This project was developed for Indra Espacio in the context of a feasibility study about the future evolution of the European SBAS system EGNOS. The goal of the project is to add the Satellite Based Augmentation System (SBAS) capabilities on an existent GPS-differential solution developed by Sílvia Navarta as an end of studies project for Indra Espacio. With this previous work as a basis, this project reads the SBAS augmentation messages from a SBAS Rinex or EMS file, computes them and incorporates them (including fast, long term and ionospheric corrections) to the navigation solution. Then, it evaluates the performances of the system – in terms of accuracy, integrity and availability - to check whether it accomplishes the Required Navigation Performances (RNP) to be used for air navigation. The SBAS simulation was developed in Matlab© in post-processing mode, following the system requirements explained in the applicable standard for SBAS systems, the RTCA-DO-229D. The obtained results have been tested using an informatics program called Pegasus to ensure their complete validity and correctness. This project is divided in four big blocks. The first of them is an introduction on satellite positioning systems for air navigation, including the basic definitions on satellite positioning and air navigation. The second chapter is a detailed explanation about the SBAS system: it includes a brief explanation on its messages, and the SBAS positioning equations. Then, the third chapter explains the basic guidelines of the simulation code, including explanations on its functions and sub functions, and the fourth shows detailed result cases, compares them with Pegasus, and evaluates the use of the system for air navigation.