Gee-H, or sometimes G-H, was a radio navigation system developed by Britain during World War II to aid RAF Bomber Command. The name, sometimes written as "GEE-H", refers to the system's use of the earlier Gee equipment, as well as its use of the "H principle" or "twin-range principle" of location determination.[1] Its official name was AMES Type 100.
Gee-H was a two station transponder-based system. Instruments in the bomber measured its range from one station and the other. Drawing arcs of that distance on a chart would produce a "fix" where the two arcs intersected. In practice, the bombers would fly at a fixed distance from one station and wait until they reached a pre-computed distance from the other, where they would drop their bombs. It could be used by up to 80 bombers at a time 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 pathfinders and markers.
Gee-H entered service in October 1943 and first used successfully in November against the Mannesmann steel works at Düsseldorf on the night of 1/2 November when about half of the sets failed leaving only 15 aircraft to bomb the factory on Gee-H. Gee-H remained in use throughout the war, although it was subject to considerable jamming from the Germans. It also remained a standard fixture of post-war RAF aircraft like the English Electric Canberra.
Gee-H was adapted by RCA into the US wartime SHORAN system with improved accuracy. The same basic concept remains in widespread use today as the civilian DME system.
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Determining your location in 2D space requires two measurements or angle or range - two angle measurements, two distance measurements, or one angle and one distance. Early radio navigation was typically based on angle measurements using radio direction finders, but these had limited accuracy on the order or tens of miles. The development of range-based systems had to wait until the invention of accurate time measurement of radio was possible, which came about as a result of the development of radar.[2]
The Luftwaffe pioneered the use of distance-measuring radio navigation systems with their Y-Gerät system in 1941. Y-Gerät used a single Knickebein-like beam for steering the bomber in the proper direction, and an on-board 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. He then radioed this information to the bomber by voice, telling them when to release their bombs.[3]
The downside to the "beam" system of navigation is that the beams are necessarily fan shaped, growing wider with increasing distance from the broadcaster. This means they have decreasing accuracy with increasing range.[4] Measurements of distance are dependant only on the accuracy of the equipment, and are independent of range. When two such signals are combined the accuracy is linear with range, as opposed to the square as is the case with beam-based or hyperbolic navigation systems.[5] However, such systems are also generally more difficult to use, as they require two range measurements to be made in quick succession, while the aircraft is moving.
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. Oboe avoided the problems with two distance measurements by using only one at a time. Before the mission the distance from one of the Oboe stations to the target was measured on a conventional chart, say 300 km, and an arc drawn on the chart. The bomber would then fly itself to one end of the arc or the other. When their transponder was activated, the Oboe station would then measure their current distance and send back a signal of either dots or dashes on the radio, allowing the pilot to adjust their path so they were at precisely the right range, where they would hear a steady tone (the "equisignal").[6]
From that point the pilot flew along the arc of the circle defined by that distance from the Oboe station, referred to as the "cat" station. They would continue to adjust their path as needed to maintain their track. A second station was also measuring the distance to the bomber, having pre-computed where it would be when it was time to drop its ordinance. When this "mouse" station saw the bomber approaching this range along the arc, it would send a morse code that would drop the bombs automatically. The same signal could also be used to update the pilot on their progress by sending letters in Morse code.[6]
The major operational problem with Oboe was that it could only be used by a single aircraft at a time. As it took about 10 minutes for the bomber to get itself properly positioned along the arc, this meant that the system could not be used for a large raid. Instead, Oboe was used to guide the target marking aircraft of the pathfinder force, giving the main force of bombers an accurate aiming point in any weather. Alternately, Oboe was sometimes used for attacks on point targets by single aircraft, or a small number dropping one after the other. In tests, Oboe demonstrated accuracies greater than those of optical bombsights during daylight bombing in good weather.[7]
Oboe's limitation to a single aircraft was due to the fact that the onboard transponder would send pulses every time the ground stations queried them. If more than one aircraft turned on their Oboe system at the same time, the ground stations would start to receive several return pulses for every query, with no way to distinguish between them.
Consider an Oboe station that randomly modifies the envelope of the signal it broadcast to the aircraft. Similar stations are situated around the UK and all of them are visible to an aircraft over Germany, so that aircraft receive and re-transmit signals from all of them. As every ground station is likewise visible to the aircraft, they will receive their own "reflected" signal as well as those being returned to other stations. The operator at a ground station can visually examine the envelope of the returned signals to identify the ones his station broadcast. Ignoring the other signals, he can make his distance measurements as before. However, as many aircraft may return his station's signal at the same time if their transponders are turned on, it can still only be used by a single aircraft at once.
Now simply reverse the location of the broadcaster and transponder - place the transmitter in the aircraft and the transponder on the ground. If every aircraft generates a different signal pattern, the operators on the aircraft can look for their own signal and ignore the others. As long as the ground station is equipped to quickly turn the signals around, and the aircraft do not query too often, the chance of more than one aircraft querying the station at the same time is low. This is the basic concept behind Gee-H.[8]
The first radio navigation system to be operated by Bomber Command was Gee. This operated by sending out two pulses of known timing which was picked up by the aircraft and read on an oscilloscope. The time between pulses was not particularly accurate, and sometimes deliberately changed, so the equipment in the bomber had a timing system that allowed it to adjust for this.[9]
When the receiver was first turned on, the pulses from the ground station would move across the display because the two time bases were not synchronized. The operator then tuned their oscillator until the pulses stopped moving, which meant that the local oscillator was now at precisely the same frequency as that in the ground station. This froze the signal on the screen, allowing signals from different ground stations could be seen and measured.[9]
In order to rapidly deploy the new system, it was decided to re-use as much of the Gee equipment as possible. This already included the oscilloscope display and the receiver unit, so all that was needed was the broadcaster unit that would trigger the ground station transceiver. This was designed to operate on the same frequencies as Gee, so that the existing receiver and display equipment in the bombers could be re-used.[5]
The new transmitter sent out pulses about 100 times a second. The timing of the pulses was slightly advanced or retarded from exactly 100 per second. This meant that every aircraft had a slightly different timing. The same signal was also sent to the Gee display unit to start the display beam moving across the face of the display. This way the received signals that did not have exactly the same inter-pulse timing would appear to move one way or the other, exactly like a mis-tuned Gee. Only the signals originating from the aircraft's own timer would line up on the display and remain motionless. This deliberate adjustment of the timing was known as "jittering".[5]
In operation, the navigator would first set the delay of the upper trace on the Gee display to a known figure that matched the radius of the arc they wanted to fly along. This would move the "blip" from the local transmitter along the face of the display. Received signals would then be sent to the display inverted. The navigator could then direct the pilot onto the right path by giving directions until the upper and lower blips aligned. Likewise, the pre-computed range from the second station to the drop point was dialled into the lower trace, and as this range was always changing the active blip would move along the display towards this value until they overlapped..[10]
The time taken by the transceiver to receive a pulse, send out the response, and return to the receiving condition was about 100 microseconds. With a pulse timing of about 100 a second, any one transceiver would be busy for 10,000 μS responding to the signals from any one aircraft in any given second of time. This would leave 990,000 ;μS free to respond to other aircraft, giving a theoretical capacity of 100 aircraft. In practice, due to the jitter", about 70 to 80 aircraft could use any single station at a time.[10]
The system had the additional advantage that each aircraft selected its own timing, which made jamming more difficult. With most pulsed navigation systems, like Gee and Y-Gerät, it is relatively easy to jam the system simply by sending out additional pulses on the same frequency, cluttering up the display and making it very difficult for the operator to read the signal. The British had used this technique to great effect against Y-Gerät, and the Germans returned the favour against Gee. In the case of Gee-H, each aircraft had its own unique timing, so unless the jammer matched the jitter exactly, the jamming signal would simply disappear off the screen. And as there were dozens of such transmitters, as opposed to just a few ground stations in the case of Oboe, the magnitude of the jamming problem became considerably more difficult.
Moreover, as the Gee-H system used the existing Gee equipment, simply turning off the transmitter turned it back into a normal Gee unit. Although Gee was susceptible to jamming, it was still useful over the UK where jamming was not an issue. Unlike Gee-H, Gee could be directly read on a map, and was extremely useful for navigating home after a mission.
Gee-H's main fault was also a side-effect of the re-use of the existing Gee equipment; using a higher frequency would allow a tighter envelope, which would allow more accurate timing measurements and thus improve accuracy.[11] Additionally, as the system re-used Gee's small oscilloscope for measurements, it did not have the same level of visual accuracy as Oboe, which used 12-inch 'scopes developed just for this purpose. In general, Gee-H achieved accuracy of about 150 yards at 300 miles.[5] And as with all VHF and UHF-based systems, Gee-H was limited to distances just out of line of sight, in this case limiting it to about 300 miles.
Gee-H was key to Operation Glimmer, a diversionary "attack" during Operation Overlord that distracted and pinned-down German defences at Calais while the real invasion fleet was 200 miles away at Normandy. Gee-H-equipped bombers of 218 Squadron flew low, in tight circles, dropping "Window" (chaff) over radar transponder-equipped small ships, in order to deceive the German radars that they were the main invasion fleet.[12]
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