In recent years, with the development of wide area differential technology, Satellite-Based Augmentation System (SBAS) has gotten extensive application in every field, such as aviation, marine, ground transportation and so on. However, with the increase of satellites serving time, the capability of satellites may degenerate. For another, as more and more satellites have been used, the probability of double satellite ephemeris fault will be increased, making the navigation system unreliable to supply Safety of Life (SOL) Service. Therefore, the availability of SBAS has been growing more and more important. It is necessary to predict the availability of SBAS in advance so that users could choose a safe and efficient navigation system. Unfortunately, existing availability algorithm based on broadcast information doesn't work in this situation, where two real-time integrity parameters are required. One is User Difference Range Error Indicator (UDREI), which is used to describe integrity related to satellite clock/ephemeris error correction for each satellite. The other is Grid Ionospheric Vertical Error Indicator (GIVEI), which is used to describe integrity related to ionospheric error correction for each Ionospheric Grid Point (IGP). In this paper, the relationship between UDREI and the number of visible Ranging and Integrity Monitoring Stations (RIMS), as well as the relationship between GIVEI and the number of visible Ionospheric Pierce Point (IPP), is analyzed through statistical analysis. With the relationship, a simplified method of predicting the availability of SBAS is proposed to solve the heavy computation and weak timeliness of the traditional algorithms. Finally, experiments are performed to evaluate the effectiveness of our proposed method. Compared with the actual performance, the prediction is encouraging and this model may be a good contender for predicting the availability of SBAS.
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