A first apparatus assembles assistance data for a satellite signal based positioning of a device, the assistance data comprising a satellite specific ionospheric model for at least one specific satellite and a time stamp that relates the satellite specific ionospheric model to particular ephemeris data for the at least one specific satellite. The assistance data is provided for transmission. A second apparatus receives the assistance data and computes a position of the device making use of the particular ephemeris data and of the satellite specific ionospheric model.