The paper presents an improved receiver clock bias forecast model.The model built with "coasting window" method,holds the real-time refresh parameters.The extrapolated future clock bias is used to augment RAIM performance.Relevant flow charts and a new algorithm for forecasting error are introduced.The data analysis proves that this method not only increases availability of RAIM,but improves the rates of fault detection and identification.The minimum fault that can be detected decreases from 50 meters to 40 meters;and the minimum fault that can be detected and identified completely dropped from 70 meters to 60 meters.