Later versions of the "Himmelbett" added two Würzburg radars, with a range of about . Unlike the early-warning Freya radar, Würzburgs were accurate (and complex) tracking radars. One would be locked onto the night fighter as soon as it entered the cell. After the Freya picked up a target the second Würzburg would lock onto it, thereby allowing controllers in the "Himmelbett" center to get continual readings on the positions of both aircraft, controlling them to a visual interception. To aid in this, a number of the night fighters were fitted with a short-range infrared device known as 'Spanner anlage' but these proved almost useless in practice.