A direct position determination method with combined TDOA and FDOA based on particle filter

The localization of a stationary transmitter using moving receivers is considered. The original Direct Position Determination (DPD) methods, with combined Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA), do not perform well under low Signal-to-Noise Ratio (SNR), and worse still, the computation cost is difficult to accept when the computational capabilities are limited. To get better positioning performance, we present a new DPD algorithm that proves to be more computationally efficient and more precise for weak signals than the conventional approach. The algorithm partitions the signal received with the same receiver into multiple non-overlapping short-time signal segments, and then uses the TDOA, the FDOA and the coherency among the short-time signals to locate the target. The fast maximum likelihood estimation, one iterative method based on particle filter, is designed to solve the problem of high computation load. A secondary but important result is a derivation of closed-form expressions of the Cramer-Rao Lower Bound (CRLB). The simulation results show that the algorithm proposed in this paper outperforms the traditional DPD algorithms with more accurate results and higher computational efficiency, and especially at low SNR, it is more close to the CRLB.     

Nonlinear Interactions between the Quasi 5-day Wave and Tides Based on Meteor Radar Observations at Maui

Nonlinear interactions between the quasi 5-day wave and tides based on meteor radar observation in the Mesosphere and Lower Thermosphere(MLT) at Maui are studied in this paper.Strong sum interaction between quasi 5-day wave and diurnal tide,and evident difference interaction between quasi 5-day wave and semidiurnal tide are observed during the time of attention.However,their difference and sum counterparts are clearly weaker.The secondary waves generated from those interactions beat with the tide and show intense modulation at the period of 5 days which confirms the existence of their interactions.Additionally,correlation coefficients among these waves are calculated to further explore their interactions and find that they can persist for several days although they are highly intermittent.The energy exchange among these waves can be reversible during the observational time.The periods when the significant difference interaction between the quasi 5-day wave and semidiurnal tide occur are much shorter than those when the significant sum interaction between the quasi 5-day wave and diurnal tide occur.Moreover,these two strong interactions can take place simultaneously.In generally,this study provides the proof of nonlinear interactions between quasi 5-day wave and tides which were seldom reported before.     

去中心化时差频差直接定位方法

针对原始利用时差频差的直接定位(DPD)方法存在数据传输量和计算量大的瓶颈,提出了两种去中心化直接定位方法。第1种方法采用去中心化配对方案,只将各观测站截获信号在站间进行一次传输,将数据传输和计算分散到各观测站间并行计算互模糊函数(CAF),构造仅满足满秩条件的互模糊矩阵(CAM)。第2种方法根据推导的任意互模糊函数间关系公式,采用归约方式去中心化的在各观测站并行计算余下互模糊函数,补全互模糊矩阵。两种方法都降低了直接定位数据传输量,提高了计算效率。性能分析和仿真实验表明本文两种方法精度性能优于两步定位方法,在低信噪比时两种方法都可达到比较理想的精度性能,在高信噪比时第2种方法与原始直接定位方法的精度性能相当。     

Effects of sector structure of the interplanetary magnetic field on the upper mesosphere–lower thermosphere dynamics

In this paper we study the influence of the interplanetary magnetic field (IMF) polarity changes caused by the Earth passing through the IMF sector boundary on the dynamic processes taking place in neutral atmosphere within the altitude interval of the upper mesosphere–lower thermosphere (83–101 km). The analysis has revealed the influence of the IMF sector structure on dynamics of the upper mesosphere–lower thermosphere. There has been a significant seasonal variation of the wind reaction to the IMF polarity changes observed. The influence of the IMF polarity changes on neutral atmosphere dynamics within the altitude range of 83–101 km is most pronounced in the zonal component of neural wind when the IMF polarity changes from negative to positive in all the seasons except for spring and when IMF polarity changes from positive to negative – in spring only.     

拓扑空间的泛函表示

本文首先指出并非所有的拓扑空间都能用连续函数表示，其次给出了一般拓扑空间的下半连续函数表示，并利用下半连续函数讨论了拓扑空间的一些性质。     

最大标准差和最大变异系数方法

本文提出的最大标准差方法和最大变异系数方法,能充分利用以往数据和经验提供的信息,大大减少了试件个数。与国际上常用的单侧容限系数法相比,可节省大量试件;而在试件个数相同的情况下,则可得到更接近母体百分位值的置信下限(如安全寿命或安全疲劳极限等)。     

线性分组码不可检错误概率新的下限

线性分组码的不可检错误概率是评价自动请求重传(ARQ)系统的重要参数。不可检错误概率的计算涉及码的重量分布,而大部分码的重量分布还不知道,因此,研究不可检错误概率的限显得非常重要。本文得到了GF()上(n,k)线性分组码不可检错误概率的两个新的下限,在一定程度上,改进了Wolf等人的结果。由于新的限与码的重量结构无关,因此可以方便地估计任何线性分组码的不可检错误概率。     

一种单星仅测TOA无源定位方法

因单星测向定位体制存在系统构成复杂、对卫星载荷和姿态要求高等问题,提出 一种单个低轨卫星仅测脉冲到达时间（TOA）实现对地表静止辐射源无源定位的方法。在研 究仅测脉冲TOA估计径向加速度的基础上,提出采用网格搜索法进行定位的方法。推导得到 了该方法定位误差的克拉美-罗下限（CRLB）及对于不同位置辐射源的定位误差几何分布（G DOP）,并分析了该方法的可实现性。理论和仿真分析表明,该方法实现简单,适用范围广 ,定位性能满足实际需求。
     

二元塞式喷管红外特征及壁面降温的红外抑制效果计算

用数值计算的方法研究了涡扇发动机二元塞式喷管在3~5μm波段的红外辐射特性,并与轴对称收扩喷管进行了对比.结果表明:无冷却措施时,相比于轴对称收扩喷管,二元塞式喷管在大部分探测角度上红外辐射增强,只在小部分的探测角度范围内具有红外抑制作用,在窄边探测面内探测角为90°方向上,其红外辐射强度降低了54%.若塞锥后部壁面温度降低300K,二元塞式喷管在大部分探测角度范围内的红外辐射强度明显低于轴对称喷管,在正尾向上的红外辐射强度降低了57%;与轴对称喷管中心锥表面降低相同温度相比,二元塞式喷管能取得更高的红外抑制效果,并且所付出的冷却代价较小.      

基于星载加速度计反演载人航天轨道大气密度

热层大气密度直接影响低轨道航天器的精密定轨,热层大气密度模型的误差是影响载人航天定轨精度的关键因素。选取400km为载人航天轨道的代表高度,利用CHAMP卫星数据修正热层大气密度模型,进而反演得到2002年的热层大气密度,统计其中长期变化特征,并分析大气密度与太阳活动和地磁活动的关系,得出热层大气密度与两种指数的总体变化趋势一致的结论,且地磁活动与大气密度的相关性更好。同时将大气密度的反演值与神舟三号飞船的实测密度值进行对比,结果显示二者有较好的一致性,其平均残差和均方根误差分别为0．03和0．24,并且地磁平静期的误差明显小于磁暴期。结果表明,利用星载加速度计数据反演载人航天轨道大气密度是一种有效的方法。