Like the United States Army Air Forces later in the war, Bomber Command had first concentrated on a doctrine of "precision" bombing in daylight. When the German defences inflicted costly defeats on British raids in late 1939, a switch to night bombing was forced upon the Command. The problems of enemy defences were then replaced with the problems of night navigation and target-finding. It was common in the early years of the war for bombers relying on dead reckoning navigation to miss entire cities. Surveys of bombing photographs and other sources published during August 1941, indicated that fewer than one bomb in ten fell within of its intended target. One of the most urgent problems of the Command was thus to develop navigational aids.