User:Gunbirddriver/Oboe (navigation)

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Oboe was used both as a marker service for the Main Force and for the precision bombing of high value targets by guidance direction of individual aircraft. It was by far the most accurate bombing system used during the war.

Background 1
Since being pushed off the continent, the weight of the British effort against Germany was being carried by Bomber Command. They had been attacking German industry and cities at night since the spring of 1940. However their efforts came at a high price, and had shown little in the way of results. Though British scientists had made substantial advancements to radar technology in primary radar for the Chain Home early warning system of Radio direction finders, or RDF, little work had been done to advance the use of radar as a guidance system for bomber aircraft. Through the course of 1941 England lagged behind Germany in guidence technologies. The release of the Butt report in August 1941 confirmed that Bomber Command pilots were largely unable to find their targets in the dark of night. Both the Royal Navy and the British Army were arguing to be given control of the aircraft of Bomber Command, and the future of the service was in question.

To address this state of affairs, Arthur Harris was made AOC of Bomber Command. His appointment instilled a new discipline and sense of purpose to his command, but the chief question remained how to make Bomber Command an effective force. The bombers had to be able to find their targets, and the means would be through improved navigation. The two main efforts toward this end was the formation of a target finding force to guide the Main Force to the target, and the development of electronic aids to help in navigation.

The first electronic navigational aid to be developed and put into operation was GEE. GEE used three timed signals that allowed the navigator on the bomber to determine their location. GEE stood for "grid", that is an electronic grid of latitude and longitude generated from the combination of the three signals. GEE was accurate on the order of kilometers, which was extremely useful for navigation and area bombing. Within several months the Germans developed jamming methods, which markedly limited GEE's effectiveness over the continent. However, heading to a target GEE would still allow a navigator to be certain of his entry point over the continent. Returning from missions, it proved very useful in stormy and foggy weather in helping crews to find their way home by confirming their aircraft's position over the North Sea, the Channel, and over England itself.

The Pathfinder Force was formed in August 1942, which in itself was a step in the improvement of the effectiveness of the bomber force. Harris placed Don Bennett in command. Like Syd Bufton, Bennett knew the problem of bomber crews finding targets over Germany at night was primarily a problem in navigation. Bennett concentrated on improving navigational skills, with intensified training of Pathfinder navigators stressing position estimations using a wind triangle combined with dead reckoning from fixed point to fixed point (weather permitting), calculating in wind speed, air speed and elapsed time. The navigator would cross check his estimates with celestial navigation using a sextant through an astrodome. Though useful, these methods had significant limitations over long distances, causing Bennett to press for the development of new electronic navigational aids.

Current article materials
The curved arc segment that appeared on the displays of German radar operators led them to call the system “der Bumerang" or boomerang, after the Australian throwing stick.

In order to accurately determine your location relative to objects on the ground you need two data points; two angles (as in triangulation), two distances (trilateration), or an angle and a distance (VOR/DME). Using radio to provide some or all of these measurements was an area of continual development leading up to the start of the war. The Germans pioneered this approach with operational systems like Lorenz beam and X-Gerät that used two narrow beam-like signals that crossed at a point in the sky to indicate a target. Later during The Blitz, the Germans introduced Y-Gerät, which combined a single Lorenz beam with a transponder-based distance measurement to fix locations. The problem with all of these systems was that they produced no information except within their narrow beams, and were not useful for general purpose navigation.

A more useful system was introduced in the RAF's Gee system, which used two timed signals that allowed the navigator on the bomber to determine their location. It could be used anywhere within line-of-sight of the transmitter stations in the UK, and generally provided a reasonable signal up to about 500 km, depending on the aircraft's altitude. Gee was read on an oscilloscope display about 3 inch across, which limited the accuracy of the timing measurements. As a result, Gee was accurate on the order of kilometers, which was extremely useful for navigation and area bombing, but did not provide the accuracy needed for pinpoint bombing.

As the accuracy of Gee was largely due to the mechanical size of the indicator unit, the accuracy could be improved by using a larger display. However, in these early days of the cathode ray tube (CRT), such displays were extremely expensive and very long, which made them unsuitable for fitting to a large number of Bomber Command aircraft.

Initial proposal
The concept of reversing the display arrangement so the display could be on the ground and the transmitter on the aircraft was obvious. It had first been suggested by Alec Reeves of Standard Telephones and Cables in 1940 and then formally presented with the help of Francis Jones in the spring of 1941.

The basic idea would be to have two ground stations that would periodically send out signals on similar but separate frequencies. The aircraft carried transponders, one for each signal, which re-broadcast the signals upon reception. By timing the total round trip time from broadcast to reception and then dividing by twice the speed of light (the signal travels to the aircraft and back again) the distance to the aircraft could be determined. This was essentially identical to radar, with the exception that the transponder greatly amplified the signals for the return journey, which aided accuracy by providing strong, sharply defined signal pulses.

A practical problem was using these range measurements to guide a bomber towards its target. In the case of Y-Gerät, a single beam was used that produced a natural path for the bomber to fly along. Only the range along this path needed to be measured and relayed to the bomber crew. In the case of a system using two range measurements, there was no inherent path in the sky for the aircraft to follow. Locations and directions could be determined by the phoning the two range measurements to a plotting room, drawing arcs from the stations at those measured distances, and then locating the intersection. However, this took time, during which the aircraft moved, making it too slow to provide the desired accuracy.

Oboe adopted a simple solution to this problem. Before the mission, a path was defined that represented the arc of a circle whose radius passed through the target as measured from one of the two stations. This station was given the name "Cat". The aircraft would then use conventional navigation techniques, dead reckoning or Gee if it was equipped, to place itself some distance north or south of the target on a point near this line. They would then begin flying towards the target, at which point an operator at Cat would call out corrections to have the aircraft fly closer or further from the station until it was flying at precisely the right range.

The Cat station continued to keep the aircraft positioned at this precise distance as it flew towards the target, causing the aircraft to fly along the pre-defined arc. The second station, code-named "Mouse", also calculated the range to the target before the mission. As the bomber approached that predetermined range they would first call out a "heads up" to tell the bomb aimer to begin the run, and then a second signal at the right time to drop it. Using this method there was no need for the two stations to compare measurements or perform any trigonometry to determine an actual location in space, both performed simple range measurements directly off their screen and sent their separate corrections to the aircraft.

In practice, ranges were not sent by voice to the aircraft. Instead, a tone generator produced Morse code dots or dashes under the control of the operators. This was similar to the beam systems like Lorenz, which the UK aircrew were already familiar with using as a blind landing aid in the pre-war period. If the aircraft was too close to the station the operator would play the dot signal, and when they were too far, dashes. The two could be mixed so that as they approached the correct range, the signal would slowly turn into a steady tone. Periodically the signal would be keyed to send out a letter to indicate how far they were from the correct range, X indicating 20 miles, Y 10 miles, and Z 5 miles. Likewise the Mouse station sent a series of keyed signals to indicate the approach, S to indicate the approach was starting, and then A, B, C and D as the aircraft approached.

Development and testing
The Oboe blind bombing system was designed by Alec Reeves at the Royal Aircraft Establishment working with Frank Jones at TRE. Oboe transponders were fitted to Mosquitoes of 109 Squadron, which developed the use of the device as part of the Pathfinder Force. The Mosquito was promoted for use with Oboe because the aircraft could reach altitudes over 30,000 feet, extending the effective range of the system. In addition, the use of Oboe required the aircraft to fly for 10 minutes on a straight and level course. Speed and altitude were essential to avoid being intercepted. The airbourne Oboe equipment were carried in three black boxes, which were fitted into the nose of the Mosquito.

There were two main problem with the use of Oboe. The main problem was limited capacity. Each pair of stations could only guide two aircraft at any given time, with each station acting as Cat for one aircraft and as Mouse for the other. Compared to the electronic grid system created with Gee, where any bomber could pick up the signals from the UK and carry out the necessary calculations.

It was used for target marking by flying one aircraft under control and having it drop flares for the following aircraft to drop on. But a more worrying concern was that the bomber aircraft would have to fly straight and level along a gently curving path while the ground operators determined its range and radioed it corrections. During this time the aircraft would be open to attack, which some considered to be almost suicidal. And finally, it was pointed out that as the British had so easily jammed Y-Gerät, even before it was widely used, there was no reason to expect the Germans would not do the same as soon as they detected the signals.

Despite widespread opposition to the use of Oboe, A.P. Rowe ordered development to begin. Development began both on the 1.5 m wavelength shared by most early UK radar systems, but also at the new "fashionable" 10 cm microwave wavelength provided by the cavity magnetron. The latter would not only provide higher accuracy, but also be largely immune to jamming unless the Germans developed their own high-power microwave devices, which only occurred in the very last days of the war.

Two stations were set up as far east as possible, one in Dover (Walmer) and a second in Cromer (RAF Trimingham)). On any given mission, one of the stations would be Cat and the other Mouse. In early testing in September 1941, an aircraft flying along the arc 130 km from Dover demonstrated an accuracy of 50 m, better than any bombing method then in use. Accuracy with bombs was not quite as good, as the bombs themselves were not identical and had slightly different trajectories. In a demonstration for senior officials on 2 July 1942, the system demonstrated a real-world accuracy of 65 m.

Oboe was first used in experimental operations by Short Stirling heavy bombers in December 1941, attacking Brest. These aircraft had a relatively limited service ceiling, and was limited to attacks at short-range where they maintained line-of-sight to the UK.

At that time there was a great debate taking place in Bomber Command over the use of "pathfinders", specialized aircraft and crews that would find the targets and use flares to mark them for attack. The same technique had first been used during the Blitz by the Germans, notably by the specialist Kampfgeschwader 100, but its effectiveness had been severely curtailed by British jamming efforts. Nevertheless, the concept had enough backing that a small force of Mosquitoes had been organized to operate as a pathfinder force, using normal optical sighting. This proved disappointing in practice, offering only a slight improvement in accuracy.

But the Mosquitoes were also the only aircraft that had the performance to fly at altitudes at which the Oboe signals could be received over Germany. At a meeting in the summer of 1942, it was agreed that the pathfinder Mosquitoes would be equipped with Oboe. Having faced opposition before, the addition of Oboe upset the argument against the specialist role, and what would become Pathfinder Force began forming over the ongoing objections.

Development 2
Among the five original squadrons assigned to the Pathfinder Force was 109 Squadron. This unit had previously been involved in the Battle of the Beams, reading German beam pulses and locating German radar stations. In April they began working with scientists at Malvern in developing a new blind bombing system. The system was called Oboe, after the musical instrument, the Oboe.

The idea for Oboe was developed at the Telecommunications Research Establishment (TRE) at Malvern. Alec Reeves was an engineer from Standard Telephone who was made director of TRE. Discussing the problem with physicist F.E. Jones, Reeves sketched out his idea on a napkin. Beam development The key was to mount a responder in the aircraft being radar guided. Typical use of radar at the time was with a reflector system, as used in H2S or in Fighter Command's Chain Home early warning radar system. In Oboe a pair of ground stations would emit trains of radar pulses, making use of the same carrier frequency but with differing recurrence frequencies. The aircraft would carry a responder, which would pick up the radar beam signals from the ground stations, delay the signal and then transmit them back. With the internal delay in the set of the target aircraft known, each station could then determine the range to the aircraft by the time it took to receive back their interrogation transmission. In addition, the train of pulses from the tracking stations were automatically modulated with a Morse signal to convey range information to the aircraft. If the aircraft was too near the ground station a series of dots were sent. If the aircraft was off range by being too far away a series of dashes were sent. When on track the pilot would hear a steady tone. The result was the creation before the aircraft of an imaginary curving beam with a width of about 50 feet. Following the gentle circle of the beam would lead the aircraft directly over the target.

For a target in the Ruhr valley, the system was accurate to within 65 feet.

Work point
CAT and Mouse

The precise range of the target from the CAT station was set up on the cathode ray tube by a strobe marker. The purpose of the CAT was to keep the returned pulse exactly in line with the target marker strobe. It did this by automatically signaling dots or dashes to the bomber; dots if the Mosquito was flying too close to the CAT station and thus was at a range less than the target range, and dashes if it was flying out past the range of the target. The dots and dashes would merge into a continuous tone, the 'equi-signal' tone, when the Mosquito was flying along at the correct range. The dot or dash signal was transmitted by width-modulation of the primary interrogating pulse. The reply transmissions from the aircraft were sent in code to ensure the signal communication was with the correct aircraft.

The sensitivity of the system was such that a deviation of 17 yards from the circular target arc would result in either dots or dashes being transmitted, enabling the pilot to steer along an invisible track some 35 yards wide in the sky. This is less than the wingspan of the Mosquito. To stay in the path the pilot had to concentrate very closely on the tones coming through his earpiece. The sound that was created was similar to the sound of the reed instrument, giving the device its name.


 * the use of Oboe was worked out at Malvern by 109 Squadron from August to December 1942.

The Oboe sets mounted in the Mosquitos were all made on sight in their own workshops. There was no outsourcing of the building of this equipment to industry. The Oboe sets were individually built by TRE at Malvern. They were not manufactured by industry. All the sets were built at Malvern on breadboards.

109 Squadron

 * 109 Squadron had been born out of the Wireless Intelligence Development Unit, which was re-designated 109 Squadron at Boscombe Down in December 1940 and tasked to identify the enemy’s use of radio beams and to develop radar aids for the RAF. In April 1942 it began work on the Oboe.
 * The squadron came together again at Stradishall with the main purpose of developing Oboe, a blind bombing device which would revolutionise bombing from March 1943 in the hands of 105 and 109 Mosquito Squadrons of the Pathfinder Force.
 * Since becoming a founder member of the Pathfinder Force in August 1942, 109 Squadron had been engaged in the pioneering work with the Oboe blind-bombing device. With the device fitted to the Mosquito, the squadron worked out the problems in the use of Oboe.

The Mosquito arrives
When work with early Oboe began in the summer of 1942 109 Squadron was using the Vickers Wellington. This aircraft could only reach 18,000 feet of altitude. To extend the range of Oboe it was hoped that it would be used in a future aircraft at much higher altitude. To test the machine under the lower pressures of high altitude a ship's boiler was placed inside a Wellington. Once in the air the boiler chamber was decompressed to mimic the low pressure seen at high altitude. When PFF AOC Don Bennett arrived to review the work being done the CO of 109 Squadron, C.C. McMullen, informed Bennett there was an airplane de Havilland was testing at their airfield at Hatfield that he should consider. Bennett called upon them. R.E. Bishop, the Mosquito's designer met with Bennett and told him the Mosquito could reach 28,000 feet and fly over 400 mph, and asked if that would that due? Bennett replied "Yes", and immediately began working to get the Mosquito released to his Oboe development unit.

The Mosquito was known at the Air Ministry as Freeman's Folly, after the aircraft's chief proponent, Air Chief Marshall Wilfrid Freeman. It was assumed that a wooden air frame had no place in a modern air force, and a bomber with no defensive armament was unlikely to be survivable. Bennett demanded the aircraft for his Pathfinder Force, but was told by the Air Ministry that it would not work. The Mosquito was said to be unsuitable, as its crew was limited to a pilot and a navigator, the plane could not be operated at night due to the large exhaust flames the engines put out, and its wooden construction made it unsuitable in the modern air combat environment. Bennett was able to counter these arguments, as he had been operating the aircraft on his own for some time, and had his engineer form exhaust masks to cover the plane's tell tail doing it every night. The Mosquito was the making of Oboe, because it was able to reach a height that could be guided from England.

Some dismay that there was no defensive armament. Aircraft had the ability to reach an altitude over 30,000 feet, which extended the effective range of Oboe.


 * the Mosquito with the Pathfinders began in August 1942. In that month No. 109 Squadron, a special duties squadron that had been working on the new Oboe radar navigation system, was transferred to the Path Finder Force (No. 8 (PFF) Group from January 1943). At the same time the squadron received Mosquito B. Mk IVs.

To equip a Mosquito with Oboe, three large pieces of electronic equipment had to be mounted in the nose of the aircraft. The pressurized transmitter was located to the rear of the bomb-bay, with a rotating box antenna below it.

Problems overcome

 * Altitude posed a problem, resulting in equipment failures that had to be worked out. Initially 109 did this by taking a Wellington and placing a ship's boiler inside to get a low pressure working environment to prove that it would operate adequately at high altitude. One such problem was arcing between the high voltage components of the sets.
 * other problems that the mechanics had to address would be equipment failure in flight. Sets would work fine on the ground and then not receive in the air. This required a mechanic going up in the air in the navigator seat and bending forward to get at the equipment. Could be a loose wire, a failure in one of the radio tubes, or a weak or dry soldering point.
 * Oboe use was interrupted by other transmissions. In particular, the use of radar by anti-aircraft units disrupted the reception of the Oboe radar

A team of aircraftsmen worked on the aircraft to modify and keep the equipment running and modify the aircraft for the equipment. As the aircraft was the wood framed Mosquito, carpenters and cabinet makers were employed on the teams to modify the Mosquito to fit the three black boxes that contained the airbourne Oboe components. The black boxes were fit into the nose of the aircraft.

Testing and training the crews
The Oboe device was tested on bomb ranges, initially in clear weather and then through a heavy cloud cover. The results were the same. The only variance was the error of the bomb fall.


 * Training was needed to develop the pilot's ability to "fly the beam", though no real beam existed to be flown.

Oboe operations
To complete an Oboe operation, a single aircraft would fly to a point in space 10 minutes flight from the target. This space was called the xyz, which the aircraft would have to reach be accurate navigation on their own. From there they would turn on their responder(?) and look for signal.

is carefully guided to a precise point in the sky up to 300 miles away by a dedicated ground-based team. At a CAT station, or tracking station, a team was made up of a controller, two radar operators and a radar mechanic. The controllers were all ex-aircrew who had completed at least one tour. The radar operators were usually WAAFs, who tracked the range and bearing of the aircraft and plotted a track for the operation on a plotting board. The team was under the control of an officer.
 * you had to maintain a steady speed and a steady altitude, while listening to the station transmissions, trying to keep in the tone, the width of which was less than the wingspan of a Mosquito. It required a tremendous amount of concentration. Most pilots lowered the seat, to help them concentrate on their instruments, the controls and the signals coming to them in their ears.
 * "All this time your ears would be tuned to the dots and dashes, waiting for them to merge in response to the control you exerted over the arcraft, so that you knew you were making an accurate course. Once on track, you had to hold whatever speed was required; also you had to maintain whatever height was necessary. The concentration was terrific, absolutely tremendous." - 'Slim' Somerville
 * When the navigator pressed the bomb release button the transponder in the aircraft would switch off and the aircraft's blip would disappear from the screens of the CAT and MOUSE stations.

Trial missions
Oboe had been shown to be effective over bombing ranges in England in all weathers, but its use over occupied territory brought new questions. Would the equipment hold up under the defensive fire of German gunners, and would the sets still work over the greater distances?
 * To try out Oboe against real targets meant 20 December 1942 first Op for Oboe. It was a practice target.
 * Target was to be a virgin target that had no prior cratering from previous attacks. It was to be a near target to minimize the trouble getting to and away from the target, and the ability to have people on the ground access the damage.
 * The choice was a power station in Lutterade, Belgium near the Dutch-German border.
 * Six Mosquitos sent to bomb the target, each carrying three 500 pound bombs. Three of the aircraft sent suffered equipment failure and were unable to bomb the target.
 * When it came time to access the accuracy of the bombing, a number of problems arose. Photo recon of the target taking the next day revealed the target was in the middle of a broad cratering, this the result of a bombing of Aachen undergone in bad weather,gone wrong and dropped short.


 * Oboe was mounted primarily in the Mosquito aircraft. It was the only aircraft that could fly above 30,000 feet, the altitude necessary to maintain radar contact at 300 miles. In addition, the Mosquitos was less likely to fall into enemy hands. In July 1944 Oboe was mounted to a Lancaster, but this experiment was abandoned a few months later, when on 23 December 1944 Oboe equipped Lancasters were used to mark a target. As they made their approach flight along the beam four of the Lancasters were shot down. The Lancaster's lower speed, lower altitude combined with the necessity to fly level and nearly straight for 10 minutes made them vulnerable to Flak and night fighters. After this disaster the Lancaster was never used again to carry an Oboe guidance system.
 * Initial use - one Mosquito would take off for its mission every 30 minutes.


 * To obtain the correct distance to target, the coordinate problems had to be worked out. This required some work, as the coordinate system in European countries varied, and they all were different than what was used in England. Dr. Calderwood of the Geographical Section of the General Staff was contacted, and a set of complex calculations were used to create a map of the continent that brought European coordinates into conformity with the British system.

Oboe operations 3
Two Oboe stations were needed to provide the beams to a single aircraft. The transponder received radar pulses from the two stations and re-transmitted them back to the stations. The transponder in the aircraft picked up the radar pulses from the stations, delayed them slightly and transmitted them back. This allowed the aircraft with the transponder to separate itself out from all other aircraft in the air. The aircraft would appear as a blip on the screen at the tracking stations. The station could determine the distance of the aircraft from the station. The two stations emitted a series of radar pulses to the target aircraft. The pulse width could be made short or long, and the controllers would change the pulse depending on whether or not the aircraft was flying along the line of approach. This was heard in the ear piece of both the pilot and navigator as a dot or dash, as one might receive in Morse. Thus the pilot and navigator on board the aircraft being tracked received a tone transmitted from the Cat station as to whether they were too near, too far or along the track of approach to the target. Dots meant they were too near, dashes meant they were too far, and a solid tone indicated they were on the track to the target. To the pilot, the effect was that before him was seemingly a beam of area that he could fly along that was slightly curving and would bring him right on track over the target. The radar pulses operated on the range from two ground stations in England across to the target. The signals went 'line of sight', and did not follow the curvature of the earth, so the further the target, the higher one needed to be. The time on the bombing run was ten minutes on a slightly curved track, as it was a system based  on range. The track to the target was extended backwards for a further five minutes. This was known as the "waiting point", but one did not actually wait there – it meant waiting for one's call-in signal. So, at zero minus 15 minutes, one turned onto the track to the target and switched on the Oboe receiver and listened out. When one heard the call-in signal we switched on the Oboe transmitter and began to receive signals from the ground station. 'The signals were heard by both pilot and navigator, and were used to track the aircraft over the target. If inside the correct line, dots were heard. If outside the line, dashes were heard. A steady note indicated we were right on track for the target. This signal was heard only by the navigator. When the release signal was received, which consisted of five dots and a two-second dash, the navigator released the markers or bombs." There was no verbal communication by radio transmission between the aircrew flying the mission and the controllers on the ground.
 * The distance from each station to the target had already been calculated. From each station a radar arc existed at the correct distance to the target. Using two stations, the two radar arcs intersected directly over the target. The Cat station, or tracking station, guided the approach of the aircraft. The Mouse station, or release station, monitored the approach of the aircraft to the intersect point and transmitted the signal to release bombs.
 * Oboe used described by John C. Sampson, a navigator in 105 Squadron:

An Oboe team was composed of a ground controller, two radar operators and a radar mechanic. The controllers were all ex-aircrew who had completed at least one tour. The radar operators were often WAAFs, who tracked the range and bearing of the aircraft and plotted a track for the operation on a plotting board. Said Frank Metcalf, an RAF officer and Oboe controller, "As the controllers were all ex-aircrew, it was much easier for us to mix with the Oboe squadron aircrew. We spoke the same language, which gave us an immediate advantage. We drank together, played poker together and came to understand one another in a manner I was convinced would pay dividends when the time came for the first ops."


 * With six pairs of stations 3 aircraft could use the Oboe at a time.

The Oboe run to the target took 10 minutes. Thus with three pairs of stations, 3 Mosquitos could be guided at a time, and with each taking 10 minutes to make their runs 18 Mosquitos could be guided over target by Oboe in one hour. Oboe Mosquitos were used primarily as marker aircraft. They also were at times used to bomb pinpoint targets.


 * Oboe use described.

An Oboe operation
 * Prior to a mission, the Cat and Mouse station would transmit information to each other through a scrambled link, but neither station would make radio contact with the directed aircraft.
 * the track or "beam" was narrower than the width of the wingspan of a Mosquito. The beam was a narrow area of airspace defined by the radar of the Cat station, or tracking station, that swept along the track over the target. Although it was referred to as flying the beam, there was no actual beam in space. The pilot would fly along the pathway until the flight intercepted with the signal from the Mouse station, or release station. At this point the navigator received the signal to drop bombs.
 * the bomb release signal was five dots followed by a dash.
 * To mark a target for an attack, more than one Mosquito was necessary in case there is equipment failure.
 * The precise release point was influenced by many factors e.g. time of bomb fall, trail distance4 (a function of bomb ballistics and airspeed), meteorological data, the velocity and heading of the Mosquito just prior to release plus instrumental corrections.5 These were taken into account by the Mouse computer (aptly named the Micestro!), the release point being continuously and automatically corrected to ensure that the predicted impact point of the bomb (or Target Indicator) was within the target zone.

Early use 4

 * The Oboe system used two radar ground stations to pinpoint the position of an aircraft, guiding it to its target. It would be used by a small number of Pathfinder aircraft, which would find the target for a raid and mark it with flares. The main bomber force would then aim at the flares.
 * No. 109 Squadron flew the first Oboe mission on 20/21 December 1942, attacking a power-station in Holland. The success of this mission led to the first combined raid on the night of 31 December 1942/ 1 January 1943. This time two Mosquitoes from No. 109 Squadron used Oboe to find Düsseldorf. Once over the target they dropped parachute flares, and a force of eight Pathfinder Lancasters bombed the flares.

Ruhr campaign, Cologne
Battle of the Ruhr began 5 March 1943. In the first attack, Oboe guided Mosquitos of the Pathfinder Force's 109 Squadron dropped their target-indicator bombs where two Oboe beams emitted from stations in England crossed in the atmosphere nearly six miles above the Krupps factory in the centre of Essen. Eight Mosquitos from 109 Squadron had been tasked with marking the Krupps works, but three of the aircraft had to turn back after suffering equipment failure. The other five were all able to successfully mark the target with their red target indicator flares. These were backed up over the course of the raid with green back-up marker flares by five Pathfinder Halifaxes and seventeen Pathfinder Lancasters. The attacked was made by 442 Main Force aircraft, which arrived over the target in three waves. During the forty minutes of the attack, Oboe-guided Mosquitoes re-marked the target at ten minute intervals. Pathfinder Lancasters flying at a lower height backed up the markers of the Mosquitos. The Oboe markers landed between 75 and 200 yards from the aiming point. Reconnaissance photographs taken the next day showed 160 acres of destruction in Essen, with 56 buildings within the Krupp works hit by bombs. The attack upon the Krupps works at Essen was the first demonstration of the accuracy of Oboe, where the Krupps works was finally hit with precision.

The Battle of the Ruhr lasted until 14 July and included forty-three separate raids, of which two thirds were against the Ruhr itself.

Bombing results beyond the range of Oboe could not achieve the same accuracy as the Ruhr attacks and accurate marking of these distant targets remained a basic problem for the remainder of the war. The Battle of Berlin, beginning 3 November 1944, was fought with H2S instead of Oboe, as Berlin was beyond Oboe range.


 * No. 109 Squadron was joined by No. 105 Squadron in June 1943 and No. 139 Squadron in July 1943. (Were these new squadrons 105 and 139 Oboe equipped?)

Special missions 5
Oboe used to guide the delivery of bombs on special targets. Examples: V-1 launch sites.

Failure against small targets 6
Oboe was put to use to guide attacks against V-1 launch sites. Unfortunately, it soon became apparent that though accurate, marking targets from 20,000 feet would never be accurate enough to guide attacks against reinforced, hardened targets. This became clear to Leonard Cheshire, the CO of 617 Squadron, in January 1944 when his squadron was tasked with the destruction of a V-1 launch site. Cheshire led the squadron against a site in the Pas de Calais region. The squadron came in at 20,000 feet. An Oboe equipped Pathfinder Mosquito working at the same altitude marked the target with a target indicator flare. 617 then proceeded to drop all their bombs within 94 yards of the marker, a remarkable feet of accuracy. The problem was the marker was 350 yards from the target. The mission was a failure. 617's failure led Cheshire to develop the low level marking techniques that 5 Group became famous for.

On the continent 7
In early 1944 Wing Commander 'Ned' Fennessey began to consider what would be needed to provide GEE, Oboe and G-H support for the upcoming invasion of the continent. He was told that pressing home attacks against Germany was the C-in-C's chief concern, and that such preparatory planning was not "a subject for immediate or popular discussion at Bomber Command."

Following the D-Day invasion and subsequent liberation of northern France, mobile Oboe trailer units were placed onto the continent to guide bombing missions deeper into Germany. A number of Oboe units were set up. They were a part of 72 Wing, part of all-Radar Navigation Aids (RNA) of 60 Group RAF, and were headquartered in Mons. Several of these units had to move rapidly in the winter of 1944 when the German counteroffensive threatened to overrun their positions. One of the last operations employing Oboe was in April 1945, when Oboe was used to guide an attack against the Berchtesgaden in the Alps. Guidance of the bombers over Hitler's retreat was disrupted by reflections off the many mountains.
 * Following the Normandy invasion, ground stations were set up on the continent, which extended the reach of Oboe guidance across the continent.
 * No. 60 Group drew up plans to form a Mobile Wing, fully equipped to be deployed to France and advance with the front to provide radar navigational aids well forward of the fighting. The unit was designated No. 72 Wing, and placed under the command of Group Captain R.L. Phillips. In addition to the navigational and bombing aid services, it was responsible for Type 11, Type 14 and Type 25 Radar stations for support of Allied fighter aircraft.
 * In preparation for the invasion of occupied Europe the previously disbanded No. 72 Wing was reactivated on 15 May 1944 to manage the mobile GEE and Oboe units of the Allied Expeditionary Air Force (AEAF).
 * In November 1944 the largest loss of radar equipment and personnel occurred when an LST transporting an Oboe unit to the continent struck a mine and sank. 14 radar officers and 224 other ranks were lost, along with 50 vehicles loaded with electronic radar equipment. All had to be relaced.
 * No. 72 Wing was reactivated under 60 Group to take all technical responsibility for the maintenance of all radar navigational ground stations. The Wing headquarters was at Mons, Belgium.


 * Mobile units on the continent were numbered as "Convoys", and each had two tractor/trailers mounting the xyz radar, and all the support needed to equip, supply and feed the unit.
 * The mobile units allowed Oboe precision guidance for targets deeper in Europe.
 * The equipment was highly secret. During the Battle of the Bulge the radar equipment of one of the Convoys nearly fell into German hands. Warned at the last minute of the advance of the German forces, the CO of the Convoy was given permission in the middle of the night to break down their camp and vacate La Roche. Moving out through deep snow, they managed to get out shortly before the town was captured by German armoured units. After the war is was learned that Field Marshal Gerd von Rundstedt had made plans to make the capture. The unit in question was located in La Roche, specifying the Type 9000 (Oboe Ground Station), a Type 100 Heavy (G-H) and a Type 7000 (GEE). The Germans were aware of the mobility of the convoys into which these technical units formed.

Late in the war, Oboe was used to assist food drops to the Dutch still trapped under German occupation, as part of Operation Manna. Drop points were arranged with the Dutch Resistance and with aiming assistance using Oboe the food canisters were dropped within about 30 m of the aiming point.

Signals wing

Summary 8
The Pathfinders made a marked improvement in navigation and target finding, but they would not have been half as effective as they were without the electronic navigational equipment that was developed for them. Of these, though H2S was made operational first, it was Oboe, arriving a few months later, that had the more dramatic effect. Oboe was the most accurate bomb guidance system developed during the war. Referring to the arrival of Oboe, Arthur Harris noted: ""At long last we were ready and equipped. Bomber Command’s main offensive began at a precise moment, the moment of the first major attack on an objective in Germany by means of Oboe. This was the night of 5/6 March 1943, when I was at last able to undertake, with any real hope of success, the task which had been given me when I took over Bomber Command at the beginning of 1942 - the task of destroying the industrial capability of Germany." OR "the task of destroying the main cities of the Ruhr.""

The accuracy of Oboe rivals that of current day smart bombs. The guidance system greatly improved the effectiveness of Bomber Command and disrupted German industry.

GEE article

 * Blanchard, W.F. The GEE System
 * It was assumed the German's would be jamming it in three months time, but in fact it took over five months before they were interfering with the signal over the continent. In fact it was one of the main aids used in the ' 1,000-bomber' raid on Cologne in May 1942.

Melinsky:Forming the Pathfinders

 * American produced hooded flares were the first available to Bomber Command.
 * Bennett
 * TRE
 * Reeves and Jones

Cumming: Beam Bombers: The Secret War of No. 109 Squadron

 * Female observers at Margam Sands: "Then, when the girls saw the bomb-bursts still appearing in the right place, they were absolutely amazed. They could not believe that bombs could be dropped with such precision from an aircraft they could not see, on a target that was invisible to the crew."


 * Bennett put an end to the work up crews posting guard duty.
 * Work up crews working 24 to 36 hours at a time attempting to fit the Oboe equipment into the Mosquito.


 * 21 wireless operator/air gunners were interviewed for the job for ground controller. Chaired by Group Captain Theak from Bomber Command HQ, the interviews were stopped halfway through as being pointless. None of these men have the experience, basic knowledge, or qualifications that the panel deemed essential. With one exception, it appeared those men interviewed "knew nothing of elementary navigation, theory of bombing, of RDF or cathode ray apparatus." WOP/AGs were not the right candidates. In the view of the panel the only personnel with the necessary background to enable them to start Oboe work at once were navigators or pilots with navigational experience. Ended up going to Harris to ask for pilots and navigators. He allowed only off service pilots.
 * the decision to use off service pilots turned out to be good on a second basis, as they were able to bridge the gap between the scientists and the airmen. TRE's pilots ended up getting along very well with Sq. 109 pilots.


 * 50 Oboe Mosqutos were put into the air on the night of 5/6 June, 25 from 109 Squadron and another 25 from 105 Squadron.
 * To put this many Mosquitos into service twenty of the older Mosquitos with Mark I Oboe were used. These were controlled by veteran Mark I Ground Stations.
 * Guidance for these 50 Oboe sorties was supplemented by the first Mobile Ground Stations, whose units had been produced at Malvern and were set up at Beachy Head in East Sussex.
 * The Mobile Units had been formed into 'Convoys', made up of two trailers with mobile ground tracking equipment, radar maintenance and repair equipment, and other support vehicles.
 * The first of the mobile units was shipped to France in late August, 1944, and performed its first guidance operation on 25 September 1944. By this time it was operating out of Belgium.
 * Six 'Convoys' were eventually operational on the continent.
 * Evacuation of equipment from La Roche, 18 December 1944.
 * The airbourne Oboe repeater system, conceptualized as a range extender for the system when Oboe was first proposed, never was successful operationally. Range extension for Oboe was accomplished with the Mobile Ground Stations, which eventually guided bombing missions to Berlin.

Goebel: Radio Navigation Systems

 * Radio Navigation Systems

Rickard: The de Haviland Mosquito with the Pathfinders

 * Modifications to the Mosquito allowed it to reach higher altitudes. The Merlin 72 engine equipped B Mk IX introduced in April 1943, followed by the pressurized cabin version known as the B Mk XVI in December 1943 gave it an operational ceiling of 35,000 feet, thereby increasing the range of Oboe.

Bowman: Mosquito Bomber/Fighter-Bomber Units 1942—45

 * 109 Squadron was the original Oboe equipped Mosquito squadron. The unit performed blind marking. They were joined by 105 Squadron, while 139 Squadron remained as a unit that provided the back up markers.
 * First Oboe missions.


 * The Oboe markers achieved such proficiency that Bennett was able to expand his Mosquito force, and in early 1943 No 8 Group (the Pathfinder force) began 'nuisance' raiding.
 * The two transmitting stations were referred to as 'Cat' and 'Mouse'
 * Operation Manna – the air-dropping of food to the starving Dutch population in German-occupied Holland. Oboe Mosquitoes were  extensively employed marking drop zones for RAF and USAAF heavy bombers,  whose bomb bays were filled with provisions instead of high explosives.


 * Jericho Raid and the loss of Percy Charles Pickard.

Ashworth: RAF Bomber Command 1936-1968

 * 109 Squadron had been working with a new navigational aid, Oboe, since the Pathfinders formed in August 1942.
 * The device would prove to be an effective aid once it became operational. With it crews could bomb targets accurately with no visual fix upon the ground below.
 * Two stations would transmit a signal. They gave the crews a point from which they were 10 minutes ETA from the target. Crews were given three pieces of information prior to each mission: a geographical target 10 minutes flying distance from the actual target (this would be the switching on point), an ETA and a two letter code. A listening watch was commenced, and when they heard their two letter code they switched on their Oboe. The Oboe device would pick up a signal from two transmitting stations. The curved course of the aircraft along the beam was called the "Bailey Beam". The pilot would follow the Bailey Beam from the switching on point to the target. If the pilot was to the left of the Bailey Beam he would hear dots transmitted to his headphones. If he was to the right of the beam he would hear dashes. By using these tones he could keep on the beam. As the aircraft aproached the release point the navigator would receive a series of signals from the Mouse station. When he reached the release point he would receive five dots followed by a long dash. This was the signal to release the marker flares or bombs. The limitation of the system was that only one aircraft could be guided at a time. For this reason Oboe was only fitted to Mosquito marker aircraft. It was first put to operational use in December. By the end of the month they had managed to hit the Krupp's Works twice, and on 31 December two Mosquitos marking for 8 PFF Lancasters dropped all their bombs on the target city of Dusseldorf in conditions which prevented any visual landmarks.
 * The navigational aides used for a raid were named after the birth places of PFF staff.

Ward: 3 Group Bomber Command



 * Six Mosquitos of 109 Squadron set off for a power station at Lutterade in Holland, led by their commanding officer, W/C Hal Bufton.
 * This night’s operation involved the first Oboe-aimed bombs as a calibration test to gauge the margin of error. Three aircraft experienced equipment malfunctions, and  bombed Duisburg instead, but W/C Bufton and two others successfully released their bombs.
 * Mechanical failure was a potential problem with any of the electronic navigational aides.
 * the advent of Oboe promised much for the future.
 * the Oboe device to negate the ever-present industrial haze blanketing and protecting the Ruhr.
 * equipment failures in the Oboe Mosquitos sometimes led to poor performance, as it did in the mission to Duisburg on the night of 26/27 December 1942 and the attack against Gelsenkirchen on the night of 25/26 June 1943 and Leverkusen on the night of 19/20 November 1943.
 * On the night of 16/17 December 1943 twenty-six Stirlings, including four from 218 Squadron, joined in the first of a series of attacks on flying bomb sites at Tilley-le-Haut near Abbeville. The attack failed, despite being marked by Oboe Mosquitos, and this highlighted the  limitations of Oboe as an aid to precision bombing. Although ideal for urban areas, where a margin of error of a few hundred yards was immaterial, a small target required pinpoint accuracy.
 * Oboe was key to Bomber Command's success against the Ruhr, as it provided a means to 'see' through the previously impenetrable barrier of haze and cloud.

Maynard: Bennett and the Pathfinders
Oboe development
 * German deceptions and disguise efforts
 * Target Indicators and Marker Flares
 * Aircraft carrying H2S were lost on the first raid the device was used, against Hamburg. Fortunately, the Germans apparently did not perceive what it was.
 * Harris picks Bennett.
 * Oboe, and the "Cat" and "Mouse" stations.
 * concern by RAF over Oboe jamming.

Mahaddie: Hamish: the memoirs of Group Captain T.G. Mahaddie DSO, DFC, AFC, CZMC, CENG, FRAeS

 * development of Oboe
 * 100 sorties was quite normal in the PFF Oboe Mosquito crews.
 * A great number of Oboe Mosquito pilots came from the Blind Approach Training instructors, with a requirement of 1,000 hours on BAT flight
 * Bennett was often down at Malvern working with the scientists at the Telecommunications Research Establishment, or rather drove them forward as he desperately needed the aid to navigation and target finding.
 * Professor R.M. Jones of the Telecommunications Research Establishment developed Oboe at Malvern, working closely with Hal Bufton, the squadron commander of 109 Squadron.
 * Accuracy and timing were the be all and end all of Pathfinding.
 * Bennett doubled the Oboe commitment by splitting the founding 109 Squadron and creating 105 Squadron.
 * The pilots selected for Oboe training were usually well experienced in Standard Beam Approach, a blind-landing radio navigation system. Most had 1,000 hours in Standard Beam Approach.
 * Crews were trained in the use of Oboe at the Pathfinder Force Navigational Training Unit at RAF Warboys.

Mahaddie interview IWM

 * Bennett got the Stirling up to 18,000 feet by lightening the aircraft. He did this by reducing the reserve petrol by half, reduced the ammunition to the guns by half, and he took out all the armour plating, save that to protect the pilot under his seat. This placed us much nearer the 20,000 feet the Halifaxes flew at, and the 22,000 feet the Lancasters were operating at.


 * The electronic aid we had at the time of the formation of the Pathfinder Force was GEE. With GEE we could get within 3 miles of the target. But after the 1,000 plan in May the Germans began to jam it.
 * Normally we weaved all the time while flying over the continent. But to get a good fix on GEE we had to fly straight and level for ten seconds or so to get a good fix. Bennett coupled this with Astro-navigation for sighting stars. This was something we really did not believe in, but Bennett insisted that with a star shot you should get withing 1 nautical mile of your position. He stressed we should use this to confirm our GEE position.


 * We couldn't even get a look at the Ruhr, as the whole thing was a tremendous mishmash of smoke and haze, with one town running into another. We couldn't even get near the Krupps works, until Oboe come along. And we knew, despite their tremendous Flak and searchlights, we knew that Oboe could drop its markers within 300 feet.
 * Everything changed when Oboe came along. With an Oboe equipped Mosquito, we could drop a marker from 30,000 feet and drop the marker on a football pitch. That is the type of accuracy we could get with Oboe.
 * Oboe repeater squadrons.
 * After the heavy losses suffered by 8th Air Force in the Schwinefurt raids, Bomber Command sent a number of experienced airman to try to convince 8th to switch over to night missions. Those sent included Hamish Mahaddie, Guy Gibson, Willie Tait, Pat Daniels (?), and Leonard Cheshire.

Good GEE fixes across the sea, and guided the rest of the way by dead reckoning getting accurate adjustments of wind

Pathfinder Craig
[https://masterbombercraig.wordpress.com/bomber-command-structure/no-8-pff-group-bomber-command/pathfinder-force-pff/pathfinder-methods/oboe/. Oboe]
 * clever story of how Bennett convinced the air ministry that Oboe was useful by targeting a small cemetery. When the Germans reported Bomber Command incompetence by blowing up dead people, it convinced all of how deadly accurate Oboe is.
 * interesting note on a heavy bomber pilot's assessment of Oboe (fly straight and level for 10 minutes over the target? Bloody suicide.)
 * doubtful that you could use any of it.
 * does describe how Oboe development was with close work with a squadron, whereas H2S was not.

Into service
The first experiments with Oboe in a combat setting over Germany began on the night of 20/21 December 1942, when a small force of six Oboe-equipped Mosquitoes were sent to bomb a power station at Lutterade in the Netherlands, on the German border. Three of the sets failed, but the three remaining aircraft, led by Squadron Leader L.E. Bufton, were able to drop properly. A follow-up reconnaissance mission the next day showed that nine of the bomb craters could be identified, all of them clustered closely together but some 2 km away from the target. It was soon realized that the issue was a coordinate problem in the maps being used. The inaccuracy was due to the differences in the surveying grids used on the continent. Once this was corrected the small raids proved to be amazingly accurate. Similar tests with small numbers of Oboe aircraft, sometimes dropping flares for small numbers of Avro Lancasters following them, were made throughout December and January.

At first, the Germans wrote off these small attacks as nuisance raids, intended to upset production by sending the workers to the air raid shelters. However, it was soon realized that something very odd was occurring; aircraft were dropping only 6 to 10 bombs, often through heavy cloud cover, and having 80 to 90% of them hit their targets, normally blast furnaces or power stations. As part of this process, the bombers released photoflash flares, which lit up the ground below the aircraft enough for photography. On 7 January 1943, Hauptmann Alexander Dahl noted these and suggested that they were using the photographs to correct for measurement errors of a new bombing system.

This was precisely what had been happening. While Oboe demonstrated accuracy on the order of tens of meters over England, over the continent the early tests were not nearly as good. It soon became clear there was a pattern to the misses. The inaccuracy was due to the differences in the surveying grids used on the continent. The solution to this problem was provided by the Germans themselves; prior to the war they had made an effort to calibrate the two systems in a series of cross-Channel measurements that the UK Ordnance Survey also received. Using these corrections they were able to address the inaccuracies almost immediately. ? Another source states a mathematician worked out the corrections needed. Please clarify which is correct.

By the late spring Bomber Command crews had practised the bomb-on-marker technique enough to begin major operations. Harris then began a series of raids known as the Battle of the Ruhr, opening with a raid on Essen on 5 March that produced rather poor results in spite of proper marking. The next major raid against the Krupp factory in Essen on 12/13 March was somewhat more successful, followed by a mix of raids that met with very different results. By May, however, the technique was tuned and a series of very large raids, typically with 500 to 800 bombers, demonstrated increasingly successful results. Among these was a late-May raid on Dortmund that caused the Hoesch steelworks to cease production, and a late July raid on Krupps that Goebbels stated had caused "complete stoppage of production in the Krupps works". Analysis of the results demonstrated that the number of bombs that fell on their targets doubled from the pre-Oboe era.

German countermeasures
German radar operators soon took notice of Oboe aircraft on their approach flights. That something odd was going on was apparent, as a single aircraft would be tracked for some 50 miles following a steady, slowly curving flight. Observing the curved path of the Mosquito, the Germans took to calling the system "Boomerang" for the gently curving nature of the flight path. The significance of the flights remained unknown for almost a year. The predictable path of the bomber was a vulnerability, compensated for by the fact that the speed and altitude of the Mosquito made it very hard to intercept. Due to the high altitude the Mosquito was flying and its high rate of speed it proved difficult to make an interception, nor did there seem to be much reason to do so. They were thought to be single bombers making nuisance attacks to disrupt the workers and generate fear. The true purpose of these flights was not worked out for a year, when engineer H. Widdra deciphered the operation of the system.

at the end of August 1943 at the RF tracking station "Maibaum", located in Kettwig near Essen, while the British bombers attacked the steelworks of "Bochumer Verein".

The Germans tried to jam 1.5 metre / 200 MHz Oboe signals, though by the time they did the British had moved on to the 10 cm / 3 GHz Mk.II Oboe and were using the old transmissions as a ruse. This was discovered in July 1944 after its operator failed to properly mark a drop using the Mk.1 signals

, who had detected the British "Pip-squeak" Identification friend or foe [IFF] system in 1940. The first attempt to jam Oboe took place at the end of August 1943 during an attack on the Bochumer Verein steelworks in Essen. A system set up at the Maibaum tracking station in Kettwig broadcast false dot and dash signals on the 1.5 m band, hoping to make it impossible for the pilot to figure out if they were at the right position. This was the same technique that the British had used against German systems during The Blitz.

However, the Oboe system had already moved to the microwave-frequency 10 cm Oboe Mk. II, but the British kept broadcasting the older signals as a ruse. The failure to jam Oboe remained a mystery until July 1944, when the older signal was apparently marking one target while a pathfinder perfectly marked another. The Germans quickly surmised that there was another signal or system in use. The Germans were well acquainted with the British microwave systems in the 10 cm area, but in April 1944 the RAF had already introduced Oboe Mk. III, which resisted German jamming efforts. Mk. III also allowed up to four aircraft to use a set of frequencies (stations) and allowed different styles of approach, not just the arc.

Technical details
Oboe used two stations at well-separated locations in England to transmit a signal to a Mosquito Pathfinder bomber carrying a radio transponder. The transponder re-transmitted the signals, which were then received by the two stations. The round-trip time of each signal gave the distance to the bomber.

Each Oboe station used radio ranging to define a circle of specific radius, with the intersection of the two circles pinpointing the target. The Mosquito flew along the circumference of the circle defined by one station, known as the "Cat", and dropped its load (either bombs or marking flares, depending on the mission) when it reached the intersection with the circle defined by another station, known as "Mouse". There was a network of Oboe stations over southern England and any of the stations could be operated as a Cat or a Mouse.

The Mark I Oboe was derived from Chain Home Low technology, operating at upper-range VHF frequencies of 200 MHz (1.5 metres). The two stations emitted a series of pulses at a rate of about 133 per second. The pulse width could be made short or long so it was received by the aircraft as a Morse code dot or dash. The Cat station sent continuous dots if the aircraft was too close and continuous dashes if the aircraft was too far and from these the pilot could make course corrections. (The Germans used a similar method with Knickebein.)

Various Morse letters could also be sent, for example to notify an aircrew their Mosquito was within a range of the target. The Mouse station sent five dots and a dash to indicate bomb release. The Mouse station included a bombsight computer, known as "Micestro", to determine the proper release time, there being no particular logic in carrying the bombsight on the Mosquito when it was under the control of the ground station.

Similar systems
The Germans created a system conceptually similar to Oboe code named Egon. It was used on a limited scale for bombing missions on the Eastern Front. The system made use of two modified Freya radars to serve in the roles of Cat and Mouse. These two Freya Egon sets were located about 150 km apart. The aircraft carried a two-channel IFF to respond to them. The bombers were directed by a controller at the station using voice radio. Despite the considerable effort the Germans put into other electronic navigation systems, they never took this concept further.

Further developments
To allow more aircraft to use the bomb guidence system at the same time, the British rethought Oboe. The new scheme developed was named "GEE-H" (or "G-H"). It was based on exactly the same technique, but reversed the equipment so that the aircraft carried the transmitter and the two ground stations were fitted with transponders. Aircraft would use the two stations in parallel. Random noise inserted into the timing of each aircraft's pulse output allowed it to sort it its own signal from those of other aircraft. The receiving gear on the aircraft could match its unique pulse pattern with that sent back by the transponders. The practical limit was about 80 aircraft at one time.

The name "GEE-H" was confusing, since the scheme was very close to Oboe and not at all like GEE. Nevertheless, the name was adopted as the system operated on the same waveband as GEE, at 15 to 3.5 metres / 20 to 85 MHz, and initially aircraft using the system read results off the GEE display and calibrator. The "H" suffix came from the system using the twin-range or 'H' principle of measuring the range from transponders at two ground stations. GEE-H was about as accurate as Oboe.

In popular culture
Oboe appears as a plot point in the "Lost Sheep" episode of the BBC television series Secret Army, which featured the search for a downed airman with technical knowledge of the system.

Storage

 * Later in the war a Gee-H system was developed, in which the transponder remained on the ground but the transmitter was mounted in the aircraft where the readout was made. This system allowed as many as 80 aircraft to be guided to the target at the same time.
 * In an attack on 5/6 March 1943, Oboe guided bombers dropped over 50% of their bombs on the Krupp factories in Essen, an enormous improvement over previous efforts that resulted in less than 10% of bombs landing on their targets. Versions using shorter wavelengths demonstrated accuracy on the order of 15 meter.
 * The system was first used in December 1941 in short range attacks over France where the necessary line of sight could be maintained. To attack the valuable industrial targets in the Ruhr, only the de Havilland Mosquito flew high enough to be visible to the ground stations at that distance.
 * A pair of stations could guide two aircraft at any given time.
 * The Oboe operators then sent radio signals to the aircraft to bring them onto their target and properly time the release of their bombs.


 * A "radio direction finder", also known as a "radio compass", used a loop antenna mounted on the back of an aircraft. If the plane of this antenna pointed to a radio beacon, the signal strength was a maximum, while a signal facing the loop induced voltages that canceled each other out, resulting in a weak or null signal. The loop antenna was connected to an amplifier, which drove an indicator on the cockpit's control panel.
 * "Automatic direction finding (ADF)" was an improvement on basic radio direction finding, which used a feedback control system to rotate a loop antenna so that a radio beacon signal was at a maximum. The control panel indicator then showed the direction of the antenna. A more sophisticated approach to ADF was to use two loop antennas, set up at a right angle to each other, and measure the ratio of the signals from each antenna to get the angle to the beacon. A simple radio beacon could only give a pilot a line from the aircraft to the radio beacon, but could not provide such useful information as whether the aircraft was north or south, east or west of the beacon. GEE was a very good navigational system, but was not accurate enough to release bombs off it's readings, and was of limited value over the continent.