The ionosphere, as the largest and least predictable error source, its behavior cannot be observed at all places simultaneously. The confidence bound, called the grid ionospheric vertical error (GIVE), can only be determined with the aid of a threat model which is used to restrict the expected ionospheric behavior. However, the spatial threat model at present widespread used, which is based on fit radius and relative centroid metric (RCM), is too conservative or the resulting GIVEs will be too large and will reduce the availability of satellite-based augmentation system (SBAS). In this paper, layered two-dimensional parameters, the vertical direction double RCMs, are introduced based on the spatial variability of the ionosphere. Comparing with the traditional threat model, the experimental results show that the user ionospheric vertical error (UIVE) average reduction rate reaches 16%. And the 95% protection level of conterminous United States (CONUS) is 28%, even under disturbed days, which reaches about 5% reduction rates. The results show that the system service performance has been improved better.