A targetless tracking device comprising: a detector configured to receive scattered radiation indicative of the presence of an object; and a processor 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 of the detector relative to the object; and generate 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; wherein the processor is further configured to: generate an image of the object based on the scattered radiation; characterised in that the processor is further configured to: select a sub-region of the image, the sub-region containing the object; display the sub-region to a user; receive input from the user, and in response to receiving the input, calculate the positional relationship by creating first and second images of the sub-region based on the scattered radiation at first and second points in time, respectively, and then comparing the first and second images in order to calculate the positional relationship. a detector configured to receive scattered radiation indicative of the presence of an object; and a processor 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 of the detector relative to the object; and generate 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; wherein the processor is further configured to: generate an image of the object based on the scattered radiation; characterised in that the processor is further configured to: select a sub-region of the image, the sub-region containing the object; display the sub-region to a user; receive input from the user, and in response to receiving the input, calculate the positional relationship by creating first and second images of the sub-region based on the scattered radiation at first and second points in time, respectively, and then comparing the first and second images in order to calculate the positional relationship. calculate a positional relationship between the device and the object based on the scattered radiation; receive an indication of a desired positional relationship of the detector relative to the object; and generate 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; wherein the processor is further configured to: generate an image of the object based on the scattered radiation; characterised in that the processor is further configured to: select a sub-region of the image, the sub-region containing the object; display the sub-region to a user; receive input from the user, and in response to receiving the input, calculate the positional relationship by creating first and second images of the sub-region based on the scattered radiation at first and second points in time, respectively, and then comparing the first and second images in order to calculate the positional relationship. generate an image of the object based on the scattered radiation; select a sub-region of the image, the sub-region containing the object; display the sub-region to a user; receive input from the user, and in response to receiving the input, calculate the positional relationship by creating first and second images of the sub-region based on the scattered radiation at first and second points in time, respectively, and then comparing the first and second images in order to calculate the positional relationship.