中文摘要：

在民航等应用领域,卫星导航系统的可用性是决定其能否作为唯一或主要导航系统的关键性能指标。主要对GPS卫星导航系统的空间信号（SIS）可用性进行评估分析,研究一种基于马尔可夫过程的卫星导航单星可用性算法模型,并研究对卫星导航星座可用性的评估模型。结合GPS实测数据和GPS标准定位服务性能标准（GPS SPS PS）中的可用性指标,证明提出的空间信号评估模型有效且符合GPS SIS可用性指标要求。研究结果对我国北斗卫星导航系统可用性的评估具有一定的参考价值。

英文摘要：

In civil aviation and other application fields, the availability of satellite navigation system is the key performance indicator for whether it can be taken as the only or the main navigation system. This paper mainly evaluates and analyzes the availability of Space-in-Signal （SIS） of GPS satellite navigation system. A model of single satellite availability algorithm based on Markov process is presented, and the model for evaluating availability of navigation satellite constellation is studied. In addition, based on the relevant data and the availability index of the latest GPS Standard Positioning Service Performance Standards （ GPS SPS PS）, it is verified that the model is effective and conforms with the availability index of GPS SIS. The result has great reference value to the study of the availability of BeiDou navigation system.