4206 20190507101003.0 10.1109/EFTF.2018.8409048 DOI SCART-2019-0161 Signorile, Giovanna Galileo and GNSS Time Offsets 2018 Galileo disseminates the GPS to Galileo Time Offset (GGTO) through the navigation message. This broadcast offset allows the user of a combined GSP/Galileo receiver to achieve a unique navigation solution by using measurements from both GPS and Galileo satellites. Nowadays, many positioning systems are available, the topic of interoperability is therefore crucial. The time offsets between the different systems should therefore be made available to the multi-GNSS users. An alternative would be that each constellation broadcasts only the time offset between its time scale and a reference common to all GNSS. In this paper we propose two alternative for, for this reference: either it can be based on the combination of the different GNSS time scales , or it can be directly UTC, as predicted and broadcast by the different systems. This paper presents a first assessment of the performances that can be achieved with each of these two approaches. NO Sesia, Ilaria Defraigne, Pascale Tavella, Patrizia Proceedings of the 32nd European Frequency and Time Forum 2018 276-280 pascale.defraigne@observatoire.be published in NONREF