The GEMMA system is a set of SW modules allowing the validation of new trajectory determination algorithms. The system is made up of measurement generators, filters and analysers, as well as trajectory generators and analysers and, as its main component, a generic platform for the optimal determination of trajectories (NAVEGA). The main purpose of the signal and trajectory generators and filters is to provide data to test and validate new navigation algorithms. Signal and trajectory analysers are used to characterize the error of data sets. NAVEGA is a software platform for the optimal determination of trajectories or paths of stochastic dynamical systems driven by observations and their associated dynamic or static models. “Configuring” NAVEGA is about “loading” components related to the specific navigation sensors and setting a number of mission related parameters. GEMMA and all its components are intended for operational as well as for development purposes. GEMMA’s target user communities are all those interested in developing new algorithms for positioning and navigation and the geodetic and navigation communities.