G-H (navigation)

G-H was a radio navigation system developed by Britain during World War II to aid RAF Bomber Command.

G-H was a two station radio direction finder system. Instruments in the bomber measured its position from one station and distance from another. It could be used by up to 80 bombers from any one pair of stations. By using more than one pair of stations, multiple targets could be attacked at the same time without the aid of pathfiders and markers. Once a major part of Bomber Command to be fitted with it, G-H became a most useful blind-bombing device.

G-H was used for the first time on the night of October 4/5 1943 when one Mosquito attacked Aachen; the trial was not a success. The second trial was on the night of October 16/17 1943 when nine Mosquitos attacked Dortmund, one was carrying out a G-H trial but its equipment failed and it had to bomb by dead reckoning. It was used for the first time in a large raid on the Mannesmann steel works at Düsseldorf on the night of November 1/2 when about half of the sets failed leaving only 15 aircraft to bomb the factory on G-H. [ [http://www.raf.mod.uk/bombercommand/oct43.html History of Bomber Command in October 1943] , from the RAF website.] [ [http://www.raf.mod.uk/bombercommand/nov43.html History of Bomber Command in November 1943] , from the RAF website.]

Development history

Along with the range restriction, an earlier system called 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 G-H (also given as "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. In this manner, it operates in a similar manner to the currently employed civilian DME system, with the aircraft following a DME arc procedure on one set, whilst using another set (tuned to a different transponder as close to 90 degrees apart as geography permits) to determine the point of bomb release.

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 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 G-H is confusing, since the scheme was very close to Oboe and not very much like GEE. The name was apparently adopted because G-H used GEE developments, operating on the same range of 15 to 3.5 meters / 20 to 85 MHz. It was about as accurate as Oboe.

ee also

List of World War II electronic warfare equipment

Footnotes