The paper is about independent performance standards for GNSS-based positioning terminals used in road ITS, developed since 2013 at CEN-CENELEC/TC5/WG1, and more specifically about the 1st European Norm which is proposing system engineering procedures for the management of performances of GNSS-based ITS systems. After having introduced the context, the authors explain the paradigm supporting this work and present the principle of the 'Sensitivity analysis', which rely on simulations using error models of the outputs of the positioning terminal. The following sections are about methods for building position error models from field data acquisition. Three different methods which have been applied to real field data are presented and discussed. The comparison is based upon the capacity of the models to represent the statistical distribution and the time dependency of the real errors.