This paper proposes a computation method for the determination of the worst user location (WUL) instead of the current grid search-based methods. The WUL is a key parameter in the estimation of the Galileo integrity data. Since a satellite clock error is the same for all users whereas the orbital error is different depending on the location of users, the WUL depends on the satellite orbit errors which can be estimated using the measurements from the Galileo sensor stations. The proposed algorithm uses the resolved orbital errors to calculate an approximate WUL on a spherical model of the Earth. The next step is to determine the WUL on a specified ellipsoid model using an iterative transformation process. Results show that the proposed method is more accurate and computationally efficient than the current search-based method. The computation time required is minimal making it valuable in meeting the real time requirements for the generation of Galileo integrity flag.
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