The European satellite navigation system GALILEO will provide radio-navigation signals for a variety of applications. Safety Of Life users will get a safe navigation service through ranging signals carrying integrity information. The Galileo Integrity Baseline algorithm includes the transmission of three parameters allowing users to monitor their integrity level. These parameters are the Signal-In- Space Accuracy (SISA: prediction of the minimum standard deviation of a Gaussian distribution overbounding the Signal-In-Space error in the fault-free case), the Signal-In-Space Monitoring Accuracy (SISMA: minimum standard deviation of a Gaussian distribution overbounding the difference between Signal-In-Space error and its estimation by ground control stations) and the Integrity Flag, which accounts for satellite status (it can be set to ?OK?, ?DON?T USE? or ?NOT MONITORED?). The work presented in this paper studies the possibility of computing SISMA using a statistically robust algorithm, so as to reject wrong measurements and decrease ground system False Alarm rate (fault-free satellites flagged ?DON?T USE?).