Gee, sometimes written GEE, was a radio-navigation system used by the Royal Air Force during World War II. It measured the time delay between two radio signals to produce a fix, with accuracy on the order of a few hundred metres at ranges up to about . It was the first hyperbolic navigation system to be used operationally, entering service with RAF Bomber Command in 1942.
Gee was devised by Robert Dippy as a short-range blind-landing system to improve safety during night operations. In the course of development by the Telecommunications Research Establishment (TRE) at Swanage, the range was found to be far better than expected. It then developed into a long-range, general navigation system. For large, fixed targets, such as cities that were attacked at night, Gee offered enough accuracy to be used as an aiming reference without the need to use a bombsight or other external references. Jamming reduced its usefulness as a bombing aid, but it remained in use as a navigational aid in the UK area throughout and after the war.
Gee remained an important part of the RAF's suite of navigation systems in the postwar era, and was included in aircraft such as the English Electric Canberra and the V-bomber fleet. It also had civilian use, and several new Gee chains were set up to support military and civil aviation across Europe. The system started to be shut down in the late 1960s, with the last station going off the air in 1970. Gee inspired the original LORAN ("Loran-A") system, which is essentially a Gee system working at much lower frequencies to give it longer range.
History
Prerequisite work
The basic idea of radio hyperbolic navigation was well known in the 1930s, but the equipment needed to build it was not widely available at the time. The main problem involved the accurate determination of the difference in timing of two closely spaced signals, differences measured in milli- and microseconds.
During the 1930s, the development of radar demanded devices that could accurately measure these sorts of signal timings. In the case of Chain Home, transmission aerials sent out signals, and any reflections from distant targets were received on separate aerials. An oscilloscope (or oscillograph as it was known in the UK) was used to measure the time between transmission and reception. The transmitter triggered a time base generator that started a "trace" moving quickly along the oscilloscope display. Any received signals caused the beam to deflect downward, forming a blip. The distance that the trace had moved from the left side of the display could be measured to accurately calculate the difference in time between sending and receiving, which, in turn, could be used to calculate the slant range to the target.
Radar can also be used as a navigation system. If two stations are able to communicate, they could compare their measurements of the distance to a target, and use basic trilateration to determine the location. This calculation could then be sent to the aircraft by radio. This is a fairly manpower-intensive operation, and while it was used by both the British and Germans during the war, the workload meant it could generally only be used to guide single aircraft.
Landing system proposal
In October 1937, Robert (Bob) J. Dippy, working at Robert Watson-Watt's radar laboratory at RAF Bawdsey in Suffolk, proposed using two synchronized transmitters as the basis for a blind landing system. He envisaged two transmitting antennas positioned about apart on either side of a runway. A transmitter midway between the two antennas would send a common signal over transmission lines to the two antennas, which ensured that both antennas would broadcast the signal at the same instant.
A receiver in the aircraft would tune in these signals and send them to an A-scope-type display, like those used by Chain Home. If the aircraft were properly lined up with the runway, both signals would be received at the same instant, and thus be drawn at the same point on the display. If the aircraft were located to one side or the other, one of the signals would be received before the other, forming two distinct peaks on the display. By determining which signal was being received first, pilots would know that they were closer to that antenna, and would be able to recapture the proper direction by turning away from it.
Watt liked the idea, but at the time, a pressing need for the system was not apparent. At the time, the RAF relied on daylight bombing by tight formations of heavily defended bombers as its primary attack force, so night landings were not a major concern. Landing aids would be useful, but radar work was the more urgent need.
Navigation system proposal
The RAF's bombing campaign plans quickly went awry, especially after the Air Battle of the Heligoland Bight in 1939. Contrary to prewar thinking, the bombers proved extremely vulnerable to both ground fire and attacking fighters. After some discussion, the best course of action was decided to be to return to night bombing, which had been the primary concept earlier in the 1930s.
This raised the need for better landing aids, and for night navigation aids in general. Dippy refined his system for this purpose, and formally presented a new proposal on 24 June 1940. The original design used two transmitters to define a single line in space, down the runway centerline. In his new concept, charts would be produced illustrating not only the line of zero-difference, where the blips were superimposed like the landing system, but also a line where the pulses were received 1 μs apart, and another for 2 μs, etc. The result would be a series of lines arranged at right angles to the line between the two stations.
A single pair of such transmitters would allow the aircraft to determine on which line they were, but not their location along it. For this purpose, a second set of lines from a separate station would be required. Ideally, these lines would be at right angles to the first, producing a two-dimensional grid that could be printed on navigational charts. To ease deployment, Dippy noted that the station in the centre could be used as one side of both pairs of transmitters if they were arranged like an L. Measuring the time delays of the two outlier stations relative to the centre, and then looking up those numbers on a chart, an aircraft could determine its position in space, taking a fix. The gridded lines on the charts gave the systems its name, "Gee" for the "G" in "Grid".
As the system was now intended to offer navigation over a much wider area, the transmitters of a single station would have to be located further apart to produce the required accuracy and coverage. The single-transmitter, multiple-antenna solution of the original proposal was no longer appropriate, especially given that the stations would be located far apart and wiring to a common point would be difficult and expensive. Instead, Dippy described a new system using individual transmitters at each of the stations. One of the stations would periodically send out its signal based on a timer. The other stations would be equipped with receivers listening for the signal arriving from the control station. When they received the signal, they would send out their own broadcasts. This would keep all the stations in synchronization, without the need for a wire between them. Dippy suggested building stations with a central "master" and three "secondaries" about away and arranged roughly 120 degrees apart, forming a large "Y" layout. A collection of such stations was known as a chain.
The system was expected to operate over ranges around , based on the widely held belief within the UK radio engineering establishment that the 30 MHz shortwave signals would have a relatively short range. With this sort of range, the system would be very useful as an aid for short-range navigation to the airport, as well as helping bombers form up at an arranged location after launch. Additionally, after flying to their cruising altitude, the bombers could use Gee fixes to calculate the winds aloft, allowing them to more accurately calculate dead reckoning fixes after the aircraft passed out of Gee range.
Experimental systems were being set up in June 1940. By July, to everyone's delight, the system clearly was usable to at least at altitudes of . On 19 October, a fix was made at at 5,000 feet.
New offensive
The discovery of Gee's extended range arrived at a pivotal point in the RAF's bombing campaign. Having originally relied on day bombing, the RAF had not invested a tremendous amount of effort on the navigation skills needed for night flying. When The Blitz night-bombing offensive started, the Germans were found to have developed a series of radio aids for this, notably the X-Gerät system. The RAF initially pooh-poohed this approach, claiming it only demonstrated the superiority of the RAF's training.
By late 1940 a number of reports were trickling back from observers in the field, who were noting that Allied bombers did not appear to be bombing their targets. In one instance, bombs reportedly fell over from their target. For some time, these results were dismissed, but calls for an official enquiry led to the Butt report, which demonstrated only 5% of the bombs sent out on a mission landed within of their targets. With these statistics, any sort of strategic campaign based on attacks against factories and similar targets was hopeless. This led to Frederick Lindemann's notorious "dehousing" paper, which called for the bomber efforts to be used against the houses of the German citizens to break their ability to work and will to resist. This became official policy of the RAF in 1942.
While the debate raged, Bomber Command dramatically lowered their sortie rate, awaiting the rebuilding of the force with the newly arriving 4-engine "heavies" such as the Handley Page Halifax and Avro Lancaster, and the deployment of Gee. The two, combined, would offer the accuracy and weight of bombs that Lindemann's calculations called for. Efforts to test and deploy Gee became a high priority, and the Chain Executive Committee was set up under the chairmanship of Robert Renwick in October 1941 to site a series of Gee stations. Gee was not the only solution being developed; it was soon joined by H2S radars and the Oboe system.
Near-compromise
As the initial availability of the Gee devices would be limited, the idea of the pathfinder force was adopted. This concept had originally been developed by the Luftwaffe for their early night raids against England. Lacking enough radio sets and the widespread training to place their radio navigation systems on all their aircraft, they collected what they had into the single group, 100 Group. 100 Group would then use their equipment to drop flares, which acted as an aiming point for following bombers.
Eager to test the Gee system, prototype sets were used on target indicator aircraft well before the production sets were available in the number required for large raids. On 15 May 1941, such a set provided an accurate fix at a range of at an altitude of . The first full transmitter chain was completed in July 1941, but in testing over the North Sea, the sets proved to be unreliable. This was traced to the power supplies and tubes, and corrections were designed and proved that summer.
On the night of 11/12 August, two Gee-equipped aircraft using Gee coordinates only delivered "uncanny accuracy" when dropping their bombs. However, on the next night on a raid over Hanover, a Gee-equipped Vickers Wellington was lost. The Gee set did not contain self-destruct systems, and it might have fallen into German hands. Operational testing was immediately suspended.