278:
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 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, 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".
244:
282:
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.
235:, so that the receiver is on the aircraft and the transmitter on the ground means that each 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.
192:. When they were near the location, the transponder was switched on 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.
185:. 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 235 mi (378 km) radius curve.
269:. 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
290:
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.
302:
Oboe, which used 12-inch oscilloscopes developed specifically for this purpose. Gee-H achieved accuracy of about 150 yd (140 m) at 300 mi (480 km), while Oboe was good to about 50 yd (46 m). As with all VHF and UHF-based systems, Gee-H was limited to distances just out of
285:
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.
65:
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
301:
Gee-H's main fault was caused by using 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
273:
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
227:
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. Swapping the
294:
unused decoy signals, the magnitude of the jamming problem was considerably worse. 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
277:
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
226:
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
221:
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
281:
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
180:
and the target would be about 235 mi (378 km); an arc with a 235 mi (378 km) 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
289:
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
212:
aircraft of the pathfinder force, giving the Main Force bombers an accurate aiming point in any weather. Oboe was sometimes used for attacks on precision targets by one or a small number of aircraft dropping one after the other. In tests, Oboe demonstrated accuracies greater than those of optical
171:
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
293:
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
146:
A failing of the beam-type system of navigation is that the beams cannot be focused perfectly and in practice are fan-shaped, growing wider with distance from the broadcaster, accuracy falling with range. Measurements of distance are dependent only on the accuracy of the equipment, and are
112:, but these were accurate only to a few degrees and only provided 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
77:
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
207:
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. Oboe was used to guide the
195:
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
298:, 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.
107:
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
274:
precisely the same pulse frequency as that in the ground station. The receiver had two systems of this type, allowing the operator to receive signals from two stations and easily compare them and make simultaneous measurements.
142:
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.
137:
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
286:
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.
243:
1237:
147:
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 measurements to provide a location
181:
point that they hit. For missions around 20,000 ft (6,100 m) in height, range is typically on the order of 1.5 mi (2.4 km) for a high-speed aircraft like the
722:
1180:
789:
365:
625:
Blanchard, W. F. (September 1991). "Air
Navigation Systems Chapter 4. Hyperbolic Airborne Radio Navigation Aids – A Navigator's View of their History and Development".
204:
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.
784:
151:, 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.
1232:
66:
aircraft at a time. Gee-H used much smaller systems on aircraft and while somewhat less accurate, could direct as many as eighty aircraft at a time.
915:
715:
794:
317:
that diverted German defences at Calais while the real invasion fleet was 200 mi (320 km) away at
Normandy. Gee-H-equipped bombers of
855:
690:
579:
865:
708:
1191:
870:
850:
860:
51:
840:
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them. One solution to this problem is to have each Oboe station send out a slightly different signal, normally by changing the
50:
equipment, as well as its use of the "H principle" or "twin-range principle" of location determination. Its official name was
1175:
325:(chaff) over radar transponder-equipped small ships, to deceive the German radars that they were the main invasion fleet.
815:
322:
91:
984:
875:
1198:
907:
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bombing system which worked along similar lines. By measuring and keeping a fixed distance to a radio station, the
1227:
835:
79:
668:. MIT Radiation Lab Series. Vol. III (online scan ed.). Lexington, Mass: Boston Technical Publishers.
663:
1170:
124:
188:
During the sortie, the bomber crew would fly to one end of the arc using any means of navigation including
1000:
820:
753:
109:
964:
253:
182:
90:
system with improved accuracy. The same basic concept remains in widespread use today as the civilian
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31:
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Gee-H entered service in
October 1943 and first used successfully in November against the
590:
552:
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pioneered the use of distance-measuring radio navigation systems with their
120:
133:-like beam for steering the bomber in the proper direction and an onboard
265:
The first radio navigation system to be operated by Bomber
Command was
611:
177:
87:
62:
572:
A Radar
History of World War II: Technical and Military Imperatives
242:
113:
16:
Radio navigation system developed by
Britain during World War II
704:
306:, in this case limiting it to about 300 mi (480 km).
83:
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station saw the bomber approaching the drop point, it sent
595:
The Services Textbook of Radio: Radiolocation Techniques
535:
533:
531:
518:
516:
503:
501:
464:
462:
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445:
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633:(3). London: Royal Institute of Navigation: 285–315.
46:. The name refers to the system's use of the earlier
334:
Gee-H Mk. I - Airborne Radio Installation (ARI) 5525
1163:
1112:
1066:
993:
927:
884:
808:
772:
736:
731:RAF strategic bombing during the Second World War
366:List of World War II electronic warfare equipment
389:
1238:Military equipment introduced from 1940 to 1944
419:The Wizard War: WW2 & The Origins Of Radar
163:developed a distance-measuring system known as
716:
176:, the distance between the Oboe station near
8:
213:bombsights during daylight in good weather.
1181:Air operations during the Battle of Europe
723:
709:
701:
597:. Vol. VII (online scan ed.).
382:
172:instance, for an attack on a target in
492:
340:Gee-H Mk. II (Tropicalised) - ARI 5696
256:RAAF as it begins its take-off run at
1176:United States Army Air Forces (USAAF)
683:Array and Phased Array Antenna Basics
539:
522:
507:
480:
468:
449:
430:
321:flew low, in tight circles, dropping
7:
1171:Aerial defence of the United Kingdom
685:. Hoboken, NJ: John Wiley and Sons.
401:
836:Combined Bomber Offensive (1943–44)
1192:Death by Moonlight: Bomber Command
14:
574:. Bristol: Institute of Physics.
313:, a diversionary "attack" during
167:which first started reaching the
1233:World War II British electronics
821:Area bombing of cities (1942–43)
749:RAF strategic bombing 1942–1945
610:Proc, Jerry (24 October 2012).
415:"Battle of the Beams: Y-GERAET"
127:system in 1941. Y-Gerät used a
57:Gee-H was used to supplant the
1:
754:Area Bombing Directive (1942)
103:Distance measuring navigation
764:Casablanca directive (1943)
553:"A Failure of Intelligence"
1254:
841:Battle of Berlin (1943–44)
785:Frederick "Prof" Lindemann
831:Battle of the Ruhr (1943)
639:10.1017/S0373463300010092
627:The Journal of Navigation
329:Air Ministry designations
250:Avro Lancaster B Mark III
80:English Electric Canberra
1155:Light Night Strike Force
681:Visser, Hubregt (2006).
662:Turner, L.; Roberts, A.
337:Gee-H Mk. II - ARI 5597
110:radio direction finders
82:. Gee-H was adapted by
795:Sir Archibald Sinclair
780:Arthur "Bomber" Harris
759:Dehousing paper (1942)
262:
940:Boston (Douglas DB-7)
826:U-boat pens (1943–44)
816:Oil targets (1940-45)
589:Haigh, J. D. (1960).
570:Brown, Louis (1999).
246:
183:de Havilland Mosquito
1186:Defence of the Reich
390:Turner & Roberts
86:into the US wartime
34:system developed by
22:, sometimes written
1089:Intruder operations
903:("Dambusters" raid)
351:Battle of the Beams
271:time base generator
98:Development history
1206:Target for Tonight
790:Sir Charles Portal
744:Butt Report (1941)
591:"Gee-H - AMES 100"
315:Operation Overlord
309:Gee-H was used in
263:
44:RAF Bomber Command
1215:
1214:
1150:No. 100 Group RAF
895:(Friedrichshafen)
692:978-0-470-87118-8
581:978-0-7503-0659-1
555:Bomber Command OR
361:Oboe (navigation)
311:Operation Glimmer
1245:
1228:Radio navigation
1140:No. 6 Group RCAF
1074:Area bombardment
1058:Target indicator
1031:Blockbuster bomb
846:Transport (1944)
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612:"The GEE System"
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40:Second World War
32:radio navigation
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1145:No. 8 Group RAF
1135:No. 5 Group RAF
1130:No. 4 Group RAF
1125:No. 3 Group RAF
1120:No. 1 Group RAF
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1104:Shuttle bombing
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1036:Earthquake bomb
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73:steel works at
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1199:Into the Storm
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1026:"Monica" radar
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542:, p. 252.
527:
525:, p. 251.
512:
510:, p. 249.
497:
485:
483:, p. 302.
473:
471:, p. 257.
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423:
421:, 1 March 2011
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217:A new approach
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190:dead reckoning
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1079:Bomber stream
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1053:Bouncing bomb
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800:Arthur Tedder
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495:, p. 65.
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413:Greg Goebel,
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304:line of sight
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296:bomber stream
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260:, August 1944
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53:
52:AMES Type 100
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45:
41:
37:
33:
29:
25:
21:
1204:
1197:
1190:
1020:
919:(Peenemünde)
916:
908:
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892:
682:
664:
630:
626:
615:. Retrieved
594:
571:
563:Bibliography
547:
488:
476:
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418:
409:
397:
385:
319:218 Squadron
308:
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292:
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284:
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264:
254:467 Squadron
229:transmitters
220:
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197:
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187:
161:Air Ministry
158:
148:
145:
140:oscilloscope
128:
118:
106:
68:
56:
27:
23:
19:
18:
1099:Pathfinders
1084:Firebombing
493:Visser 2006
356:Lorenz beam
248:G–H Leader
135:transponder
38:during the
1222:Categories
1046:Grand Slam
994:Technology
980:Wellington
960:Manchester
885:Operations
617:17 January
540:Haigh 1960
523:Haigh 1960
508:Haigh 1960
481:Brown 1999
469:Haigh 1960
450:Haigh 1960
431:Brown 1999
372:References
202:Morse code
174:Düsseldorf
130:Knickebein
75:Düsseldorf
71:Mannesmann
1006:H2S radar
955:Lancaster
909:Hurricane
893:Bellicose
876:The Hague
866:Pforzheim
856:Heilbronn
809:Campaigns
737:Overviews
674:123180755
655:130079994
647:0373-4633
603:504108531
402:Proc 2012
377:Citations
233:receivers
121:Luftwaffe
1164:See also
970:Stirling
965:Mosquito
935:Blenheim
928:Aircraft
901:Chastise
345:See also
224:envelope
94:system.
30:, was a
1067:Tactics
1041:Tallboy
985:Whitley
975:Ventura
950:Hampden
945:Halifax
871:Dresden
851:Hamburg
773:Leaders
125:Y-Gerät
42:to aid
36:Britain
1016:"Oboe"
911:(1944)
861:Kassel
689:
672:
653:
645:
601:
578:
323:Window
178:Walmer
88:SHORAN
63:bomber
1113:Units
1021:Gee-H
1001:Chaff
917:Hydra
651:S2CID
239:Gee-H
198:mouse
114:radar
28:GEE-H
20:Gee-H
687:ISBN
670:OCLC
643:ISSN
619:2022
599:OCLC
576:ISBN
231:and
165:Oboe
159:The
155:Oboe
119:The
59:Oboe
1011:Gee
635:doi
267:Gee
252:of
149:fix
92:DME
84:RCA
48:Gee
26:or
24:G-H
1224::
649:.
641:.
631:44
629:.
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530:^
515:^
500:^
457:^
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417:,
116:.
54:.
724:e
717:t
710:v
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