System for sensing at least two points on an object for determining the position and orientation of the object relative to another object. Two light emitters mounted in spaced relation to each other on an external portion of an invasive probe remaining outside an object into which an invasive tip is inserted are sequentially strobed to emit light. Three light sensors or detectors, the positions of which are known with respect to a predetermined coordinate system, detect the positions of the two light emitters positioned on the probe. A computer connected to the probe and to the light sensors receives data from the sensors and determines the position and orientation of the probe relative to the predetermined coordinate system. The computer then determines the position and orientation of the invasive portion of the probe inside the object by correlating the position of the invasive portion of the probe relative to the predetermined coordinate system with the position of a model of the object defined in relation to the predetermined coordinate system. A display device connected to the computer indicates the location of the invasive portion of the probe in the object by displaying a representation of the location of the invasive portion of the probe with respect to the model of the object.