Method, computer program to implement method, storage medium to store the program and apparatus for computing Signal-in-Space Accuracy (SISA) parameters in a regional or Global Navigation Satellite System (GNSS). The method includes at least of: determining individual Signal-in-Space Error (SISE) vectors; mapping individual SISE vectors to a service area; accumulating empirical sample sets; processing density functions of estimations of accumulated sample sets; individually overbounding density functions in one of an overbounding sense related to the regional or GNSS or in a paired overbounding with excess mass sense; and selecting a worst case according to predefined requirements.