4214 20190508174507.0 CTALK-2019-0115 Signorile, Giovanna Galileo and GNSS Time Offsets 2018 2018-04-11 Since April 2013, Galileo started to disseminate the GPS to Galileo Time Offset (GGTO), by transmitting the conversion parameters through the navigation message. This broadcast offset is allowing the user of a combined GSP/Galileo receiver to achieve a unique navigation solution by using measurements from both GPS and Galileo satellites, thus allowing interoperability between the two navigation satellite systems. Nowadays, many positioning systems are available, the topic of interoperability is therefore crucial to provide better capabilities at user level than the ones that would be achieved by relying solely on one system. The time offsets between the different systems should therefore be made available to the multi-GNSS users. An alternative would be that each constellation broadcast the time offset between its time scale and a reference common to all GNSS. In this paper, we investigate this scenario. We propose, as reference common to all GNSS, either a combination of the predictions of the GNSS time scales of the different systems, or directly the UTC predictions broadcast by the different systems. The link between this common GNSS reference time scale and BIPM is also investigated. From these approaches, we present a first assessment of the performances that can be achieved by a single user equipped with a not calibrated multi-GNSS receiver when estimating the offset between Galileo and other GNSS time scales, therefore addressing the interoperability of Galileo with other GNSS. NO Sesia, Ilaria, Defraigne, Pascale Tavella, Patrizia 32nd European Frequency and Time FOrum pascale.defraigne@observatoire.be Contributed CTALKCONT