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 Post Office Equipment for Radar

Means had therefore to be found for signalling height measurements across to the controller as quickly as possible and at the same time to show the difference between the heights of the target and fighter. Also the heights had to be displayed in other parts of the operations room. To enable these facilities to be given the height-signalling equipment was developed.

The Problem.

The requirements may be summarised as follows:-

(a) To provide a means whereby the height information as read by the height finder could be recorded in the form of a remote lamp display.
(b) To provide a means of determining the difference in the heights of the fighter and target and display this information.
(c) To enable (a) and (b) to be carried out for four separate interceptions simultaneously.

Circuit Principles.

To enable the information to be recorded and displayed, plunger keys were incorporated in the radar equipment and lamp displays installed as required. The information keyed was stored on groups of relays, each number being translated into a "Radix 2" expression. By such representation, subtraction of the two numbers was conveniently effected, because any number expressed in radix 2 terms is made up entirely of the digits 1 or 0, which is readily expressible on relays, since 1 or 0 can be represented by the operated or unoperated condition of the relay respectively. Then by testing the operated and unoperated conditions the difference can be recorded on a further group of storage relays.

The control equipment, consisting entirely of 3,000 type relays, was mounted on a standard 2,000 type rack 8 ft. 6 in. by 2 ft. 9 in. Strip-mounted relay sets were used and cables to the displays and controls were distributed from connection strips mounted on the rack. A fully equipped rack accommodated four interception circuits and one operations room display.

The Computer Type 60A

This equipment was designed for use in bomber control stations whose function was to transmit signals to pathfinder aircraft in the vicinity of the target, and thereby control the plane during its "run up" and release of target indicators, etc. Two control stations were involved in the operation and were known as Cat and Mouse stations. The Cat kept the bomber on a specified approach course and the Mouse gave instructions as to when the indicators should be released. The latter requirement involved an accurate timing device and facilities for sending code signals to the aircraft. For this purpose the computer was designed.

The Problem and Circuit Principles.

At the Mouse station a trace was provided on a cathode ray tube closely embracing the target area. This trace is represented by Fig. 6. A, B and C are artificial echoes and represent points on the approach run. A plane flying along the prescribed course is displayed as a further echo on the trace.

The problem was to measure the time taken for the aircraft to fly from A to B, and then determine the point v at which the release signal must be given to the aircraft. Also to send code signals to the aircraft at certain intervals, giving warning of approach to point y.

The circuit principle followed was

(a) To step a uniselector under the control of a fixed impulse rate as the plane echo travelled from A to B.
(b) To step a further uniselector an equivalent time, less the time of bomb fall (y to C), at which point a release signal was sent to the aircraft. The time of bomb fall was preset on a group of keys and adjusted according to the height of the aircraft.

The "approach" messages were sent by the depression of keys mounted on the receiver, the actual code being decided by relay circuits.

The equipment was mounted in duplicate on a special apparatus rack at the Mouse station. The control keys, etc., were mounted on the radar equipment.


Successful accomplishment of the Air Ministry's requirements was made possible by the co-operation of the parties involved. In this respect mention should be made of the Air Ministry Telecommunications Research Establishment from whom many of the basic ideas originated, of the Post Office Factories Department, Messrs. Siemens Bros. and others who were responsible for production, and of the Engineer-in-Chief's Circuit Laboratory and the Regions who carried out the installation work.

Permission from the Director of Signals, Air Ministry, to publish this article is gratefully acknowledged.

This article was reproduced from The Post Office Electrical Engineers Journal, Vol.38, Part 4, January 1946

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Updated 06/11/2001

Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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