 To make up for the lack of visual aids initially, converted bomber pilots had to be used, because they already had experience with navigating at night.Another navigation aid was simply the Allied bombing target; a city illuminated as it burned would guide the fighters to their target.The week-long Battle of Hamburg in July 1943 proved disastrous for the "Luftwaffe", when the first use of Window by Bomber Command knocked out the "Himmelbett" radar defence system.