Oboe was a British aerial blind bombing system in World War II, based on radio transponder technology. The system consisted of a pair of radio transmitters on the ground, which sent signals which were received and retransmitted by a transponder in the aircraft. By comparing the time each signal took to reach the aircraft, the distance between the aircraft and the station could be determined. The Oboe operators then sent radio signals to the aircraft to bring them onto their target and properly time the release of their bombs.
The system was first used in December 1941 in short-range attacks over France where the necessary line of sight could be maintained. To attack the valuable industrial targets in the Ruhr, only the de Havilland Mosquito flew high enough to be visible to the ground stations at that distance. Such operations began in 1942, when Pathfinder squadron Mosquitos used Oboe both to mark targets for heavy bombers, as well as for direct attacks on high-value targets. In an attack on 21 December 1942, Oboe-guided bombers dropped over 50% of their bombs on the Krupp factories in Essen, an enormous improvement over previous efforts that resulted in less than 10% of bombs landing on their targets. Versions using shorter wavelengths demonstrated accuracy on the order of 15 meters (49 ft).
Oboe was used extensively by Pathfinder marker aircraft during the Battle of the Ruhr in 1943. In December 1943 Bomber Command began the Battle of Berlin, which was beyond the range of Oboe. For this campaign, Bomber Command was forced to rely on H2S instead, which never was able to provide the consistent accuracy of Oboe. A later development was the Gee-H system, in which the transponder remained on the ground but the transmitter was mounted in the aircraft where the readout was made. This system allowed around 80 aircraft to be guided at the same time. Neither H2S nor Gee-H could provide the accuracy of Oboe, which demonstrated the highest average bombing accuracy of any system in the war.
Bearing – The horizontal direction of a line of sight between two objects on the surface of the earth. See also absolute bearing and relative bearing.