass range from 10^-3 to and covering a meteor velocity range from 11 to , we numerically modeled both thermal ablation and sputtering ablation during atmospheric flight. We considered three meteoroid models believed to be representative of asteroidal ( mass density), cometary () and porous cometary () meteoroid structures. Atmospheric profiles which considered the molecular compositions at different heights were use in the sputtering calculations. We find that while in many cases (particularly at low velocities and for relatively large meteoroid masses) sputtering contributes only a small amount of mass loss during atmospheric flight, in some cases sputtering is very important. For example, a porous meteoroid at will lose nearly 51% of its mass by sputtering, while a asteroidal meteoroid at will lose nearly 83% of its mass by sputtering. We argue that sputtering may explain the light production observed at very great heights in some Leonid meteors. We discuss methods to observationally test the predictions of these computations. A search for early gradual tails on meteor light curves prior to the commencement of intense thermal ablation possibly represents the most promising approach. The impact of this work will be most dramatic for very small meteoroids such as those observed with large aperture radars. The heights of ablation and decelerations observed using these systems may provide evidence for the importance of sputtering.     

Radiative Heat Transfer and Hydrostatic Stability in Nocturnal Fog

We have performed a one-dimensional and transient radiative heat transfer analysis in order to investigate interaction between atmospheric radiation and convective instability within a nocturnal fog. The radiation element method using the Ray Emission Model (REM²), which is a generalized numerical method, in conjunction with a line-by-line (LBL) method, is employed to attain high spectral resolution calculations for anisotropically scattering fog. The results show that the convective instability has a strong dependence on radiative properties of the fog. For the condition of a 20-m droplet diameter and liquid water content of 0.1 × 10^–3 kg m^–3;, the temperature profile within the fog becomes S shaped, and a convective instability layer forms in the middle or lower level of the fog. However, for the same water content and a 40-m diameter droplet, no strong convective instability layer forms, whereas for a 10-m diameter droplet a strong convective instability is observed.     

采样数据密度及栅格尺寸对高程中误差的影响分析

利用全数字摄影测量得到的高精度点数据,获取不同密度的点数据,建立基于ANUDEM和TIN方法的两种DEM,比较两种DEM的高程中误差,分析数据密度、栅格尺寸与高程中误差的关系。研究结果表明,建立1∶100 00比例尺,具有高程中误差2m左右、栅格尺寸5m以下的ANUDEM,需要的点密度水平为大于7 000个/km2,而TINDEM则为8 500个/km2。同时,当栅格尺寸小于20m、数据密度介于7 000～9 400个/km2时,无论是ANUDEM还是TINDEM,高程中误差都处于一个较稳定的状态。     

地区级旅游卫星帐户构建的国际经验及启示

地区级旅游卫星帐户(R-TSA)的构建在世界范围内普遍面临着机构、统计、框架、方法等方面的障碍。有些国家已经在R-TSA的数据收集、统计体系规范、TSA框架构建、影响效应估算等方面进行了一些探索和试验,以丹麦、安达卢西亚、澳大利亚的R-TSA的编制实践为例,探讨其经验与不足,并总结提炼出其对中国省市自治区级旅游卫星帐户的编制启示。     

中国区域土地利用/覆被变化对陆地碳收支的影响

准确估计土地利用/覆被变化（LUCC）对陆地生态系统碳收支的影响已成为当前全球变化和全球碳循环研究的重点内容。本文通过文献调研和数据的整合分析方法总结讨论了近年来中国区域LUCC时空特征及其对陆地碳收支影响,为合理评价中国区域陆地碳平衡以及确定未来研究发展方向提供参考。已有大量研究对近年来中国区域LUCC主要特征进行了探讨,并分别利用卫星遥感方法和IPCC清单法对中国区域陆地碳源汇影响进行了评估。结果表明,目前全国土地利用活动,特别是农林活动正对陆地生态系统碳收支产生了比较显著的积极作用,但基于以上两种方法的研究结论之间差异很大,反映出中国LUCC导致陆地碳收支变化的评估结果仍存在着较大的不确定性。通过分析认为,中国未来的研究工作应着重于发展和利用基于土地利用相互转化面积的计量方法,以提高对中国区域LUCC导致陆地碳收支变化评估的准确性。