Road transport, after Location-Based Services, is the second application domain of GNSS. Measuring the end-toend performance of intelligent transportation systems (ITS)and how this depends on the embedded GNSS chipset-makes the core of the ongoing European standardization initiative at CEN-CENELEC TC5/WG1. This article first presents the methodology, called "Sensitivity analysis", onto which the future norm will rely. Then the problem of modelling the GNSS positioning error is addressed, with some practical solutions, and the choice of one in particular, which is developed in an experimental context for validation purpose. The signal processing made from the full scale experiment in order to figure out the parameters of the model is presented. The use of this model in a simulation process is shown and analysed, so that first conclusions are drawn and research perspectives designed.