Abstract: Receiver autonomous integrity monitoring(RAIM) uses the redundant observations to detect faults and provide timely alerts when it should not be used for navigation user.RAIM can monitor the satellite faults,moreover,its calculation time is short.In this paper,the insufficiency of previous RAIM availability algorithms have been pointed out,which only considered the user horizontal position error caused by the satellite breakdown(non-random factors) without considering the user horizontal position error caused by the pseudorange noise(random factors) under a certain confidence.To this purpose,a RAIM algorithm based on circular error probable(CEP) is put forward and designed according to both random factors and non-random factors.Firstly,the user horizontal position deviation is calculated based on satellite breakdown of GPS and a certain false alarm probability and missing alarm rate.Then,the CEP mathematical model has established under certain confidence and RAIM availability is obtained by comparing the horizontal protection level(HPL) and the horizontal alert limit(HAL).Finally,simulation computation indicates the new RAIM availability algorithm is effective and useful.