Oboe (navigation)

Oboe was a British aerial blind bombing targeting system in World War II, based on radio transponder technology. Prior to a mission, a circle was drawn around one of the Oboe transmitters so that it passed over the selected target. The bombers, one at a time, would then attempt to fly along this path towards the target. The Oboe operator in England would use the equipment to see if the bomber strayed from the path, and give the pilot instructions on how to regain it. The path was only 35 yards wide, allowing for much greater accuracy than competing systems like Gee. The curved path was quite evident to German radar operators, who came to call the system "Boomerang" after the arc segment left on their displays as the aircraft appeared and disappeared out of range.

The system was first used in December 1941, about the same time as H2S radar was introduced and a few months after the first use of Gee. As it could only guide a single aircraft at a time, Oboe was used to guide the pathfinder bombers to drop their target indicators for the initial stages of the raid; following bombers would use the markers as a reference for conventional bombsights. Individual aircraft were also used to bomb point targets. Gee, a passive system that could be used by an unlimited number of aircraft, remained in use for local guidance over the UK even after Oboe became widely used for bombing.

Oboe's limitation of one aircraft at a time led to several new systems. Among these were the Gee-H system, which used the existing Gee equipment with Oboe-like transmitters. Gee-H could guide about 80 aircraft at the same time. In the US, SHORAN was similar, with a limitation of about 20 aircraft at once.

Technical details
Oboe used two stations at different and well-separated locations in England to transmit a signal to a Mosquito Pathfinder bomber carrying a radio transponder. The transponder re-transmitted the signals, which were then received by the two stations. The round-trip time of each signal gave the distance to the bomber.

Each Oboe station used the radio ranging to define a circle of specific radius, with the intersection of the two circles pinpointing the target. The Mosquito flew along the circumference of the circle defined by one station, known as the "Cat", and dropped its load (either bombs, or marking flares, depending on the mission) when it reached the intersection with the circle defined by another station, known as "Mouse". There was a network of Oboe stations over southern England, and any of the stations could be operated as a Cat or a Mouse as the need demanded.

The initial "Mark I" Oboe was derived from Chain Home Low technology, operating at upper-range VHF frequencies of 1.5 meters / 200 MHz. The two stations emitted a series of pulses at a rate of about 133 per second. The pulse width could be made short or long so it was received by the aircraft as a Morse code dot or dash. The Cat station sent continuous dots if the aircraft was too close and continuous dashes if the aircraft was too far, and from these the pilot could make the needed course corrections. (The Germans used a similar method with Knickebein.)

Various Morse letters could also be sent, for example to notify the aircraft crew that the Mosquito was within a specific range of the target. The Mouse station sent five dots and a dash to indicate bomb release. The Mouse station included a bombsight computer, known as "Micestro", to determine the proper release time, there being no particular logic in carrying the bombsight on the Mosquito when it was under the control of the ground station.

Although Oboe had been tested against Essen in January 1943, Oboe was rarely used for "big industrial plants" such as those in the Ruhr Area. The basic idea of Oboe was dreamt up by Alec Reeves of Standard Telephones and Cables Ltd, and implemented in a partnership with Frank Jones of the TRE and had as part of the team Dr Denis Stops who later became a leading physicist at the University College London. Denis Stops' role in the development of Oboe was so secret that he was drafted into the RAF Pathfinder Squadron as a Wing Commander to conduct his work. His role was largely to develop the systems on the aircraft in conjunction with the land based radar systems. The system worked by using triangulation to pin-point the target.

Operational history
Oboe was first used by Short Stirling heavy bombers in December 1941, attacking Brest.

In December 1942, Oboe on Mosquitos was trialled at Lutterade. Half of the Oboe units malfunctioned in some way. This was about the same time as H2S was introduced. The Germans, observing the curved path of the Mosquito, called the system "Boomerang". The predictable path of the bomber was a vulnerability, compensated for by the fact that the speed and altitude of the Mosquito made it very hard to intercept. The major limitation of Oboe was that it was a line-of-sight system; the curvature of the Earth therefore allowed it to be useful for attacking the Ruhr industrial area, but not targets deeper inside Germany.

Oboe was extremely accurate. In his book, Most Secret War, British physicist R. V. Jones wrote, "As it turned out, Oboe was the most precise bombing system of the whole war. It was so accurate that we had to look into the question of the geodetic alignment of the Ordnance Survey with the Continent, which effectively hinged on triangulation across the straits of Dover." With an error radius of about 110 m at a range of 400 km, Oboe was about as good as optical bombsights. Late in the war it was used for humanitarian purposes to assist food drops to the Dutch still trapped under German occupation, as part of Operation Manna. Drop points were arranged with Dutch Resistance contacts and the food canisters were dropped within about 30 m of the aiming point thanks to Oboe.

It took the Germans more than a year to discover the mystery of the system. Oboe was cracked by engineer H. Widdra (who had already detected the British "Pip Squeak" (IFF) in 1940) at the end of August 1943 at the RF tracking station "Maibaum", located in Kettwig near Essen, while the British bombers attacked the steelworks of "Bochumer Verein".

The Germans tried to jam 1.5 metre / 200 MHz Oboe signals, though by the time they did the British had moved on to the 10 cm / 3 GHz Mk.II Oboe and were using the old transmissions as a ruse. This was discovered in July 1944 after its operator failed to properly mark a drop using the Mk.1 signals.

The Mk.III of April 1944, was more sophisticated. Four aircraft could operate on one frequency and the system could accommodate approaches other than simple radial ones.

Similar systems
Interestingly, the Germans improvised a system conceptually similar to Oboe, code named Egon, to perform bombing on the Eastern Front on a limited scale. It used two modified Freyas to play the roles of Cat and Mouse; these two Freya Egon sets were located about 150 km apart, and the aircraft carried a two channel IFF to respond to them. Voice radio directed the bombers. Despite the considerable effort the Germans put into other electronic navigation systems, they never took this concept farther.

Along with the range restriction, Oboe had another limitation: it could only really be used by one aircraft at a time. As a result, the British rethought Oboe, and came up with a new scheme named "GEE-H" (or "G-H") based on exactly the same thinking, differing only in having the aircraft carry the transmitter and fitting ground stations with the transponder.

Multiple aircraft could use the two stations in parallel because random noise was inserted into the timing of each aircraft's pulse output. The receiving gear on the aircraft could match its own unique pulse pattern with that sent back by the transponder. Each receive-reply cycle took the transponder 100 microseconds, allowing it to handle a maximum of 10,000 interrogations per second and making "collisions" unlikely. The practical limit was about 80 aircraft at one time.

The name "GEE-H" is confusing, since the scheme was very close to Oboe and not very much like GEE. The name was adopted because the system was based on GEE technologies, operating on the same range of 15 to 3.5 meters / 20 to 85 MHz, and initially used the GEE display and calibrator. The "H" suffix came from the system using the twin-range or 'H' principle of measuring the range from transponders at two ground stations. It was about as accurate as Oboe.

In popular culture
Oboe appears as a plot point in the "Lost Sheep" episode of the BBC television series Secret Army, which featured the search for a downed airman with technical knowledge of the system.