1. A registration method for navigation-guided medical interventions at a patient, with no patient-associated markers, comprising the steps of: disposing a support mount relative to an imaging X-ray device, having a measurement volume, and fastening a carrying arm to said support mount; fastening an X-ray calibration phantom at a defined location of said carrying arm at which said X-ray calibration phantom is penetrated by an X-ray beam emitted by an X-ray source of said X-ray device, thus producing a known coordinate transformation between a coordinate system allocated to said support mount and a coordinate system allocated to said X-ray calibration phantom; acquiring a series of 2D projections of said X-ray calibration phantom from different projection directions using said X-ray device; and determining, based on said series of 2D projections, a coordinate transformation between the coordinate system allocated to the measurement volume and the coordinate system allocated to the calibration phantom, and from said coordinate transformation and said known coordinate transformation, determining a coordinate transformation between the coordinate system allocated to the support mount and a coordinate system allocated to said measurement volume.