Oboe (navigation)
- For the musical instrument of the woodwind family see Oboe
Oboe was based on radio transponder technology. The was to use two stations, each at a different and well-separated location in England, to transmit a signal to a Mosquito pathfinder (RAF) carrying a radio transponder. The transponder reflected the signals, which were then received by the two stations. The round-trip time of each signal gave the distance to the bomber. The system went live in December 1942, about the same time 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 marking flares when it reached the intersection with the circle defined by another station, known as "Mouse". There was a network of Oboe stations littered 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.
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 slavish control of the ground station.
- 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. Oboe's operational use in December 1942, about the same time 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, useful for attacking the Ruhr industrial area but not targets deeper inside Germany.
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 feet) of the aim point.
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 10 cm / 3 GHz "Mark III" Oboe and were simply using the old transmissions as a ruse.
Interestingly, the Germans improvised a system conceptually similar to Oboe to perform bombing on the Eastern front on a limited scale, using two Freyas to play the roles of Cat and Mouse, and using voice radio to direct the bombers. Despite the considerable effort the Germans put into other electronic navigation systems, they never took the concept farther than that.
- 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" 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 the its own unique pulse pattern with that shot 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 leveraged off Gee technologies, operating on the same range of 15 to 3.5 meters / 20 to 85 MHz. It was about as accurate as Oboe.
