北斗卫星导航系统采用8参数电离层模型用于单频用户的电离层改正。在导航信号格式设计中，每个模型参数需分配相应的位长以及量化单位。利用12年欧洲定轨中心（CODE：Center for OrbitDetermination in Eumpe）发布的格网电离层模型（GIM：Grid Ionospheric Model）数据，分别基于北斗卫星导航系统不同信号频率，拟合每一天的电离层预报模型参数，分析各参数的大小与变化情况。分析结果表明，部分参数的大小与导航信号频率及太阳活动程度有关，导航信号频率越大，参数幅值越小；太阳活动越剧烈，参数幅值越大。在参数变化特性分析基础上，提出了导航电文中电离层预报模型参数的设计原则，分析了导航电文参数量化对用户电离层延迟修正带来的误差。
Geostationary(GEO) satellites form an indispensable component of the constellation of Beidou navigation system(BDS). The ephemerides, or predicted orbits of these GEO satellites(GEOs), are broadcast to positioning, navigation, and timing users. User equivalent ranging error(UERE) based on broadcast message is better than 1.5 m(root formal errors: RMS) for GEO satellites. However, monitoring of UERE indicates that the orbital prediction precision is significantly degraded when the Sun is close to the Earth's equatorial plane(or near spring or autumn Equinox). Error source analysis shows that the complicated solar radiation pressure on satellite buses and the simple box-wing model maybe the major contributor to the deterioration of orbital precision. With the aid of BDS' two-way frequency and time transfer between the GEOs and Beidou time(BDT, that is maintained at the master control station), we propose a new orbit determination strategy, namely three-step approach of the multi-satellite precise orbit determination(MPOD). Pseudo-range(carrier phase) data are transformed to geometric range(biased geometric range) data without clock offsets; and reasonable empirical acceleration parameters are estimated along with orbital elements to account for the error in solar radiation pressure modeling. Experiments with Beidou data show that using the proposed approach, the GEOs' UERE when near the autumn Equinox of 2012 can be improved to 1.3 m from 2.5 m(RMS), and the probability of user equivalent range error(UERE)<2.0 m can be improved from 50% to above 85%.
LI XiaoJieZHOU JianHuaHU XiaoGongLIU LiGUO RuiZHOU ShanShi
