The objectives of this paper are manifold. First, it presents the evolution of the architectures for the System Time Generation Function (STGF) along with the different generations of Radio Navigation Satellites Systems. Here, the goal is to demonstrate that an STGF incorporating satellite clocks (also called "Flying clocks") represents a continuity of the past and current STGF architectures based on master clocks, or on an ensemble of ground clocks. Then, the advantages and drawbacks for this new architecture are highlighted. A description of the functional flow of the STGF for Satellite Based Augmentation Systems (SBAS) is then given, together with the main timing requirements allocated to this specific STG function. An example of SBAS Network Time (SNT) generation based on stations clocks (Low Cost Rubidium) and GPS satellite clocks (Cesium and Rubidium) is proposed to illustrate the expected stability performances of the SNT and also to provide first quantitative values for the SNT Retrieval Error (SNTRE). First models for the propagation of jumps affecting (non-) participative satellites or station clocks, onto the SNTRE and up to PVT solution at user level are proposed, and are also supported with simulations. A final part concentrates on additional aspects of an STGF for SBAS and based on flying clocks, such like the possibility to incorporate clocks from different GNSSs.
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