Signal-In-Space Error (SISE) is important for the system integrity of GNSS. Real-time monitoring SISE has been one of the effective solutions used for improving GNSS system-level integrity. As one of the typical solutions, the Galileo integrity concept uses Signal-In-Space Accuracy (SISA) and Signal-In-Space Monitoring Accuracy (SISMA) respectively to transmit the quality of SIS and the integrity alarming information to users. As an important component of the Galileo Integrity Concept, Worst User Location (WUL) is of great importance for the computation of SISA and SISMA. An improved WUL algorithm based on a geometrical model is given in this paper, which needs no iterative search and is simple when compared with the existing models. The WUL accuracy requirement is discussed based on an analysis of the magnitude of the orbit errors and SIS ranging resolution and error. The conclusion is that, considering the metre-level orbital error observed in the orbit determination process, as long as the orientation error of the WUL is less than three degrees, the projection error of SISE can be kept to centimetre levels. This will satisfy the requirements of most users. The algorithm is further optimised to omit the non-linear iterative parts based on the above assumption. The optimisation reduces computation load and enhances the real-time capability. GPS raw measurements are used to validate the correctness and reliability of the algorithms.