For the first half of the war the RAF struggled with the accuracy of its bombing and target marking. Surveys suggested that a large proportion of bombs were not coming within miles of the target.

Late in 1942 RAF Bomber Command established the 'Path Finder' Force led by D.C.T. Bennett. It was a controversial move, resisted by some who felt there was no need for an elite group of navigators who would mark the target for the main force of heavy bombers. Their early operations did not suggest that such specialisation would make much difference.

Then technology intervened. The OBOE navigation aid was to produce incredibly accurate target position information. The aircraft flew along a radius guided by a signal transmitted by one of the two participating ground stations - the tone changed if they flew too far inside the circle or too far outside it. Then as they flew over the target, a second signal from the other ground station indicated the intersection with the radius that marked the target. It was a system that was not only very accurate but also very hard to jam.