Oboe was a British aerial blind bombing targeting system in World War II, based on radio transponder technology. The system went live in December 1942, about the same time as H2S radar was introduced.
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 1.5 meters / 200 MHz. The two stations emitted a series of pulses at a rate of about 133 times per second. The pulse width could be made short or long so that 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.
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.
Oboe was extremely accurate, with an error radius of about 110 meters (120 yards) at a range of 400 kilometers (250 miles), about as good as optical bombsights. Late in the war it was used for humanitarian purposes to perform food drops for the Dutch still trapped under German occupation. Drop points were prearranged with Dutch resistance contacts and the food canisters were dropped within about 30 meters (100 ft) of the aim point.
It took the Germans more than a year to discover the mystery of the system. Oboe was cracked by the German RF engineer H. Widdra (who had already detected the British "Pip Squeak" (IFF) method 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 "Bochumer Verein".
The Germans tried to jam 1.5 meter / 200 MHz Oboe signals, though by the time they did so, the British had moved on to the 10 cm / 3 GHz Mk.II Oboe and were simply using the old transmissions as a ruse. This ruse was discovered in July 1944 after its operator failed to properly mark a drop using the Mk.1 signals.
The Mk.III system, from April 1944, was more sophisticated. Four aircraft could operate on one frequency, and the system could accommodate approaches other than simple radial ones.
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" (also given as "G-H") based on exactly the same logic, differing only in that the aircraft carried the transmitter and the ground stations were fitted 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 up 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 apparently adopted because the system was based on GEE technologies, operating on the same range of 15 to 3.5 meters / 20 to 85 MHz. It was about as accurate as Oboe.