Intrinsic GNSS Metrological Time Traceability to UTC

In many documents describing Global Navigation Satellite System (GNSS) features at user level, the wording "traceability to UTC" appears, UTC being the Coordinated Universal Time computed monthly by the Bureau International des Poids et Mesures (BIPM). This should be read as "metrological traceability to UTC", but it implies numerous requirements. We propose to discuss what would be an intrinsic time traceability to UTC by using a GNSS signal according to metrological rules. After a reminder of what these metrological rules are, we describe and analyze in a generic approach the relationship to UTC of a user time scale through GNSS. We provide a more detailed analysis of two examples from European GNSS. First with the European Geostationary Navigation Overlay Service (EGNOS), whose relationship to UTC is based on an Earth station located in Observatoire de Paris (OP) and connected to UTC(OP), the local realization of UTC. Second with Galileo, whose relationship to UTC is based on time transfer between the Galileo Precise Time Facilities (PTF), where Galileo System Time (GST) is generated, and five European National Metrology Institutes (NMI) or Designated Institutes (DI) providing UTC(k), the local realizations of UTC.