It proves that least square residual method is equal to parity space method in detection rule. A novel GPS(global positioning system) RAIM(receiver autonomous integrity monitoring) algorithm is presented. The results show that it provides high availability, low missed detection and sensitive to fault detection. It fulfils the need of the reliability of high precision guidance system. The approach using the barometer and receiving GLONASS(global navigation satellite system) signal to realize GPS RAIM when lacking satellite number-in-view is studied,and it has been demonstrated by simulation.