A new method has been proposed for assessing availability of satellite navigation system. Software has been compiled to simulate BeiDou-2 (BD-2) constellation with 12 satellites and GPS constellation. Based on 27 stations of Crustal Moment Observation Network of China, satellite visibility, dilution of precision (DOP), and availability of BD-2, GPS and integrated BD-2/GPS are calculated and analyzed in detail. Simulation results reveal that the visibility of BD-2 is superior to that of GPS and the variation of DOP of the former is more stable in China. The system availability of BD-2 is superior to that of GPS. And the integrated BD-2/GPS system is superior to any single system in all respects of positioning performance. Without other aided technical measures, GPS is insufficient for the requirement of availability of the required navigation performance (RNP), and BD-2 and integrated BD-2/GPS can meet the requirement of RNP under some conditions.