The Chain Home radar system
The 25 Hz sinewave output from the Dippy oscillator was passed to a waveform generator and timing unit which produced the trigger for the transmitter, the time-base and bright-up waveform for the display and the trigger for the calibrator unit. The calibrator unit generated a sequence of 10 mile marker pips which could be mixed in with signal, allowing the time base to be accurately aligned to the range scale.
The waveform generator also produced a 12.5 Hz blanking pulse to suppress alternate trigger pulses to the transmitter and display unit when, under bad ionospheric conditions, the operator selected the alternative p.r.f. of 12.5 p.p.s.
In the early years of the war, aircraft were manually plotted by reading off range from the calibrated range scale of the display and the bearing from the goniometer, and then tracked locally on a grid-referenced plotting board. The grid references were then passed by the track-teller, via high quality landlines, to the Filter Room. Groups of stations were directly connected to the Filter Room each having their own dedicated plotter. The quality of the plots varied considerably, depending on the range and bearing of the target in relation to particular stations; it was the job of the plotters to sort out, or 'filter', the sometimes-conflicting information and endeavour to form a true track. New plots were assigned an 'X' number until positively identified and then allocated either an 'H' or 'F' prefix for hostile or friendly. The Filter Rooms were linked to the Fighter Control networks (GCI's, Sector Control, etc.) for appropriate action.
At a later stage, manual plotting was superseded by automatic plotting, using an electro-mechanical calculator, a 'fruit machine', figure 15, into which the various corrections resulting from calibration were programmed.
As the war progressed, many improvements were made to the equipment and to the reporting procedure to reduce the time taken to pass information to the various control centres at times when there were many raids in progress. For example, modifications to the receiver console were made to enable the operator after "D/F ing" the target and moving a cursor along the time-base to correspond to the target range, simply to press buttons for the data to be passed directly and automatically to the control centre.
Many ingenious devices, including optical converters, and calculators, too numerous to describe here, were introduced in the latter stages of the war which made the Chain Home system extremely efficient and reliable.
(This article is taken from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.)
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