Editing Gee (navigation) (section)

=== Navigation system proposal ===

The RAF's bombing campaign plans quickly went awry, especially after the [[Battle of the Heligoland Bight (1939)|Air Battle of the Heligoland Bight]] in 1939. Contrary to prewar thinking, the bombers proved extremely vulnerable to both ground fire and attacking [[Fighter aircraft|fighters]]. After some discussion, the best course of action was decided to be to return to night bombing, which had been the primary concept earlier in the 1930s.

This raised the need for better landing aids, and for night navigation aids in general. Dippy refined his system for this purpose, and formally presented a new proposal on 24 June 1940.{{sfn|Blanchard|1991|p=297}}{{sfn|Campbell|2000|p=5}}{{sfn|Brown|1999|p=288}} The original design used two transmitters to define a single line in space, down the runway centerline. In his new concept, charts would be produced illustrating not only the line of zero-difference, where the blips were superimposed like the landing system, but also a line where the pulses were received 1&nbsp;μs apart, and another for 2&nbsp;μs, etc. The result would be a series of lines arranged at right angles to the line between the two stations.{{sfn|Brown|1999|p=288}}

A single pair of such transmitters would allow the aircraft to determine on which line they were, but not their location along it. For this purpose, a second set of lines from a separate station would be required. Ideally, these lines would be at right angles to the first, producing a two-dimensional grid that could be printed on navigational charts. To ease deployment, Dippy noted that the station in the centre could be used as one side of both pairs of transmitters if they were arranged like an L. Measuring the time delays of the two outlier stations relative to the centre, and then looking up those numbers on a chart, an aircraft could determine its position in space, taking a fix. The gridded lines on the charts gave the systems its name, "Gee" for the "G" in "Grid".{{sfn|Brown|1999|p=288}}

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. Instead, Dippy described a new system using individual transmitters at each of the stations. One of the stations would periodically send out its signal based on a timer. The other stations would be equipped with receivers listening for the signal arriving from the control station. When they received the signal, they would send out their own broadcasts. This would keep all the stations in synchronization, without the need for a wire between them. Dippy suggested building stations with a central "master" and three "secondaries" about {{convert|80|miles|km}} away and arranged roughly 120&nbsp;degrees apart, forming a large "Y" layout. A collection of such stations was known as a chain.{{sfn|Blanchard|1991|p=298}}{{sfn|Brown|1999|p=288}}

The system was expected to operate over ranges around {{convert|100|miles|km}}, based on the widely held belief within the UK radio engineering establishment that the 30&nbsp;MHz [[shortwave]] signals would have a relatively short range. 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. Additionally, after flying to their cruising altitude, the bombers could use Gee fixes to calculate the winds aloft, allowing them to more accurately calculate [[dead reckoning]] fixes after the aircraft passed out of Gee range.{{sfn|Blanchard|1991|p=298}}

Experimental systems were being set up in June 1940. By July, to everyone's delight, the system clearly was usable to at least {{convert|300|miles|km}} at altitudes of {{convert|10000|feet|km}}. On 19 October, a fix was made at {{Convert|110|mi|km|abbr=}} at 5,000&nbsp;feet.{{sfn|Campbell|2000|p=5}}