北斗卫星导航系统预计2020年将实现全球无源服务能力。如果仅利用区域性的地面站观测数据来确定导航卫星轨道，难以满足高精度的轨道确定以及导航的完好性、连续性等要求。建立星间链路，减小卫星导航系统对地面站的依赖是北斗卫星导航系统实现全球无源服务能力的关键。本项目将针对北斗系统实时星地联合定轨过程中所存在的关键及难点问题展开研究，主要研究内容包括：（1）利用赫尔默特方差分量估计实现星间与星地数据的合理融合；（2）提出一种解算效率更高的参数半分解滤波算法；（3）提出一种适用于各历元观测信息不同维情况下的自适应Kalman滤波。本项目以期通过以上研究从理论上解决星地联合定轨中的重点及难点问题，完善与改进星地联合定轨理论算法，并在此基础上研制出一套可用于北斗系统实时星地联合定轨的仿真计算软件，为北斗系统星间链路的建立提供分析验证平台，为北斗卫星导航系统全球无源服务能力的实现做出贡献。

The Compass Navigation Satellite System (BDS for short) is expected to realize Global Passive Service Ability in 2020.Since it’s hard to meet the requirements(such as integrity and continuity etc.) of high-precision satellite orbits determination and navigation by utilizing only the observations of regional ground stations, to develop inter-satellite crosslink and reduce the dependence of GNSS on the ground station plays an important role in the realization of Global Passive Service Ability of BDS. This project will engage in a research on the key and difficult issues of real-time Combined Orbit Determination of BDS, the main research contents include: (1) to realize the fusion of inter-satellite and satellite-station observations by using method of Hermert variation component estimation; (2) proposed a half-decomposed parameter filter algorithm, which is much more effective than conventional method;(3) proposed a robust-adaptive Kalman filter which applies to observations of different dimensionality from each epoch. This research project aims at resolving the difficult issues of Combined Orbit Determination, improving the theoretic algorithm of Combined Orbit Determination as well as developing a simulation software which could be applied to real-time Combined Orbit Determination of BDS and thus contribute to the realization of Global Passive Service Ability of BDS.