一种基于分组截短PN码的SOFDM信道估计方法

基于PN序列的信道估计方法的特点是计算过程简明,已被广泛应用在地面无线局域网系统中。在卫星正交频分多路复用传输体制(SOFDM)系统中引入传统PN序列信道估计方法的主要问题在于卫星信道的多径延时远大于SOFDM数据符号的持续时间,信道特性比较复杂,使得传统方法的估计精度严重下降。提出了一种改进的方法,根据信道的近似周期特性对数据帧进行合理的等长度分组,并把长PN序列改为短PN码来跟踪信道局部特性的变化,再对分组后的数据子帧分别进行信道估计。对该方法的设计思想进行了理论分析,并通过仿真验证了该方法的估计精度比传统方法至少提高了10倍。     

Proposal for ionospheric tomography technique using a new HF radio source

Measurements of the line integral of the electron density along satellite-to-ground ray paths (i.e. TEC) using differential phase or Doppler of two coherent VHF/UHF signals transmitted from NNSS or GPS satellite networks have been used in the ionospheric tomography for mapping large-scale ionospheric images over region of interest. In this paper, we present theoretical studies of using a new signal source HF frequency in tomographic imaging. The initial phase problem inherent in the phase measurement can be eliminated by measuring Faraday rotations. Relative rotation on two adjacent HF frequencies is used to solve the ambiguity problem. A second-order approximation of the Faraday rotation incorporated with ray-tracing technique improves the reconstruction degradation due to ionospheric refractions. CASSIOPE is a multi-purpose small satellite that receives HF signals from ground radar facilities and it is scheduled for launch in early 2008. Simulations have demonstrated the potential applications of the ionospheric tomography in CASSIOPE/e-POP satellite experiment.     

Use of two-line element data for thermosphere neutral density model calibration

Traditional empirical thermospheric density models are widely used in orbit determination and prediction of low-Earth satellites. Unfortunately, these models often exhibit large density errors of up to around 30% RMS. Density errors translate into orbit errors, adversely affecting applications such as re-entry operations, manoeuvre planning, collision avoidance and precise orbit determination for geodetic missions. The extensive database of two-line element (TLE) orbit data contains a wealth of information on satellite drag, at a sufficiently high spatial and temporal resolution to allow a calibration of existing neutral density models with a latency of one to two days. In our calibration software, new TLE data for selected objects is converted to satellite drag data on a daily basis. The resulting drag data is then used in a daily adjustment of density model calibration parameters, which modify the output of an existing empirical density model with the aim of increasing its accuracy. Two different calibration schemes have been tested using TLE data for about 50 objects during the year 2000. The schemes involve either height-dependent scale factors to the density or corrections to CIRA-72 model temperatures, which affect the density output based on a physical model. Both schemes have been applied with different spherical harmonic expansions of the parameters in latitude and local solar time. Five TLE objects, varying in perigee altitude between 280 and 530 km, were deliberately not used during calibration, in order to provide independent validation. Even with a single daily parameter, the RMS density model error along their tracks can already be reduced from the 30% to the 15% level. Adding additional parameters results in RMS errors lower than 12%.     

可变直径轮月球探测车运动学建模与分析

针对月球探测的特殊要求,设计了具有滚动扭转转向特性的四轮直径可变月球探测车.通过对可变直径轮的原理进行分析,推导出车轮等效半径的公式.根据该月球探测车的结构特点,以闭链坐标变换和瞬时重合坐标法为基本工具,推导了四轮可变直径轮月球车基于三维地形环境的正、逆运动学模型,分析了相关的运动学特征,提出了运动学逆解的理论解法和实用解法.实用解法有效地降低了计算量.并提出了用车轮等效半径调整探测车辆俯仰角度的方法,提高了月球车的稳定性和通过性能.该研究结果为四轮月球探测车的结构分析与运动控制提供了有力的基础.      

基于预测滤波器的GPS姿态估计

采用预测滤波器,利用全球定位系统的双差相位观测量实现载体姿态估计。该方法对初值的选取具有鲁棒性,可以在天线和可见星较少的情况下快速收敛到姿态精确解。仿真结果显示,该方法的姿态估计结果可以与基于非线性最小二乘的姿态估计结果相比拟,且受初值的影响小;采用双差相位测量比采用单差相位测量可以得到更佳的姿态估计效果;当使用三基线的天线阵时,在飞行机动较大的情况下采用近似正交布局的天线结构可以更好地估计姿态。     

卫星多波束天线赋形波束的最优化计算

主要介绍了卫星多波束天线的基本原理和数学模型 ;说明了卫星多波束天线赋形问题的本质是一个优化问题 ;用两种可行有效的优化方法——最小 p乘法和最小最大法分别对矩形区域和中国本土进行了赋形 ,并在最后分析了两种算法的优缺点     

敏捷导弹气动力/侧向推力复合控制特性分析研究

讨论了敏捷导弹的气动力与姿控发动机侧向推力复合控制.提出将控制系统分成控制律和复合逻辑两部分进行设计的方法,对敏捷导弹进行复合控制,并通过具体的仿真实例比较了在纯气动控制以及姿控发动机在连续和脉冲形式下的控制效果.最后分析了姿控发动机到拦截器质心的距离对控制效果的影响.      

现代卫星导航系统技术的研究进展

首先 ,文章详细地分析了美国GPS系统的技术特征及其现代化进程 ;然后 ,论述俄罗斯的GLONASS系统现状与不足 ,及其与GPS系统的差异 ,并介绍了欧洲的Galileo系统的建设进展及相关技术 ;最后 ,阐述北斗卫星导航系统尚不能满足中国各行业广大用户的需求 ,初步提出发展中国第二代卫星导航系统的总体构想及其关键技术     

Ka波段卫星ATM系统差错控制编码方案

Ka波段通信是当前卫星通信发展的热点 ,卫星异步传递模式 (ATM)技术代表了宽带数字卫星通信网络的研究方向 ,文章研究了适合Ka波段卫星ATM系统的差错控制编码方案。首先分析了卫星信道的特点以及对ATM信元传输所带来的影响 ,并给出了突发误码与信元丢失率 (CLR)关系的仿真曲线 ,然后讨论了六种具有代表性的卫星ATM差错控制编码方案。针对Ka波段宽带卫星通信的特点对六种方案进行了分析比较 ,其中的CASI/ALA以及RM Turbo方案为两种较好的选择。     

舱外机动装置控制技术的实时仿真研究

舱外载人机动装置的姿态控制为非线性时变,并且带有不确定参数的系统控制问题,传统的控制理论难以处理该问题.为了验证自适应控制方法对该问题的处理以及该方法在工程上的实用性,因此在dSPACE平台上建立了半物理仿真系统,该系统真实模拟了现实环境并且仿真出真实可靠的结果,从而验证了自适应控制方法对于控制舱外机动装置具有良好的控制特性.