It is disclosed a device for determining an axis joining origins of two reference systems of a rigid body. The device comprises first and a second inertial sensors constrained to the rigid body and for defining the first and second reference systems. The device comprises a processing unit comprising an acquiring module of first data of angular velocities and second data of linear accelerations of the rigid body, a first module for calculating third data of angular accelerations of the rigid body as a function of the first acquired data, a second module for calculating a relative orientation between the first and the second reference system as a function of the first data, a third module for calculating a vector joining origins of the first and second reference system, wherein the primary direction of the primary vector is representative of the axis joining the origins of the two reference systems.
