The detection of gravitational waves from the merger of two neutrons stars and, soon after, the discovery of its electromagnetic emission, GRB170817, confirmed that binary neutron star (BNS) mergers can be the progenitors of short Gamma Ray Burst (sGRB). This discovery probed that the bright optical--NIR thermal transient (the kilonova) is produced by the radioactively heated ejecta launched before and during the merger. Intriguingly, this event posed the question if, as expected in GRBs, a jet successfully broke out of the BNS ejecta or a more isotropic outflow is responsible for the non--thermal emission (from the X--ray to the radio band) observed for nearly one year post merger. Modelling the evolution of the lightcurve is insufficient to tell these two scenarios apart. High resolution global VLBI radio imaging (project GG084) show that the size of the source at 207.4 days is smaller than what expected in the case of a cocoon and is consistent with a narrow structured jet. This result implies that at least 10\% of BNS should produce a successful jet.