Oboe was a British bomb aiming system developed to allow their aircraft to bomb targets accurately in any type of weather, day or night. Oboe coupled radar tracking with radio transponder technology. The guidance system used two well separated radar stations to track the aircraft. Each Oboe tracking station used radio ranging to define a circle, the radius of which was the distance from the station to the target, the third point in the triangulation. The two circles intersected at the target. Radar pulses from each station were picked up by a transponder mounted in the aircraft. The aircraft transponder transmitted the signals back to the stations, after a slight delay. By assessing the time it took for the signal to return the distance between the station and the aircraft could be determined. One tracking station, the Cat station, was used to adjust the aircraft's flight path. The other station, the Mouse, was used to key the bomb release point. Oboe, in essence, was a ground-controlled, blind bombing system. The system was developed in 1942 by the Telecommunications Research Establishment at Malvern in Worcestershire, working in close association with 109 Squadron. By December 1942 a working system had been developed. The first major use of Oboe was in March 1943 when the system was used to mark the Krupp Works in an attack against Essen. Over the course of the month the system was used with great success to mark targets for the Main Force against the German industrial center of the Ruhr and for attacks against Cologne. Through November 1943 Oboe was used with good success against targets within its 250 mile range. In December 1943 Bomber Command entered what was hoped to be a war winning campaign with the Battle of Berlin. Berlin was over the horizon for even the highest flying RAF aircraft, and thus beyond the range of Oboe. The campaign had to depend upon straight navigation and H2S. Bomber Command's efforts against Berlin over the next four months were unsuccessful. At the end of March Bomber Command was directed to serve under SHAEF to make preparations for the invasion of occupied Europe. These missions to northern France allowed Oboe to again demonstrate its value in the precision delivery of markers or bombs, regardless of weather or the visibility of the target. Neither H2S nor Gee-H could provide the accuracy of Oboe. By guidance direction of