Gee (navigation)

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 350 miles (560 km).

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.

[1] At the time, the RAF relied on daylight bombing by tight formations of heavily defended bombers as its primary attack force, so night landings were not a major concern.

[5] As the system was now intended to offer navigation over a much wider area, the transmitters of a single station would have to be located further apart to produce the required accuracy and coverage.

The single-transmitter, multiple-antenna solution of the original proposal was no longer appropriate, especially given that the stations would be located far apart and wiring to a common point would be difficult and expensive.

Dippy suggested building stations with a central "master" and three "secondaries" about 80 miles (130 km) away and arranged roughly 120 degrees apart, forming a large "Y" layout.

With this sort of range, the system would be very useful as an aid for short-range navigation to the airport, as well as helping bombers form up at an arranged location after launch.

Having originally relied on day bombing, the RAF had not invested a tremendous amount of effort on the navigation skills needed for night flying.

When The Blitz night-bombing offensive started, the Germans were found to have developed a series of radio aids for this, notably the X-Gerät system.

By late 1940 a number of reports were trickling back from observers in the field, who were noting that Allied bombers did not appear to be bombing their targets.

For some time, these results were dismissed, but calls for an official enquiry led to the Butt report, which demonstrated only 5% of the bombs sent out on a mission landed within 5 mi (8 km) of their targets.

This led to Frederick Lindemann's notorious "dehousing" paper, which called for the bomber efforts to be used against the houses of the German citizens to break their ability to work and will to resist.

While the debate raged, Bomber Command dramatically lowered their sortie rate, awaiting the rebuilding of the force with the newly arriving 4-engine "heavies" such as the Handley Page Halifax and Avro Lancaster, and the deployment of Gee.

On 18 August 1941, Bomber Command ordered Gee into production at Dynatron and Cossor, with the first mass-produced sets expected to arrive in May 1942.

[16] One illustration of Gee's routine employment by Bomber Command in navigation tasks was its use (albeit a limited one) in Operation Chastise (commonly known as the "Dam Buster Raid") in May 1943.

In his memoir, Enemy Coast Ahead,[17] Guy Gibson, the leader of the raid, briefly mentions his navigator, F/O 'Terry' Taerum, RCAF, employing what Gibson calls Taerum's "G Box" to determine groundspeed while flying very low at night over the North Sea from Britain to Holland, en route to Germany.

[19] After the end of the war in Europe, Britain planned to send Lancasters to the Japanese theatre as part of Tiger Force and to use Gee for the passage of flights to Asia.

Preparations began for Gee transmitters in Nablus (in Palestine) guiding the flights across the Middle East, but the surrender of Japan removed the need for this chain.

[20] Later in the war, Bomber Command wanted to deploy a new navigation system not for location fixing, but to mark a single spot in the air.

The result was a set of four chains, South Western, Southern, Scottish, and Northern, which have continuous coverage over most of the UK out to the northeastern corner of Scotland.

[1] Instead of using two separate pairs of stations, the system can be simplified by having a single master and two secondaries located some distance away from each other so their patterns overlap.

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 time for ten cycles of this 150 kHz oscillation, 66.66 μs, was called a Gee unit and corresponded to a range difference of 12.4 miles (20.0 km).

When two such signals from a single chain are considered, the resulting pattern of lines becomes increasingly parallel as the baseline distance becomes smaller in comparison to the range.

This is the basic accuracy of the Gee system, at least at shorter ranges and at locations near the center of the baselines where the hyperbolic lines were close to perpendicular.

However, navigating to a target using such a system would be complex; multiple fixes would have to be taken over time and then averaged in order to calculate the ground speed and direction.

The pilot would then fly the aircraft along the circular arc that would take them over the aim point, with periodic corrections from the navigator as needed to realign the two blips.

Gee was highly susceptible to jamming; all the Germans had to do was radiate spurious pulses that made it impossible to determine which was a real signal from the stations and which was being broadcast from a jammer.

The Eastern chain became operational (as opposed to conducting tests) from March 1942, and was used in "Bomber" Harris's major and unprecedentedly successful raids on Lübeck and Cologne that spring, Its HQ and monitoring station, initially at Great Bromley, moved on to Barkway that November.

Wing Commander Phillips, assisted by Squadron Leader Allerston and the scientific side Edward Fennessey, were then in charge.