1. An image guided surgery system, comprising: a surgical instrument comprising a flat or elongated portion; a tracking system configured to locate and track an associated object having a flat or straight portion in a surgical environment, the tracking system comprising a tracking camera system configured to track a reference array or reference marker arranged on the associated object, the tracking system being further configured to provide tracking data and image data of the associated object; and a medical navigation system comprising a computer, the medical navigation system configured to process the tracking data and the image data of the associated object, and to output the processed image data of the associated object and positional information based on the processed tracking data on a display, wherein the surgical instrument, the tracking system including the tracking camera system, and the medical navigation system including the display are integrated in one single portable unit comprising a generally rectangular housing accommodating the medical navigation system and the display, the surgical instrument being rigidly attached or attachable to the housing in a known position relative to the housing, wherein the flat or elongated portion of the surgical instrument extends in a direction substantially transverse to the generally rectangular housing, wherein the flat or elongated portion of the surgical instrument is of a known positional relationship relative to the housing, in order to detect an orientation of the flat or straight portion of the associated object relative to the reference array or reference marker.