Tracking Device

A targetless tracking device has a detector configured to receive scattered radiation indicative of the presence of an object. The device also has a processor which is configured to calculate a positional relationship between the device and the object based on the scattered radiation, receive an indication of a desired positional relationship to the object, and output an error signal indicative of the positional relationship versus the desired positional relationship so that the device position is able to be controlled to reduce the error signal. The processor creates first and second images at first and second points in time, respectively, and compares the first and second images in order to calculate the positional relationship.