Optical navigation system for determining the position of a patient's anatomy of interest , comprising : - a locating device with at least two optical sensors , - a control unit , - a patient reference comprising at least three optical markers , the respective positions of the optical markers relative to one another being known a priori by the control unit , said patient reference being intended to be positioned on the patient at the anatomy of interest, said optical navigation system being characterized in that : - the optical navigation system further comprises a reflective device whose position in a reference frame of the locating device can be determined by the control unit , - when a direct line of sight between the patient reference and each optical sensor is available, the optical sensors are configured to measure, for each optical marker of the patient reference , a quantity representative of the position of said optical marker in the reference frame of the locating device from optical radiation originating from said optical marker and having for each optical sensor a direct path between said optical marker and said optical sensor , - when a direct line of sight between the patient reference and an optical sensor is intersected by an obstacle , the optical sensors are configured to measure, for each optical marker of the patient reference , a quantity representative of the position of said optical marker in the reference frame of the locating device from optical radiation originating from said optical marker and having a path reflected by the reflective device to each optical sensor , - the control unit is configured to determine, from the measurements made by the optical sensors , the position of the patient reference in the frame of reference of the locating device , and to deduce therefrom the position of the anatomy of interest in said frame of reference. - a locating device with at least two optical sensors , - a control unit , - a patient reference comprising at least three optical markers , the respective positions of the optical markers relative to one another being known a priori by the control unit , said patient reference being intended to be positioned on the patient at the anatomy of interest, - the optical navigation system further comprises a reflective device whose position in a reference frame of the locating device can be determined by the control unit , - when a direct line of sight between the patient reference and each optical sensor is available, the optical sensors are configured to measure, for each optical marker of the patient reference , a quantity representative of the position of said optical marker in the reference frame of the locating device from optical radiation originating from said optical marker and having for each optical sensor a direct path between said optical marker and said optical sensor , - when a direct line of sight between the patient reference and an optical sensor is intersected by an obstacle , the optical sensors are configured to measure, for each optical marker of the patient reference , a quantity representative of the position of said optical marker in the reference frame of the locating device from optical radiation originating from said optical marker and having a path reflected by the reflective device to each optical sensor , - the control unit is configured to determine, from the measurements made by the optical sensors , the position of the patient reference in the frame of reference of the locating device , and to deduce therefrom the position of the anatomy of interest in said frame of reference. <EOS>