Gee (navigation)

Gee, sometimes written GEE, was a radio-navigation system used by the Royal Air Force during World War II. It measured the time delay between two radio signals to produce a fix, with accuracy on the order of a few hundred metres at ranges up to about . It was the first hyperbolic navigation system to be used operationally, entering service with RAF Bomber Command in 1942. Gee was devised by Robert Dippy as a short-range blind-landing system to improve safety during night operations. In the course of development by the Telecommunications Research Establishment (TRE) at Swanage, the range was found to be far better than expected. It then developed into a long-range, general navigation system. For large, fixed targets, such as cities that were attacked at night, Gee offered enough accuracy to be used as an aiming reference without the need to use a bombsight or other external references. Jamming reduced its usefulness as a bombing aid, but it remained in use as a navigational aid in the UK area throughout and after the war. Gee remained an important part of the RAF's suite of navigation systems in the postwar era, and was included in aircraft such as the English Electric Canberra and the V-bomber fleet. It also had civilian use, and several new Gee chains were set up to support military and civil aviation across Europe.…

Gee, sometimes written GEE, was a radio-navigation system used by the Royal Air Force during World War II. It measured the time delay between two radio signals to produce a fix, with accuracy on the order of a few hundred metres at ranges up to about . It was the first hyperbolic navigation system to be used operationally, entering service with RAF Bomber Command in 1942. Gee was devised by Robert Dippy as a short-range blind-landing system to improve safety during night operations. In the course of development by the Telecommunications Research Establishment (TRE) at Swanage, the range was found to be far better than expected. It then developed into a long-range, general navigation system. For large, fixed targets, such as cities that were attacked at night, Gee offered enough accuracy to be used as an aiming reference without the need to use a bombsight or other external references. Jamming reduced its usefulness as a bombing aid, but it remained in use as a navigational aid in the UK area throughout and after the war. Gee remained an important part of the RAF's suite of navigation systems in the postwar era, and was included in aircraft such as the English Electric Canberra and the V-bomber fleet. It also had civilian use, and several new Gee chains were set up to support military and civil aviation across Europe. The system started to be shut down in the late 1960s, with the last station going off the air in 1970. Gee inspired the original LORAN ("Loran-A") system, which is essentially a Gee system working at much lower frequencies to give it longer range. ===Gee chains=== Gee chains used an arrangement with one master and two or three secondaries. The transmitters had a power output around 300 kW and operated in four frequency bands between 20 and 85 MHz. The Gee signal for any given chain consisted of a series of pulses of radio signal with a roughly inverted-parabolic envelope about 6 microseconds in duration. In a three station system, the master sent a single pulse, referred to as A, followed 2 milliseconds (ms) later by a double pulse, A′ (A Prime). The first secondary sent a single pulse 1 ms after the master's single pulse, labeled B, and the second sent a single pulse 1 ms after the master's double pulse, labeled C. As the receiver did not have any means of automatically synchronizing to the master signal, the A′ double pulse allowed the order sequence to be identified by the navigator operating the receiver. The whole sequence repeated on a 4 ms cycle (i.e. 250 times per second), with the pattern A-B-A′-C. In the case of a four station system, the cycle above would be repeated, with the addition of the D station, which would broadcast another double pulse. To allow this to be identified, the D station was timed at 166 times per second, such that its pulse would move from the A-B trace to the A'-C trace, not appearing on either trace and back to the A-B trace. The cycle therefore was A-B-D-A′-C-A-B-A′-C-D-A-B-A′-C.... The D pulse appearing on both traces meant that a fix could be made using the combinations AB/AC, AB/AD, or AC/AD, giving a wider area of high precision coverage than the three station system. The triggering of the A pulses was timed at 150 kHz by a stable local oscillator at the master station, but the timing was sometimes deliberately changed. The time for ten cycles of this 150 kHz oscillation, 66.66 μs, was called a Gee unit and corresponded to a range difference of . ===Decoding the signals=== On board the aircraft, the signals from the three or four stations were received and sent to the display. The description below is with reference to a three station system, but the D pulse would be substituted for either the B or C pulse in a four station system. In the "Main" time base setting, the CRT display was configured to display the signal across two lines, each showing half of the time of the signal. A local oscillator of much less complexity than the one at the master station was used to trigger the display sweep. When first activated, it would be unlikely to have exactly the same timing as the master station, so the operator would see the pattern of blips traveling across the screen. A control knob that adjusted the oscillator was used to tune the local oscillator frequency until the blips on the display were stationary which meant the local and master oscillators now had the same timing (simulated waveform "Step 1"). The pulses would be identified, then a delay control was adjusted to move the blips back and forth in order to bring the double A′ pulses to the left of the bottom trace, directly below the A on the upper trace (simulated waveform "Step 2"). Rotary switches followed by fine adjustment were used to position markers under the B and C pulses (the markers would invert the pulses on the display) (simulated waveform "Step 3"), and then the time base was switched to a "fast" position, which would add additional lines to the display, and show the A and A′ pulses above the inverted B and C pulses respectively (simulated waveform "Step 4"). The fine adjustment would be used to position the B pulse directly beneath the A pulse, and the C pulse directly beneath the A′ pulse (simulated waveform "Step 5"). A switch, referred to as the "clearing switch", was thrown, and the time of the fix noted. The clearing switch changed the display from displaying the pulses to displaying an internally generated scale. This scale would be read in the "fast" time base position for the decimal readings, followed by the whole numbers which would be read with the display in the "Main" time base setting (simulated waveforms "Step 6" and "Step 7"). The respective numbers from the A-B and A′-C readings would were then looked up on a Gee chart. Signals from different chains were closely spaced in frequency, close enough that the wide-band R1355 receiver would often tune in more than one chain at a time. For station identification, the A′ signals were only sent periodically. After the display was stabilized so the pulse trains were appearing in a single location on the screen, the A′ pulses could be seen blinking on and off with a set pattern (thus "ghosting" on the display). This allowed the operator to determine the identity of the master signal, and thus select the chain they wanted to use by positioning its associated A′ signal on the bottom left. ====Southern chain==== Source: Virginia: 48.75 MHz. Eastern and Southern (Virginia) chains could not operate simultaneously. Master, Bulbarrow Hill Truleigh Hill West Prawle Carolina: 44.90 MHz. Used by Coastal Command and Combined Operations. Same Master and Slave sites. ====Rheims Chain==== Source: Operational 5 October 1944. 83.5 MHz Master Rheims AMES 7912 became AMES 7913 B Slave La Capelle AMES 7925 became AMES 105 C Slave Ligny AMES 7926 became AMES 128 D Slave Estissac AMES 7924 became AMES 124 Monitor Mourmelon AMES 7931 ==Notes== ==References== ===Citations=== ===Bibliography=== A modified version is Jerry Proc, "The GEE System", 14 January 2001 Adapted from Air Ministry Confidential Document CD1136, 1956. Also published in the US as The Wizard War: British Scientific Intelligence 1939¹–1945, Coward, McCann and Geoghegan, 1978 ===Further reading=== Gee at the International Bomber Command Centre Digital Archive.

Text derived from https://en.wikipedia.org/w/index.php?oldid=1373873650 under CC BY-SA 4.0; modifications have been made.