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.