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. Two circles were created before the mission, one around each station, such that they intersected at the bomb drop point. The operators used the radars, aided by transponders on the aircraft, to guide the bomber along one of the two circles and drop the bombs when they reached the intersection.
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 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 individual aircraft, Oboe was used both to guide marker aircraft for the Main Force and for bombing aircraft making precision bombings of high value targets. It was by far the most accurate bombing system used during the war.
History
Background In order to accurately determine one's location relative to objects on the ground one needs two data points; two angles (as in triangulation), two distances (trilateration), or an angle and a distance (VOR/DME). Using radio to provide some or all of these measurements was an area of continual development leading up to the start of the war. The Germans pioneered this approach with operational systems like Lorenz beam and X-Gerät that used two narrow beam-like signals that crossed at a point in the sky to indicate a target using triangulation. Later, during The Blitz, the Germans introduced Y-Gerät, which combined a single Lorenz beam with a transponder-based distance measurement to fix locations. The problem with all of these systems was that they produced no information except within their narrow beams, and were not useful for general purpose navigation.
A more useful system was introduced in the RAF's Gee system, which used two timed signals that allowed the navigator on the bomber to determine their location using trilateration. It could be used anywhere within line-of-sight of the transmitter stations in the UK, and generally provided a reasonable signal up to about , depending on the aircraft's altitude. Gee was read on an oscilloscope display about across, which limited the accuracy of the timing measurements. As a result, Gee was accurate on the order of kilometers, which was extremely useful for navigation and area bombing, but did not provide the accuracy needed for pinpoint bombing.
As the accuracy of Gee was largely relative to the mechanical size of the indicator unit, the accuracy could be improved by using a larger display. However, in these early days of the cathode-ray tube (CRT), such displays were extremely expensive and very long, which made them unsuitable for fitting to a large number of Bomber Command aircraft.
Initial proposal The concept of reversing the display arrangement so the display could be on the ground and the transmitter on the aircraft was obvious. It had first been suggested by Alec Reeves of Standard Telephones and Cables in 1940 and then formally presented with the help of Francis Jones in the spring of 1941.
The basic idea would be to have two ground stations that would periodically send out signals on similar but separate frequencies. The aircraft carried transponders, one for each signal, which re-broadcast the signals upon reception. By timing the total round trip time from broadcast to reception and then dividing by twice the speed of light (the signal travels to the aircraft and back again) the distance to the aircraft could be determined. This was essentially identical to radar, with the exception that the transponder greatly amplified the signals for the return journey, which aided accuracy by providing strong, sharply defined signal pulses.
A practical problem was using these range measurements to guide a bomber towards its target. In the case of Y-Gerät, a single beam was used that produced a natural path for the bomber to fly along. Only the range along this path needed to be measured and relayed to the bomber crew. In the case of a system using two range measurements, there was no inherent path in the sky for the aircraft to follow. Locations and directions could be determined by the phoning the two range measurements to a plotting room, drawing arcs from the stations at those measured distances, and then locating the intersection. However, this took time, during which the aircraft moved, making it too slow to provide the desired accuracy.
Oboe adopted a simple solution to this problem. Before the mission, a path was defined that represented the arc of a circle whose radius passed through the target as measured from one of the two stations. This station was given the name "Cat". The aircraft would then use conventional navigation techniques, dead reckoning or Gee if it was equipped, to place itself some distance north or south of the target on a point near this line. They would then begin flying towards the target, at which point an operator at Cat would call out corrections to have the aircraft fly closer or further from the station until it was flying at precisely the right range to keep it on the circle.
The Cat station continued to keep the aircraft positioned at this precise distance as it flew towards the target, causing the aircraft to fly along the pre-defined arc. The second station, code-named "Mouse", also calculated the range to the target before the mission. As the bomber approached that predetermined range they would first call out a "heads up" to tell the bomb aimer to begin the run, and then a second signal at the right time to drop it. Using this method there was no need for the two stations to compare measurements or perform any trigonometry to determine an actual location in space, both performed simple range measurements directly off their screen and sent their separate corrections to the aircraft.
In practice, ranges were not sent by voice to the aircraft. Instead, a tone generator produced Morse code dots or dashes under the control of the operators. This was similar to the beam systems like Lorenz, which the UK aircrew were already familiar with using as a blind landing aid in the pre-war period. If the aircraft was too close to the station the operator would play the dot signal, and when they were too far, dashes. The two could be mixed so that as they approached the correct range, the dots would fill in the gaps between the dashes and form a steady tone.
Periodically the signal would be keyed to send out a letter to indicate how far they were from the correct range, X indicating , Y , and Z . Likewise, the Mouse station sent a series of keyed signals to indicate the approach, S to indicate the approach was starting, and then A, B, C and D as the aircraft approached.
Development and testing
There were some obvious problems with this approach, however. One of the most obvious is that any given ground station could only track a single aircraft at a time, compared to Gee where any bomber could pick up the signals from the UK and carry out the necessary calculations. This did not immediately eliminate it as a useful system; Y-Gerät had the same limitation, so it was used for target marking by flying one aircraft under control and having it drop flares for the following aircraft to drop on. The British adopted the same solution.
A more worrying concern was that the bomber aircraft would have to fly straight and level along a gently curving path while the ground station monitored its range as it flew towards the target. During this time the aircraft would be open to attack, which some considered to be almost suicidal. Lastly, the British had no difficulty jamming the German Y-Gerät system, even before it was widely used. There was no reason to expect the Germans would not do the same with Oboe as soon as they detected the signals.
Despite widespread opposition to the use of Oboe, A. P. Rowe ordered development to begin. Development began both on the shared by most early UK radar systems, but also at the new "fashionable" provided by the cavity magnetron. The latter would not only provide higher accuracy, but also be largely immune to jamming unless the Germans developed their own high-power microwave devices. This only occurred in the very last days of the war.
Two stations were set up as far east as possible, one in Dover (Walmer) and a second in Cromer (RAF Trimingham). On any given mission, one of the stations would be Cat and the other Mouse. In early testing in September 1941, an aircraft flying along the arc from Dover demonstrated an accuracy of , better than any bombing method then in use. Accuracy with bombs was not quite as good, as the bombs themselves were not identical and had slightly different trajectories. In a demonstration for senior officials on 2 July 1942, the system demonstrated a real-world accuracy of . In contrast, even using advanced visual bombsights like the Norden, average accuracies in 1942 were on the order of .
Oboe was first used in experimental operations by Short Stirling heavy bombers in December 1941, attacking Brest. These aircraft had a relatively limited service ceiling, and was limited to attacks at short-range where they maintained line-of-sight to the UK.Hooton 1994, p. 114.
At that time there was a great debate taking place in Bomber Command over the use of "pathfinders", specialized aircraft and crews that would find the targets and use flares to mark them for attack. The same technique had first been used during the Blitz by the Germans, notably by the specialist Kampfgruppe 100, but its effectiveness had been severely curtailed by British jamming efforts. Nevertheless, the concept had enough backing that a small force of Mosquitoes had been organized to operate as a pathfinder force, using normal optical sighting. This proved disappointing in practice, offering only a slight improvement in accuracy.
But the Mosquitoes were also the only aircraft that had the performance to fly at altitudes at which the Oboe signals could be received over Germany. At a meeting in the summer of 1942, it was agreed that the pathfinder Mosquitoes would be equipped with Oboe. Having faced opposition before, the addition of Oboe upset the argument against the specialist role, and what would become Pathfinder Force began forming over the ongoing objections.
Into service