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This paper presents the results of a computer simulation, which models a combined GPS, satellite/pseudolite navigation system for precision landing. The goal of this system is to meet the CAT III requirements exploiting a combined system of satellites/pseudolites under severe geometry, weather, and jamming conditions. The simulation includes the aircraft position/velocity estimation and probability of false alarm and misdetection utilizing the satellite and/or the pseudolite navigation system. Aircraft trajectory, GPS satellite constellation, pseudolite location, and the system noise input parameters form the system inputs. The system output is given in terms of the position/velocity estimation error vs. simulation time. Performance measure of the system integrity are also computed and presented. The overall system performance is the combination of the system navigation accuracy and integrity. It is well known that the GPS satellite navigation system lacks vertical information, which results in poor vertical accuracy. On the other hand a pseudolite navigation system provides very good vertical information thus improving the vertical accuracy which is crucial for CAT III precision landing. Both satellite and pseudolite systems are all weather navigation systems, which guarantees optimal performance under any weather conditions. The pseudolite system can perform navigation in the S band or bands different from the L band; therefore, enabling the aircraft landing under severe jamming conditions of the L band.