Editing Gee (navigation) (section)

== 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 in milli- and microseconds.{{sfn|Blanchard|1991|p=297}}

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){{sfn|Blanchard|1991|p=297}} 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.{{sfn|Blanchard|1991|p=297}}

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.<ref>{{Cite journal | doi = 10.1049/ji-3a-1.1946.0133| title = Oboe: A precision ground-controlled blind-bombing system| journal =  Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation| volume = 93| issue = 2| pages = 496–511| year = 1946| last1 = Jones | first1 = F. E. | author-link1 = Francis Jones (physicist)}}</ref>{{sfn|Jones|1978|pp=172–178}}

=== 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 [[Instrument approach|blind landing]] system. He envisaged two transmitting antennas positioned about {{Convert|10|mi|km|abbr=}} 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.{{sfn|Blanchard|1991|p=297}}

A receiver in the aircraft would tune in these signals and send them to an [[Radar display#A-Scope|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.{{sfn|Blanchard|1991|p=297}}{{efn|Although it is not mentioned in available sources, some method of telling the two signals apart would be needed. The solution used on Gee, periodically turning one of them on and off to cause it to oscillate on the display, would work here as well.}}

Watt liked the idea, but at the time, a pressing need for the system was not apparent.{{sfn|Blanchard|1991|p=297}} 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.{{sfn|Blanchard|1991|p=297}}

=== Navigation system proposal ===

The RAF's bombing campaign plans quickly went awry, especially after the [[Battle of the Heligoland Bight (1939)|Air Battle of the Heligoland Bight]] in 1939. Contrary to prewar thinking, the bombers proved extremely vulnerable to both ground fire and attacking [[Fighter aircraft|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.{{sfn|Blanchard|1991|p=297}}{{sfn|Campbell|2000|p=5}}{{sfn|Brown|1999|p=288}} 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&nbsp;μs apart, and another for 2&nbsp;μs, etc. The result would be a series of lines arranged at right angles to the line between the two stations.{{sfn|Brown|1999|p=288}}

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".{{sfn|Brown|1999|p=288}}

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 {{convert|80|miles|km}} away and arranged roughly 120&nbsp;degrees apart, forming a large "Y" layout. A collection of such stations was known as a chain.{{sfn|Blanchard|1991|p=298}}{{sfn|Brown|1999|p=288}}

The system was expected to operate over ranges around {{convert|100|miles|km}}, based on the widely held belief within the UK radio engineering establishment that the 30&nbsp;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.{{sfn|Blanchard|1991|p=298}}

Experimental systems were being set up in June 1940. By July, to everyone's delight, the system clearly was usable to at least {{convert|300|miles|km}} at altitudes of {{convert|10000|feet|km}}. On 19 October, a fix was made at {{Convert|110|mi|km|abbr=}} at 5,000&nbsp;feet.{{sfn|Campbell|2000|p=5}}

=== 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 [[Battle of the Beams#X-Gerät|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 {{convert|50|mi|km|abbr=on}} 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 {{cvt|5|mi|km|0}} 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, 1st Baron Renwick|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 radar]]s and the [[Oboe (navigation)|Oboe]] system.

=== Near-compromise ===

As the initial availability of the Gee devices would be limited, the idea of the [[Pathfinder (RAF)|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, [[Kampfgeschwader 100|KG100]]. KG100 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 {{convert|400|mi|km}} at an altitude of {{convert|10,000|ft|m}}. 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.{{sfn|Campbell|2000|p=5}} 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.{{sfn|Jones|1978|p=218}} Operational testing was immediately suspended.{{sfn|Campbell|2000|p=5}}

[[R. V. Jones]] responded by starting a disinformation campaign to hide the existence of the system. First, the use of the codename 'Gee' in communications traffic was dropped, and false communications were sent referring to a fictitious system called 'Jay'; it was hoped the similarity would cause confusion. A double agent in the [[Double Cross System|Double Cross system]] reported to German Intelligence a fictional story of hearing a couple of [[Royal Air Force|RAF]] personnel talking carelessly in a hotel about Jay, and one dismissing it as it was "just a copy" of the German [[Knickebein (navigation)|Knickebein]] system. Jones felt this would flatter the Germans, who might consider the information more reliable as a result. Extra antennae were added to the Gee transmitters to radiate false, unsynchronized signals. Finally, false Knickebein signals were transmitted over Germany.{{sfn|Jones|1978|pp=219–221}} Jones noted all this appealed to his penchant for practical joking.

In spite of these efforts, Jones initially calculated only 3 months would be needed before the Germans would be able to jam the system. As it turns out, jamming was not encountered until five months into the campaign, and it was much longer before it became a serious concern.{{sfn|Jones|1978|p=221}}

=== Into service ===

Even with limited testing, Gee proved itself to be easy to use and more than accurate enough for its tasks. On 18 August 1941, Bomber Command ordered Gee into production at [[Dynatron Radio Ltd|Dynatron]] and [[A.C. Cossor|Cossor]], with the first mass-produced sets expected to arrive in May 1942. In the meantime, a separate order for 300 hand-made sets was placed for delivery on 1 January 1942,{{sfn|Campbell|2000|p=6}} which was later pushed back to February. Overall, 60,000 Gee sets were manufactured during World War II, used by the RAF, [[USAAF]], and [[Royal Navy]].<ref>[http://www.flightglobal.com/pdfarchive/view/1946/1946%20-%201465.html Cossor Advertisement], ''Flight International'', 1 August 1946, Advertisements 11]</ref>

The first operational mission using Gee took place on the night of 8/9 March 1942, when a force of about 200 aircraft attacked [[Essen]]. It was installed on a [[Vickers Wellington|Wellington]] of [[No. 115 Squadron RAF|No. 115 Squadron]] from [[RAF Watton]] captained by Pilot Officer Jack Foster, who later said, "targets were found and bombed as never before".<ref>[http://www.flightglobal.com/pdfarchive/view/1970/1970%20-%200586.html "Gee Chain Closure"], ''Flight International'', 26 March 1970, p. 536</ref> [[Krupp]], the principal target, escaped bombing, but bombs did hit the southern areas of the city. In total, 33% of the aircraft reached the target area, an enormous advance over earlier results.<ref name=black/>

The first completely successful Gee-led attack was carried out on 13/14 March 1942 against [[Cologne]]. The leading crews successfully illuminated the target with flares and incendiaries and the bombing was generally accurate. Bomber Command calculated that this attack was five times more effective than the earlier raid on the city. The success of Gee led to a change in policy, selecting 60 German cities within Gee range for mass bombing using 1,600–1,800 tons of bombs per city.<ref name=black>Henry Black, [http://www.lancaster-archive.com/bc_gee.htm "A Short History of 'GEE' Air Navigation"], 2001</ref>

To provide coverage of the entire UK, three Gee chains were constructed under the direction of [[Edward Fennessy]].<ref>[https://www.telegraph.co.uk/news/obituaries/technology-obituaries/6819456/Sir-Edward-Fennessy.html "Sir Edward Fennessy"], ''The Telegraph'', 15 December 2009</ref> The original chain started continuous operation on 22 June 1942, followed by a chain in Scotland later that year, and the southwest chain in 1943. Even as German jamming efforts took hold, Gee remained entirely useful as a short-range navigation system over the UK. Only 1.2% of Gee-equipped aircraft failed to return to their base, as opposed to 3.5% of those without it.{{sfn|Campbell|2000|p=7}} Gee was considered so important that an unserviceable Gee set would ground an aircraft.{{sfn|Campbell|2000|p=8}}

One illustration of Gee's routine employment by Bomber Command in navigation tasks was its use (albeit a limited one) in [[Operation Chastise]] (commonly known as the "Dam Buster Raid") in May 1943. In his memoir, ''[[Enemy Coast Ahead]]'',<ref>Guy Gibson, ''Enemy Coast Ahead'', Michael Joseph, 1946</ref> [[Guy Gibson]], the leader of the raid, briefly mentions his navigator, F/O 'Terry' Taerum, [[RCAF]], employing what Gibson calls Taerum's "G Box" to determine groundspeed while flying very low at night over the North Sea from Britain to Holland, en route to Germany.

=== Upgrades ===
[[File:Mobile Gee station in Holland - CH 16520.jpg|thumb|Low-level photo of a light mobile Gee station operating in a field near Roermond, Holland. These forward stations provided Gee coverage deeper into Germany, as well as strong signals for aircraft returning to bases in Western Europe.]]

The first serious jamming was encountered on the night of 4/5 August 1942. This grew in strength as the bombers approached their target at Essen, and the signals became unusable at {{convert|10|to(-)|20|mi|km}} from the target. The newly formed southern chain was not yet known to the Germans and continued to be useful. On 3/4 December, a fix was made from this chain over [[Turin]] in Italy, at a range of {{Convert|730|mi|km|abbr=}}. This remained the operational record for Gee, bested only by a freak reception over [[Gibraltar]] at a range of {{Convert|1000|mi|km|abbr=}}.{{sfn|Campbell|2000|p=8}}

Counter-jamming efforts had already been considered, and resulted in the Gee Mk. II. This replaced the original receiver with a new model where the oscillators could be easily removed and swapped out to provide a range of operational frequencies. These included the original 20–30 MHz band, as well as new bands at 40–50, 50–70, and 70–90&nbsp;MHz. The navigator could replace these in flight, allowing reception from any active chain. Gee Mk. II went into operation in February 1943, at which point it had also been selected by the US [[8th Air Force]].{{sfn|Campbell|2000|p=9}}

On 23 April 1942, the go-ahead was given to develop mobile stations for Gee in preparation for the invasion of Europe. This would not only extend the range of the system eastward, but also allow stations to move and suddenly appear elsewhere if jamming became an issue. The first of an eventual three such mobile chains was formed up on 22 November 1943. This was put into operation on 1 May 1944 at [[Foggia]] in Italy, and was used operationally for the first time on 24 May. Other units were sent into France soon after [[D-Day]]. The mobile units in France and Germany were later replaced by fixed stations, the "heavies".{{sfn|Campbell|2000|pp=10–11}}

After the end of the war in Europe, Britain planned to send Lancasters to the Japanese theatre as part of [[Tiger Force (air)|Tiger Force]] and to use Gee for the passage of flights to Asia. Preparations began for Gee transmitters in [[Nablus]] (in Palestine) guiding the flights across the Middle East, but the [[surrender of Japan]] removed the need for this chain. This work was being carried out by MEDME, Cairo, under Air Vice-Marshal R. S. Aitken.

German bombers also used the Gee system for attacks on the UK; captured Gee receivers provided the electronics.{{sfn|Jones|1978|p=397}}

=== Gee-H ===
{{main|Gee-H (navigation)}}

Later in the war, Bomber Command wanted to deploy a new navigation system not for location fixing, but to mark a single spot in the air. This location would be used to drop bombs or target indicators for strikes by other bombers. The [[Oboe (navigation)|Oboe]] system provided this already; Oboe sent an interrogation signal from stations in the UK, "reflected" them from [[transceiver]]s on the aircraft, and timed the difference between the two signals using equipment similar to Gee. However, Oboe had the major limitation that it could only guide a single aircraft at a time and took about 10 minutes to guide a single aircraft to its target. A system able to guide more aircraft at once would be a dramatic improvement.

The result was a new version of the same basic Oboe concept, but reversed so that it was driven by the aircraft and reflected from ground-based transceivers. This would require equipment on the aircraft that could receive and measure the time difference between two signals. The reuse of the existing Gee equipment for this purpose was obvious. The new '''Gee-H''' system only required a single modification, the addition of a new transmitter that would send signals out for reflection from ground-based transceivers. With this transmitter turned off, the system reverted to being a normal Gee unit. This allowed it to be used in Gee-H mode during attacks, and then Gee mode for navigation back to their home airfields.

=== Postwar use ===

Gee was of such great utility that the hurried deployments during the war were rationalized as the basis for an ongoing and growing navigational system. The result was a set of four chains, South Western, Southern, Scottish, and Northern, which have continuous coverage over most of the UK out to the northeastern corner of Scotland. These were joined by a further two chains in France, and a single chain in the UK occupation zone in northern Germany.{{sfn|Blanchard|1991|loc=See map, page 301.}}