Gee - H ( navigation ) Gee - H , sometimes written G - H or GEE - H , was a radio navigation system developed by Britain during World War II to aid RAF Bomber Command . 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 bombing system , both of which worked along similar lines . By measuring the distance to a radio station , the bomber was able to navigate along an arc in the sky , dropping their bombs when they reached a set distance from another station . The main difference between Oboe and Gee - H was the location of the equipment ; Oboe used very large displays in ground stations to take very accurate measurements but could only direct one aircraft at a time . Gee - H used much smaller gear on board the aircraft and was somewhat less accurate but could direct as many as 80 aircraft at a time and was passive . Gee - H entered service in October 1943 and first used successfully in November against the Mannesmann steel works at Düsseldorf on the night of 1 / 2 November when about half of the sets failed leaving only 15 aircraft to bomb the factory on Gee - H . 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 . Gee - H was adapted by RCA into the US wartime SHORAN 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 or angle or range - two angle measurements , two distance measurements , or one angle and one distance . Early radio navigation was typically based on angle measurements using radio direction finder s , but these had limited accuracy on the order or tens of miles . The development of range - based systems had to wait until the invention of accurate time measurement of radio was possible , which came about as a result of the development of radar . The Luftwaffe pioneered the use of distance - measuring radio navigation systems with their Y - Gerät system in 1941 . Y - Gerät used a single Knickebein - 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 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 " system of navigation is that the beams are necessarily fan shaped , growing wider with increasing distance from the broadcaster . This means they have decreasing accuracy with increasing range . Measurements of distance are dependant only on the accuracy of the equipment , and are independent of range . When two such signals are combined the accuracy is linear with range , as opposed to the square as is the case with beam - based or hyperbolic navigation systems . However , such systems are also generally more difficult to use , as they require two range measurements to be made in quick succession , while the aircraft is moving . Oboe . The Air Ministry developed a distance - measuring system known as Oboe 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 , the distance between a station near London and a target in Düsseldorf would be about 300 miles , so an arc with a 300 mile radius around the station would be drawn , passing through Düsseldorf . Now the " range " of the bombs being dropped would be calculated . Range is the distance between the point where the bombs are released and the point where they impact . For missions around 20 , 000 ft altitude , range is typically on the order of 1 . 5 miles for a high - speed aircraft like the de Havilland Mosquito . The mission planners would then calculate the location along that 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 , considering winds , atmospheric pressure , even the tiny centrifugal force generated by the aircraft following the 300 mile radius curve . During the mission , the bomber would fly itself to one end of the arc or the other , using any means of navigation including dead reckoning . When they were near the pre - arranged location , the transponder was activated , and the Oboe station would measure their current distance . The station would then send out a radio signal of either dots or dashes , allowing the pilot to adjust their path so they were at precisely the right range , where the Oboe station would send out a steady tone , the " equisignal " . Operators would continue to watch the position of the aircraft , sending out correcting signals as needed so the pilot could continue to adjust their path along the arc . Meanwhile a second station was also measuring the distance to the bomber . This station was equipped with the bomb ' s range value calculated earlier , and had used this to calculate the range between their station and the bomber at the point where the bomb ' s needed to be dropped . When this " mouse " station saw the bomber approaching the drop point , it sent a series of Morse code signals to inform the pilot that the drop point was approaching . At the precise instant , it would send another morse signal that would drop the bombs automatically . The main constraint of Oboe was that it could only be used by one aircraft at a time . As it took about 10 minutes for the bomber to get itself properly positioned along the arc , this delay meant that the system could not be used for a large raid with multiple 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 . Alternately , Oboe was sometimes used for attacks on point targets by single aircraft , or a small number 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 Oboe , 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 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 reverse the location of the transmitters and receivers , so that the receiver is on the aircraft and the transmitter on the ground . Now every aircraft generates a different signal pattern , 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 which were picked up by the aircraft and read on an oscilloscope . The time between pulses was not particularly accurate and sometimes deliberately changed , so the equipment in the bomber had a timing system that allowed it to adjust for this . 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 frequency as that in the ground station . This " froze " the signal on the screen , allowing signals from different ground stations to be seen and measured . In order rapidly to deploy the new system , it was decided to re - 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 the 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 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 . 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 mis - tuned Gee . Only the signals originating from the aircraft ' s timer would line up on the display and remain motionless . This deliberate adjustment of the timing was known as " jittering " . In operation , 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 sent to the display inverted . The navigator could then direct the pilot onto the right path by giving directions until the upper and lower blips aligned . The range computed in advance , from the second station to the drop point , was dialled into the lower trace and as this range was always changing , the active blip would move along the display towards this value until they overlapped . 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 , 000 μS responding to the signals from any one aircraft in a second of time . This would leave 990 , 000 μS 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 more difficult . 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 . In the case of Gee - H , each aircraft had unique timing , so unless the jammer matched the jitter , the jamming signal would simply disappear off the screen . As there were dozens of such transmitters , and a few ground stations in the case of Oboe , the magnitude of the jamming problem became considerably more difficult . As the Gee - H system used Gee equipment , turning off the transmitter turned it back into a normal Gee unit . Although Gee was susceptible to jamming , it was still useful over the UK where jamming was not an issue . Unlike Gee - H , Gee could be directly read on a map and was extremely useful for navigating home after a mission . 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 . As 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 for this use . Gee - H achieved accuracy of about 150 yards at 300 miles . 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 , a diversionary " attack " during Operation Overlord 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 , in order to deceive the German radars that they were the main invasion fleet .