Gee-H (navigation)
By measuring and keeping a fixed distance to a radio station, the bomber could navigate along an arc in the sky.
Y-Gerät used a Knickebein-like beam for steering the bomber in the proper direction and an onboard transponder for distance measurements.
A special signal was periodically sent from a ground station, and on reception, the transponder would send out an answering pulse after a known delay.
A ground operator used an oscilloscope to measure the time between broadcast and reception and deduced the range in a fashion similar to conventional radar systems.
[3] A failing of the beam-type system of navigation is that the beams cannot be focused perfectly and in practice are fan-shaped, growing wider with distance from the broadcaster, accuracy falling with range.
[5] The Air Ministry developed a distance-measuring system known as Oboe which first started reaching the Pathfinder Force in late 1941 and was used experimentally in 1942.
Before the mission, the distance from one of the Oboe stations to the target was measured and an arc of that radius drawn on a conventional navigation chart.
For missions around 20,000 ft (6,100 m) in height, range is typically on the order of 1.5 mi (2.4 km) for a high-speed aircraft like the de Havilland Mosquito.
This calculation, carried out on the ground, could be as time-consuming as required, allowing for the consideration of winds, atmospheric pressure, even the tiny centrifugal force generated by the aircraft following the 235 mi (378 km) radius curve.
[6] During the sortie, the bomber crew would fly to one end of the arc using any means of navigation including dead reckoning.
This "cat" station would then send out a voice-frequency radio signal of either dots or dashes, allowing the pilot to adjust the path to be at the right distance, where the transmission would be a steady tone, the "equisignal".
[6] Operators would watch the position of the aircraft, sending out correcting signals as needed so the pilot could adjust the path along the arc.
As it took about ten minutes for the bomber to get onto the arc, this delay meant that the system could not be used for a large raid with aircraft in succession.
In tests, Oboe demonstrated accuracies greater than those of optical bombsights during daylight in good weather.
[7] Oboe was limited to one aircraft because the onboard transponder would send pulses every time the ground stations queried them.
If more than one aircraft turned on their transponder, the ground stations would start to receive several return pulses for every query, with no way to distinguish between them.
Similar stations with different signal modifications can be situated around the UK, so that all of them are visible to an aircraft over Germany.
This operated by sending out two pulses of known timing from ground stations which were picked up by the aircraft and read on an oscilloscope.
By the late war period, Gee was generally useless for bombing and used primarily as a navigational aid when returning to England.
As there were dozens of transceivers as well, many unused decoy signals, the magnitude of the jamming problem was considerably worse.
