Navigational Aids
Gee AMES Type 7000
Gee was a navigational system that utilised accurately phased pulses from ground based transmitters and it was the brainchild of R. J. (Bob) Dippy, who first set down his ideas on paper in 1937. However the rush to get the Chain Home system operational meant that Dippy's paper lay forgotten until 1940 when an urgent need for Bomber Command to have a reliable system of navigation arose. Dippy's system became operational in early 1942.
Colin Brown (ex-72nd entry) kindly sent me this circuit diagram for the AMES Type 7000 - Gee (Ground) - Transmitter T.1365.
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Major system characteristics |
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System type |
Hyperbolic navigation system |
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Frequency band |
20 - 85 Mc/s |
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Individual system band width |
1/2 Mc/s |
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Cycle repetition frequency |
250 c/s |
The text that follows in this section is taken from "The Services Textbook of Radio, Volume 7, Radiolocation Techniques" by Brig. J. D. Haigh, O.B.E., M.A., M.I.E.E., Edited by the staff of "Wireless World", H.M.S.O., London, 1960 pages 242 - 249. This volume was known to the British armed forces as Admiralty B.R.600(7), War Office 10224(7) and Air Ministry A.P.3214(7).
Extended-Field Navigational Systems
The deliberate withdrawal of normal navigational aids during the last war led to the development of various navigational systems for the assistance, particularly, of bombers trying to locate targets in blacked-out conditions and subsequently finding their way back to their home aerodromes.
In developing these systems there was a complete breakaway from classical direction-finding techniques because it was realised that the irreducible errors inherent in such methods could not possibly give the required accuracy of position fixing over Germany, for example, when the only d.f. stations available were in Great Britain. Apart from the types of direction-finding error which have been described in earlier chapters, yet another effect had come to be recognized which could give errors of up to 90o. This effect is known as the Heiligtag effect after its discoverer and arises when two trains of waves arrive at the direction-finding station from slightly different directions such as might be the case, for example, if both a direct ray and an ionospheric ray which had suffered lateral deviation were present together. A simple explanation of how the errors arise can be given by referring to Fig. l7.1 which is an exaggerated diagram showing a direct ray, TR and a laterally deviated ray TIR.
On arrival at R the ray TIR can be resolved into two components one parallel to TR and the other at right angles to TR. If the component parallel to TR happen to be in antiphase to the direct ray and of the same amplitude the direct ray will be cancelled and there will remain only the component at right angles to TR. Attempts to take a bearing in these conditions will clearly lead to a 90o error. This, of course, is an extreme case, but very large errors are probable in these conditions.
Another factor which influenced the departure from the usual d.f. techniques was the need for the aircraft to maintain wireless silence which meant that all direction finding would have had to be done from the aircraft itself which would have meant relying on medium frequency d.f. as it is not possible to carry out v.h.f. direction finding in an aircraft (see Chapter 3).
The solution to the problem was found in relying not on the rectilinear propagation of wireless waves but on the constancy of their velocity of propagation. Several systems of navigation were evolved which can conveniently be divided into range difference systems, twin-range systems and c.w. systems.
