Gee was the code name given to a radio navigation system used by the Royal Air Force during World War II.[Notes 1] It was the first hyperbolic navigation system to be used operationally. Gee was designed to improve aircraft navigation accuracy, thereby increasing the destructiveness of raids by Avro Lancasters and various other bombers. For large fixed targets, like the 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 reference.
Gee was devised by Robert Dippy and developed at the Telecommunications Research Establishment (TRE) at Swanage. Dippy later went to the United States where he worked on the development of the LORAN system, a system that was similar to Gee but using a longer wavelength. LORAN was used by the US Navy and Royal Navy during World War II, and after the war came into common civilian use worldwide for coastal navigation, until GPS made it obsolete.
Gee was also known as AMES Type 7000, for "Air Ministry Experimental Station". AMES was a common universal naming system in the UK, similar to the "AN" series in the US.
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The basic idea of 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 was the accurate determination of the difference in timing of two closely spaced signals, signals that were travelling at the speed of light and thus measured in mili and microseconds.[1]
During the 1930s the development of radar demanded devices that could accurately measure the difference in time of these sorts of signals. In the case of Chain Home, signals were sent from transmission towers and any reflections off of distant targets that were received on separate antennas. The oscilloscope (or oscillograph as it was known in the UK)[1] was used to read these signals. A trigger signal sent from the transmitter started the "trace" moving quickly along the display, and any received signals caused the beam to deflect upward, 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 range to the target.[1]
In October 1937, Robert (Bob) J. Dippy, working at Robert Watson-Watt's radar laboratory at RAF Bawdsey, proposed using two synchronized transmitters as the basis for a blind landing system. He envisioned two transmitters positioned about 10 miles apart on either side of a runway. A common signal would be sent to both from a transmitter in the center of the two antennas, which would ensure that they would be broadcast from both antennas at the same instant.[1]
A receiver in the aircraft would tune in these signals and send them to an oscilloscope for display. If the aircraft was 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 was 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, the pilot would know that they were closer to that antenna, and would be able to recapture the proper direction by turning away from it.[1]
Watt liked the idea, but at the time there did not appear to be a pressing need for the system.[1] At the time the RAF was relying 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.[1]
The RAFs bombing campaign plans quickly went awry, especially after the Air Battle of the Heligoland Bight in 1939. Contrary to pre-war thinking, the bombers proved extremely vulnerable to both ground fire and attacking fighters. After some discussion, it was decided the best course of action would 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 navigation aids in general.
By this time Dippy had refined his system, and formally presented a new proposal on 24 June 1940.[2][3][1] Instead of guiding the aircraft along a single line towards the airport, his new concept used a number of long-range transmitters to produce several sets of lines. These were arranged so they would cross each other, forming a 2D grid that would be printed on navigational charts. By measuring their position relative to the lines generated by two pairs of stations, the aircraft could calculate their position in space, taking a "fix". The gridded lines on the charts gave the systems its name, "Gee" for the "G" in "Grid".[3]
Drippy's earlier proposal used two closely spaced stations to generate a single "line" in the sky. For his new system there would need to be more stations to generate multiple lines, and these stations would have to be spaced much further apart to provide the necessary coverage. The single-transmitter two-antenna solution from the landing system was no longer appropriate, especially given that the stations would be located far apart. Instead, he described a new system using individual transmitters at each of the stations. One of the stations would periodically send out a 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 broadcast. This would keep all the stations in sync, without the need for a wire between them. Drippy suggested building stations with a central "master" and three "slaves" about 80 miles away and arranged roughly 120 degrees apart. A collection of such stations was known as a "chain".[4][3]
It was expected that the system would operate over ranges of about 100 miles, based on the widely held belief within the UK radio engineering establishment that the 30 MHz signals would have relatively short range. It was believed this would be very useful as an aid for short-range navigation to the airport, helping bombers form up after launch, as well as a way to more accurately calculate the winds for dead reckoning after the aircraft passed out of Gee range.[4] Experimental systems were already being set up in June 1940, and by July it was clear, to everyone's delight, that the system was usable to at least 300 miles at altitudes of 10,000 feet. On 19 October they made a fix at 110 miles at only 5,000 feet.[2]
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 these were put into service, it was found the requirements for accurate dead reckoning rarely existed in practice, and the navigational problem was far harder than originally believed. The Germans had developed a series of radio aids for this, notably the Knickebein system, but the RAF initially pooh-poohed this approach, claiming it only demonstrated the superiority of their own approach.
By late 1940 a number of reports were trickling back from observers in the field who were noting bombers did not appear to be bombing their targets. In one instance, it was reported bombs fell over 50 mi (80 km) from their target. For some time these results were dismissed out of hand, but calls for an official enquiry lead to the Butt report, which demonstrated only 5% of the bombs sent out on a mission landed within 5 mi (8.0 km) of their target. With these sorts of 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 in order 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", 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 would soon be joined by H2S radars and the Oboe system.
As the initial availability of these devices would be limited, the idea of the pathfinder force was adopted. This had been used by the Luftwaffe against England for similar reasons, with the radio-equipped aircraft of KG100 dropping flares for the following bombers to use as an aim point. Eager to test the system, prototype Gee 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 400 miles at an altitude of 10,000 feet. The first full 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 bombed using Gee coordinates only and delivered "uncanny accuracy".[2] 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 was possible that it had fallen into German hands.[5] Operational testing was immediately suspended.[2]
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. Extra antennae were added to the Gee transmitters to radiate false, unsynchronized signals. A double agent in the Double Cross system reported to German Intelligence a fictional story of hearing a couple of RAF personnel talking carelessly in a hotel about Jay, and one dismissing it as it was "just a copy" of the German Knickebein system. Jones felt this would flatter the Germans, who might consider the information more reliable as a result. Finally, false Knickebein signals were transmitted over Germany.[6] Jones noted all this appealed to his penchant for practical joking.
In spite of these efforts, Jones initially calculated it would be only three months before the Germans would be able to jam the system. As it turns out, it was not until five months into the campaign that jamming was encountered, and longer before it became a serious concern.[7]
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 and 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[8] 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.[9]
The first operational Gee mission took place on the night of 8/9 March 1942 when a force of about 200 aircraft attacked Essen. It was installed on a Wellington of No. 115 Squadron from RAF Watton captained by Pilot Officer Jack Foster who later said "targets were found and bombed as never before".[10] Krupps, 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.[11]
The first completely successful Gee-led attack was carried out on 13/14th March 1942 against Cologne. The leading crews successfully illuminated the target with flares and incendiaries and the bombing was generally accurate. Bomber Command reckoned this attack was 5 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.[11]
In order to provide coverage of the entire UK, three Gee chains were constructed. 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 was widely used for navigation in the UK. Only 1.2% of Gee-equipped aircraft failed to return to their base, as opposed to 3.5% of those without it.[12] Gee was considered so important that a non-working Gee set would ground an aircraft.[13]
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 10 to 20 miles 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 730 miles. This remained the operational record for Gee, with the exception of a freak reception over Gibralter at a range of 1,000 miles.[13]
Counter-jamming efforts had already been considered, and produced the Gee Mk. II. This replaced the original receiver with a new model where the oscillators could be easily removed and swapped out in order 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 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.[14]
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. The first of these 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".[15]
After the end of the war in Europe, Britain planned to send Lancasters to the Japanese theatre as part of 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 Marshall Aitken.
German bombers also used the Gee system for attacks on the UK; captured Gee receivers provided the electronics.[16]
Later in the war Bomber Command desired 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 system was already in use, which sent an interrogation signal from stations in the UK, "reflected" them from transceivers 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 re-use 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 returned 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.
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 the UK occupation zone in northern Germany.[17]
Hyperbolic navigation systems can be divided into two main classes, those that calculate the time difference between two radio pulses, and those that compare the phase difference between two continuous signals. Here we will consider the pulse method only.
Consider two radio transmitters located at a distance of 300 km from each other, which means the radio signal from one will take 1 millisecond to reach the other. One of these stations is equipped with an electronic clock that periodically sends out a trigger signal. When the signal is sent, this station, the "master", sends out its transmission. 1 ms later that signal arrives at the second station, the "slave". This station is equipped with a receiver, and when it sees the signal from the master arrive, it triggers its own transmitter. This ensures that the master and slave send out signals precisely 1 ms apart, without the slave needing to have an accurate timer of its own. In practice, a fixed time typically is added to account for delays in the electronics.[4]
A receiver listening for these signals and displaying them on an oscilloscope will see a series of blips on the display. By measuring the distance between them, the delay between the two signals can be calculated. If that delay is 1 ms, that implies the delay is zero from one station and 1 ms from the other, and that the difference in distance to the two stations is 300 km. Only two locations could meet this condition, the location of the master or slave. If the receiver moves to another location they might measure a delay between the signals of 0.5 ms. This implies that the difference in the distance to the two stations is 150 km. In this case there is an infinite number of locations where that delay could be measured - 75 km from one and 225 from the other, or 150 km from one and 300 from the other, and so on. When plotted on a chart, the collection of possible locations for any given difference forms a hyperbolic curve. The collection of curves for all possible measured delays forms a set of curved radiating lines, centred on the line between the two stations, known as the "baseline".[4]
In order to take a fix, the receiver takes two measurements based on two different stations. The intersections of the two sets of curves normally results in two possible locations. Using some other form of navigation, even dead reckoning, can eliminate one of these possible positions, and thus provide an exact fix. Instead of using two entirely separate pairs of stations, this can be simplified by having a single master and two slaves located some distance away so their patterns overlap. A collection of stations is known as a "chain".[1]
Gee chains used an arrangement with one master and (typically) three slaves. The signals from the master and either of the two slaves would be active at any time, with the third station and a chain monitor station providing backup and signal quality measurements. The transmitters had a power output of about 300 kW and operated in four frequency bands between 20 and 85 MHz.[4]
The Gee signal for any given chain consisted of a series of pulses of radio signal with a roughly inverted-parabolic envelope about 6 microseconds in duration.[18] The master sent a single pulse, referred to as "A", followed 2 ms later by a double pulse, "D". These were used to identify the start and end of the broadcast cycle. The first slave station sent a single pulse 1 ms after the master's single pulse, labeled "B", and the second slave sent a single pulse 1 ms after the master's double pulse, labeled "C".[4] The whole sequence repeated on a 4 ms cycle, with the pattern "ABD... ACD... ABD..."[Notes 2] The triggering of the A pulses was timed at 150 kHz by a local oscillator at the master station,[19] and was subject to drift and sometimes deliberately changed.
In Gee, all of the signals in a given chain were sent on the same frequency, and there was no attempt to modify the pattern of the individual envelopes from the slaves to allow identification. This meant that it would normally be possible to confuse the "ABD" for the "ACD", as they would look identical on the screen. In order to resolve these problems, the master station periodically sent a third pulse, "A prime" or the "ghost A". These were folded into the signal only on the "ACD" cycle, allowing the operator to visually identify the B and C portions of the cycle.[20]
On board the aircraft, the signals from the three stations were received and sent to the oscilloscope display. When first turned on, the operator would see a repeating pattern of blips travelling across the screen. A local oscillator of much less complexity than the one at the master station was used to trigger the display sweep, and when first activated it would be unlikely to have the exact same timing as the master station. A control was used to tune the local oscillator frequency until the blips on the display stopped moving across the screen, which meant the local and master oscillators now had the same timing.[20]
The sweep of the beam across the display was initially set 1/10th the basic frequency of the local oscillator, so approximately five complete signal chains would be seen on the display. Once the operator was happy with the display, a switch was thrown to increase the beam speed 10 times and reducing the amount of the signal display on the line to one complete 2 ms A...D cycle. A fine control could then be used to slightly speed or slow the beam so the cycle exactly filled the screen with the D signal on the right.[20] The time for ten cycles of this 150 kHz oscillation, 66.66 μS was called a Gee unit and corresponded to a range difference of 12.4 miles.[19]
The Gee display also included a second control unit to create a second line on the lower portion of the screen. This was set up in the same fashion, but this time the delay was tuned so that the AA' signal was placed on the extreme left. In this case the upper line displayed the "ABD" signal, and the lower the "ACD". This allowed the two delays to be read simultaneously against the scale, and then looked up on the navigational chart.[20] Once set up, little adjustment was needed to keep the system running, and Gee could be used for long periods.
Signals from different chains were closely spaced in frequency, close enough that the wide-band R1355 receiver would often tune in more than one chain at a time. For station identification, the A' signals were only sent periodically. After the display was stabilized so the pulse trains were appearing in a single location on the screen, the A' pulses could be seen blinking on and off with a set pattern (thus "ghosting" on the display). This allowed the operator to determine the identity of the master signal, and thus select the chain they wanted to use by positioning its associated "ABD" signal on the left.[20]
At long ranges the hyperbolic lines approximate straight lines radiating from the center of the baseline. When two such signals from a single chain are considered, the resulting pattern of lines becomes increasingly parallel as the baseline distance becomes smaller in comparison to the range. Thus at short distances the lines cross at angles close to 90 degrees, and this angle steadily reduces with range. As the accuracy of the fix depends on the crossing angle, all hyperbolic navigation systems grow increasingly inaccurate with increasing range.[21]
When examining an expanded delay, timing was based on 1/10th of a Gee unit, or 6.66 μS. This corresponds to a distance of 1.24 miles. It was assumed that an operator under good conditions could measure the peaks of the pulse envelope within a 1/10th of a calibration mark, or 0.124 miles. This is the basic accuracy of the Gee system, at close ranges and at locations directly perpendicular to the baseline. In practice the accuracy was a function of range from the transmitters, varying roughly with the square of distance.[22] At short ranges accuracies of 165 yards were reported, while at long range over Germany it was quoted to about 1 mile.[20]
The airborne side of the Gee Mk. II system consisted of two portions, the R1355 radio receiver, and the Indicator Unit Type 62 (or 62A) oscilloscope. The two were attached together by two thick cables, one of which carried the signal, and the second carried the power to the oscilloscope, the power supply being built into the receiver to save space on the display side.[23] A "tropicalized" version of the system was also produced, with the R3645 receiver and Indicator Unit Type 299, which moved the later's power supply into the display unit.
The R1355 was designed to allow the radio receiver to be easily swapped out in flight. This allowed the navigator to select different Gee chains in-flight, an operation that took a few minutes. This was also used to avoid jamming, as the Germans would not be aware which signals were actively being used.
Unlike the German beam systems where the bombers flew to their targets along the beam, Gee pulses were radiated in all directions so if detected, they would not reveal the bombers' destinations.[1] As the system was passive, unlike H2S, there were no return signals which could give away the bombers' positions to night fighters. Additionally, this meant all of the aircraft could use the system at the same time.
Gee was highly susceptible to jamming since all the Germans had to do was radiate surplus pulses that made it impossible to determine which was a real signal from the slave and which was being broadcast from a jammer. This could be easily arranged by locating another slave station in France or the Netherlands and modifying its delay and signal strength to appear to be similar to one of the stations in the UK. However, this was suitable for use only over Germany; when flying over the UK the signal would appear too weak and/or from the wrong direction. Using conventional radio receivers and loop antennas for direction finding, radio operators could determine which of the signals was false.
In the case of Gee-H, the use of the system was changed only slightly. Instead of the sweep being timed by the local oscillator in the display unit, the trigger signal was instead sent from the onboard transmitter. The signal was also sent out and reflected from the distant ground stations, and received on the existing Gee receiver for display. In theory this could be used to calculate a fix in exactly the same fashion as Gee, but using different charts. However, navigating to a target using such a system would be complex, similar to dead reckoning, with periodic fixes needed to update the windage calculations.
Instead, Gee-H was used in a fashion similar to the earlier Oboe system. The navigator would first pick a station to use as the "cat" signal, the main navigation beacon. The range from the cat station to the target was calculated, and then the signal delay at that range. During the mission, the navigator would dial in the desired range to the transmitter, which would send out an "A"-like pulse at the set timing. The received pulse from the "cat" station would be displayed on the same trace. By instructing the pilot to turn left or right, the navigator would guide the bomber until the two traces were precisely overlapped, meaning that the bomber was flying a precise distance from the station, along a circular arc that would take them over the aim point. The signal received from the second station, "mouse", was likewise set up for display on the lower trace. In this case the distance would continue to change as the aircraft flew along the arc of the "cat" station. When this signal overlapped the pre-set range from "mouse", the payload was dropped.
Using this method of operation greatly reduced the workload for the navigator. For much of the mission he simply had to keep the blips on the upper trace aligned on the display, and then periodically watch the lower blips for timing. Additionally, due to the measurements always being measured as direct lines from the station, as opposed to hyperbolic curves, the accuracy fell off linearly instead of with the square of distance. Gee-H could this guide the bomber to within 120 yards over Germany, a dramatic improvement over Gee's approximately 1 mile accuracy at the same distance.
The Eastern chain operated from 22 June 1942. Virginia[24] 48.75 MHz[24]
Virginia 48.75 MHz. Eastern and Southern (Virginia) chains could not operate simultaneously
Carolina 44.90 MHz Used by Coastal Command and Combined Operations [24] Same Master and Slave sites
Carolina 44.90 MHz. Used by Combined Operations.
The Northern Gee chain operated from late 1942 until March 1946.
Worth Matravers was used after the war as a training base for Gee operators.
Operational 18 April 1944
A Western chain was planned but cancelled
Operational for about six months in 1945
Planned
Operational
There was another chain Indiana using 46.79 MHz but it was not in use by 1943.
An emergency frequency (XF) of 50.5 MHz, codeword Zanesville, was allocated.
Operational 5 October 1944. 83.5 MHz
Operational 9 October 1944, replace by heavy mobile equipment 23 October 1944 and became the Ruhr Chain.
80.5 MHz (?)
Operational 21 March 1945 using light Type 100 units. Replaced with heavy units from the Rheims Chain. 50.5 MHz
Planned but decided that no longer needed though decided to go ahead as part of the post-war Gee organisation. Operational 26 April 1945
Proposed
Deployed as
After World War II the Gee system was used as a navigational aid for civil aviation though mainly from new sites.
Post-World War II the RAF re-sited two of the three wartime Gee chains in England. Eastern and South Western chains (four stations each) and Southern chain of three stations. The Southern chain became a four station London chain and Eastern chain became a Midland chain. This was planned for 1948.[26]
This continued post-World War II using existing sites, two on the North coast of Scotland, one North of Aberdeen and one in the Shetlands.[26]
Opened around 1948 and closed early 1969.[27]
A chain of Gee stations was opened after the war in North Germany. Stations were at Winterberg, Ibug, Nordhorn and Uchte.
There were several stations during the 1955-1959 period that appeared to be more of a deception than really operational. They were 550 SU at Fort Spijkerboor outside of Pumerand, Holland; 889 SU at Eckendforde in North Germany; and 330 SU outside of Ingolstadt in Bavaria, Germany. These stations were rarely if ever operation in the late 1950s.
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