Gee-H (navigation)
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Gee-H, sometimes written G-H or GEE-H, was a
radio navigation Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination. The basic principles a ...
system developed by
Britain Britain most often refers to: * The United Kingdom, a sovereign state in Europe comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands * Great Britain, the largest island in the United King ...
during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
to aid
RAF Bomber Command RAF Bomber Command controlled the Royal Air Force's bomber forces from 1936 to 1968. Along with the United States Army Air Forces, it played the central role in the strategic bombing of Germany in World War II. From 1942 onward, the British bo ...
. The name 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. Its official name was AMES Type 100. Gee-H was used to supplant the
Oboe The oboe ( ) is a type of double reed woodwind instrument. Oboes are usually made of wood, but may also be made of synthetic materials, such as plastic, resin, or hybrid composites. The most common oboe plays in the treble or soprano range. A ...
bombing system which worked along similar lines. By measuring and keeping a fixed distance to a radio station, the
bomber A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching aerial torpedo, torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped ...
could navigate along an arc in the sky. The bombs were dropped when they reached a set distance from a second station. The main difference between Oboe and Gee-H was the location of the equipment; Oboe used large displays in ground stations to take very accurate measurements but could only direct one aircraft at a time. Gee-H used much smaller systems onboard aircraft and while somewhat less accurate, could direct as many as 80 aircraft at a time. Gee-H entered service in October 1943 and first used successfully in November against the
Mannesmann Mannesmann was a German industrial conglomerate. It was originally established as a manufacturer of steel pipes in 1890 under the name "Deutsch-Österreichische Mannesmannröhren-Werke AG". (Loosely translated: "German-Austrian Mannesmann pi ...
steel works at
Düsseldorf Düsseldorf ( , , ; often in English sources; Low Franconian and Ripuarian: ''Düsseldörp'' ; archaic nl, Dusseldorp ) is the capital city of North Rhine-Westphalia, the most populous state of Germany. It is the second-largest city in th ...
on the night of 1/2 November, when about half of the sets failed leaving only 15 aircraft to bomb the factory. 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 The English Electric Canberra is a British first-generation, jet-powered medium bomber. It was developed by English Electric during the mid- to late 1940s in response to a 1944 Air Ministry requirement for a successor to the wartime de Havil ...
. Gee-H was adapted by
RCA The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
into the US wartime
SHORAN SHORAN is an acronym for SHOrt RAnge Navigation, a type of electronic navigation and bombing system using a precision radar beacon. It was developed during World War II and the first stations were set up in Europe as the war was ending, and was op ...
system with improved accuracy. The same basic concept remains in widespread use today as the civilian DME system.


Development history


Distance measuring navigation

Determining your location in 2D space requires two measurements of angle or range - two angle measurements, two distance measurements, or one angle and one distance. Early radio navigation was typically based on taking two angle measurements using
radio direction finder Direction finding (DF), or radio direction finding (RDF), isin accordance with International Telecommunication Union (ITU)defined as radio location that uses the reception of radio waves to determine the direction in which a radio station ...
s, but these were accurate only to a few degrees and provided limited accuracy on the order of tens of miles. The development of range-based systems had to wait until the invention of accurate time measurement of radio signals were possible, which came about as a result of the development of
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
. The
Luftwaffe The ''Luftwaffe'' () was the aerial-warfare branch of the German ''Wehrmacht'' before and during World War II. Germany's military air arms during World War I, the ''Luftstreitkräfte'' of the Imperial Army and the '' Marine-Fliegerabtei ...
pioneered the use of distance-measuring radio navigation systems with their
Y-Gerät The Battle of the Beams was a period early in the Second World War when bombers of the German Air Force (''Luftwaffe'') used a number of increasingly accurate systems of radio navigation for night bombing in the United Kingdom. British scientific ...
system in 1941. Y-Gerät used a single
Knickebein The Battle of the Beams was a period early in the Second World War when bombers of the German Air Force (''Luftwaffe'') used a number of increasingly accurate systems of radio navigation for night bombing in the United Kingdom. British scientific ...
-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 An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
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. The downside to the beam-type system of navigation is that the beams cannot be focused perfectly and in practice are fan-shaped, growing wider with increasing distance from the broadcaster. This means they have decreasing accuracy with increasing range. Measurements of distance are dependent only on the accuracy of the equipment, and are independent of range. This means their accuracy is a fixed percentage of the measurement, and so is linear with range. It is possible to use two such measurements to provide a location ''fix'', but such systems are generally difficult to use, as they require two range measurements to be made in quick succession, while the aircraft is moving.


Oboe

The
Air Ministry The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of State ...
developed a distance-measuring system known as
Oboe The oboe ( ) is a type of double reed woodwind instrument. Oboes are usually made of wood, but may also be made of synthetic materials, such as plastic, resin, or hybrid composites. The most common oboe plays in the treble or soprano range. A ...
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 and an arc of that radius drawn on a conventional navigation chart. For instance, for an attack on a target in
Düsseldorf Düsseldorf ( , , ; often in English sources; Low Franconian and Ripuarian: ''Düsseldörp'' ; archaic nl, Dusseldorp ) is the capital city of North Rhine-Westphalia, the most populous state of Germany. It is the second-largest city in th ...
, the distance between the Oboe station near Walmer and the target would be about ; an arc with a radius around the station would be drawn, passing through Düsseldorf. Now the "range" of the bombs being dropped would be calculated, the distance between the point where the bombs are released and the point that they hit. For missions around altitude, range is typically on the order of for a high-speed aircraft like the
de Havilland Mosquito The de Havilland DH.98 Mosquito is a British twin-engined, shoulder-winged, multirole combat aircraft, introduced during the Second World War. Unusual in that its frame was constructed mostly of wood, it was nicknamed the "Wooden Wonder", or ...
. The planners would calculate the place along the arc where the bombs would need to be dropped to hit the target. 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 radius curve. During the mission, the bomber crew would fly to one end of the arc using any means of navigation including dead reckoning. When they were near the location, the transponder was activated and the Oboe station would measure their distance. 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". Operators would watch the position of the aircraft, sending out correcting signals as needed so the pilot could adjust the path along the arc. A second station would also measure the distance to the bomber. This station was equipped with the bomb's range value calculated earlier and had used this to calculate the distance between their station and the bomber at the point where the bombs should be dropped. When this ''mouse'' station saw the bomber approaching the drop point, it sent
Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
signals to inform the pilot that the drop point was approaching. At the right moment, it would send another morse signal that would drop the bombs automatically. The main constraint with Oboe was that it could only be used by one aircraft at a time. 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. 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. Oboe was sometimes used for attacks on point targets by one or a small number of aircraft dropping one after the other. In tests, Oboe demonstrated accuracies greater than those of optical bombsights during daylight in good weather.


A new approach

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. One solution to this problem is to have each Oboe station send out a slightly different signal, normally by changing the
envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card. Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a shor ...
of the signal it broadcasts to the aircraft. Similar stations with different signal modifications can be situated around the UK, so that all of them are visible to an aircraft over Germany. An aircraft that turns on its transponder will receive and re-transmit signals from all of them. Although all of the ground stations will receive all of the signals, they can pick out their own by looking for their unique signal. This change allows many Oboe stations to be operational at the same time, although it does not help the situation if more than one aircraft turns on their transponder. Now consider reversing the location of the
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
s and receivers, so that the receiver is on the aircraft and the transmitter on the ground. Now every aircraft generates a different signal pattern, and the operators on the aircraft can look for their own signal and ignore the others. Any number of aircraft can use the same station at the same time. 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 very same time is low. This is the basic concept behind Gee-H.


Gee-H

The first radio navigation system to be operated by Bomber Command was Gee. 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. The timing between transmissions was not fixed and varied from station to station, so the equipment in the bomber had a system that allowed it to adjust for this. The receiver had a local oscillator that provided a
time base generator A time base generator (also timebase or time base) is a special type of function generator, an electronic circuit that generates a varying voltage to produce a particular waveform. Time base generators produce very high frequency sawtooth waves spec ...
that could be adjusted. 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 pulse frequency as that in the ground station. The receiver had two complete systems of this type, allowing the operator to receive signals from two stations and easily compare them and make simultaneous measurements. In order rapidly to deploy the new design, it was decided to use as much of the Gee equipment as possible. Gee already included the oscilloscope display and the receiver unit, so all that was needed was a 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 used. 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, instead of using Gee's manually tuned oscillator. This way, the received signals that did not have the same inter-pulse timing would appear to move one way or the other, exactly like a mistuned Gee. Only the signals originating from the aircraft's own transmitter would line up on the display and remain motionless. This deliberate adjustment of the timing was known as "jittering". The delay from the original Gee was still used; 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 inverted and sent to the display. The navigator could then direct the pilot onto the right path by giving directions until the upper and lower blips aligned. The same was done for the second channel, setting it to the computed range where the bombs should be dropped. Since they stayed the same distance from one station the operator only had to check that periodically, while watching the ever-moving lower trace as the active blip moved slowly along the display towards the timer blip until they overlapped and the bombs were dropped. 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, a transceiver would be busy for 10 ms out of every second responding to the signals from any one aircraft. This would leave 990 ms 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 a station at a time. The system had the additional advantage that each aircraft selected its timing, which made jamming harder. 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. By the late war period, Gee was generally useless for bombing and used primarily as a navigational aid when returning to England. In the case of Gee-H, each aircraft had unique timing; to jam the receiver, the jammer would also have to have similar timing. As one signal might be used by dozens of aircraft, dozens of jammers set to slightly different times would be needed. As there were dozens of transceivers as well, many unused decoy signals, the magnitude of the jamming problem was considerably more difficult. As the Gee-H system used Gee equipment, turning off the interrogation transmitter turned it back into a normal Gee unit. On a typical mission, the set would be used for Gee while leaving England and forming up into a
bomber stream The bomber stream was a saturation attack tactic developed by the Royal Air Force (RAF) Bomber Command to overwhelm the nighttime German aerial defences of the Kammhuber Line during World War II. The Kammhuber Line consisted of three layers of ...
, for Gee-H during the mission and back to Gee on the return flight for finding its airbase. Since Gee could be directly read on a map, it was extremely useful for general navigation, while Gee-H was only practically used to navigate to one place. Gee-H's main fault was also a side-effect of the use of Gee equipment; using a higher frequency would allow a tighter envelope, which would allow more accurate timing measurements and thus improve accuracy. Because the system used Gee's small oscilloscope for measurements, it did not have the same visual accuracy as Oboe, which used 12-inch oscilloscopes developed specifically for this purpose. Gee-H achieved accuracy of about 150 yards at 300 miles, while Oboe was good to about 50 yards. 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 Operations Taxable, Glimmer and Big Drum were tactical military deceptions conducted on 6 June 1944 in support of the Allied landings in Normandy. The operations formed the naval component of Operation Bodyguard, a wider series of tactical a ...
, a diversionary "attack" during
Operation Overlord Operation Overlord was the codename for the Battle of Normandy, the Allies of World War II, Allied operation that launched the successful invasion of German-occupied Western Front (World War II), Western Europe during World War II. The operat ...
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, to deceive the German radars that they were the main invasion fleet.Freeman Dyson
"A Failure of Intelligence"
Bomber Command OR


Air Ministry designations

* Gee-H Mk. I - Airborne Radio Installation (ARI) 5525 * Gee-H Mk. II - ARI 5597 * Gee-H Mk. II (Tropicalised) - ARI 5696


See also

* Battle of the Beams *
Lorenz beam The Lorenz beam was a blind-landing radio navigation system developed by C. Lorenz AG in Berlin. The first system had been installed in 1932 at Berlin-Tempelhof Central Airport, followed by Dübendorf in Switzerland (1934) and others all over the ...
*
Oboe (navigation) Oboe was a British bomb aiming system developed to allow their aircraft to bomb targets accurately in any type of weather, day or night. Oboe coupled radar tracking with radio transponder technology. The guidance system used two well separated ...
* List of World War II electronic warfare equipment


References


Citations


Bibliography

* * * ** Adapted from * * {{RAF WWII Strategic Bombing Radio navigation World War II British electronics Military equipment introduced from 1940 to 1944