The European Commission recently announced the development of a new satellite navigation system called &Galileo'. It shall be a system compatible with GPS but shall have an improved performance compared to existing systems. Results of a study under a contract of the European Space Agency (ESA) with respect to achievable improvements of ephemeris and clock state determination as well as of integrity by use of intersatellite links and board-autonomous measurements to ground reference stations are presented. Simulation and analyses for two di!erent aspects were performed. The "rst aspect was the determination of the satellite state using intersatellite links and a conventional ground network. In the second aspect, additional two-way ground-referenced links were added and the performance analysis was repeated for such a con"guration with emphasis on autonomous measurements on board the satellites. The simulations were performed for a global GEO/IGSO and a global GEO/LEO satellite constellation, but we expect that the main results are similarly applicable to the GEO/MEO or MEO constellation, which is suggested to be used with Galileo. With respect to the integrity performance, the MEO constellation was analysed separately.