Non-invasive tracking of bones for surgery

There is described a device for use with a position sensing system to register position and orientation in a reference-coordinate system, the device comprising a set of at least three base units removably and non-invasively attachable to a skin surface covering a bone, each of the base units having a reference marker attached thereto, the reference markers being one of passive and active devices recognized by the position sensing system and positioned and oriented in the reference coordinate system with respect to a fixed reference, the base units adapted to measure a distance between the skin surface and the bone in conjunction with an ultrasound component.