Before each surgery the accuracy of the navigation system is verified using the two elements on the left of the figure. A bucket and a measuring pen (manufactured by ourselves), the latter containing an electromagnetic sensor with six degrees of freedom at its upper end. Reading must be corrected by the offset between the sensor and the measuring pen tip. This calibration is done by pivoting around a fixed point as shown in figure. In that process the sensor crosses a spherical surface, “m” readings are taken and a lineal system of type Rp = d is solved. R is a 3mx6 matrix containing the angular component of the measurement, d is a 3mx1 vector with the translational component of the measure. The vector p contains two unknowns: the offset and the coordinates of the pivot point. The system is solved using SVD to obtain the Moore-Penrose pseudoinverse. VNL library has been used for that purpose.