The utilization of hybridized INS/GNSS systems for space launcher applications presents big advantages with respect to the conventional navigators (as INS alone) used in such applications. However, for its integration it is necessary to cope with some challenging facts when using a GNSS receiver in a space launcher: the extreme dynamics as accelerations and jerks caused by the own launcher propulsion and the separation of the different stages, the environmental conditions caused by engines vibration, the rapidly changing environment caused by fast altitude increments, the launcher continuously rotating in roll and its effects on satellite visibility, the changing elevation angles, etcetera. In the frame of the project NAVIGA-VNE, SENER Aeroespacial (as project prime and Data Fusion design authority - DF), Deimos Space (as GNSS receiver design authority), and AVIO as the final integrator, are jointly working on the development of the Vega-C Navigation Equipment (VNE), an INS/GNSS navigation system aimed for the precise estimation of the PVT trajectory and attitude of AVIO’s VEGA-C space launcher. The objective of this paper is the description of the tools developed to support the VNE GNSS and Data Fusion and verification. It is overviewed the elements of the environment that have been modelled. Then a description of the GNSS performance model integrated in the Data Fusion Functional Engineering Simulator (DF-FES) tool is performed, focusing on the geometrical and signal propagation, the way the receiver logic has been simulated and the error models that have been included for the generation of the simulated GNSS raw observations. It is also explained how the GNSS model has been validated using a semi-analytical GNSS simulator that has also been developed in the frame of this project from tools already available at Deimos Space. After that, the integration of the performance model with other elements of the DF-FES is described; focusing on the iteration of the GNSS model with the integrated GNSS PVT estimator also developed by Deimos Space (also known as Navigation Processor or NAVPRO), which among other features integrates a velocity estimator based on Time Difference Carrier Phase (TDCP) algorithm. Finally, some relevant results of the GNSS performance model, with NAVPRO PVT output and DF PVT output will be included.