A method and system for estimating a position of a mobile device is disclosed. In particular, a method and system in which the position of a mobile device is determined using measurements of received Global Navigation Satellite System, GNSS, satellite signals is disclosed. The present invention therefore proposes to qualify a received satellite signal based on whether a signal propagation characteristic of this signal falls within an expected range of this characteristic. The expected range is determined using information about the satellite that sent the signal. The position of the mobile device is computed based on the validated satellite signals.