Early Centimetric Ground Radars - A Personal Reminiscence
These coastal ship-watching stations had two main functions: first, to ensure that shipping moving up and down the coast navigated safely along the 'swept channel' between the inner and outer minefields, and second, to warn of the presence of 'E-boats', which were the enemy equivalent of the British MTBs (motor torpedo boats). These boats were fast (40-50 knots), of shallow draft and heavily armed for their size. A few such boats could wreak havoc on an undefended convoy of coasters in a few minutes, and then escape at high speed.
To fulfil these functions, the operations room was fitted with two displays, an A-scope, showing the amplitude of the echo against its range, and a PPI (plan position indicator) on which the echo showed as a bright 'sausage' against a grid map from which its position could be read off directly. Such a display was based on a linear radial timebase, rotating in synchronism with the antenna: on it, of course, the coastline and any inland features showed up clearly, to give an easily recognizable map of the surroundings.
The rotation of the antenna was controlled by the A-scope operator, and could be either continuous at 4 revs/mm., or as controlled by a small hand-wheel, by which the operator could turn the antenna backwards and forwards over any angle. In this way, any desired ship could be 'D/Fed' for a maximum A-scope signal, and its range and bearing read off. In accordance with naval gun-ranging practice, the A-scope range was measured in thousands of yards (1 yard is c. 0.9m). The range and bearing of each ship in a convoy, or perhaps only of the leading and trailing ships, would be converted into grid positions and passed to the local naval plotting room, routinely every fifteen minutes, but more often if required; for instance, as the convoy approached a turning point in the swept channel.
All ships within range also appeared, of course, on the PPI, from which their position could in principle have been read-off at once, thus making the laborious plotting from the A-scope unnecessary Unfortunately, the PPIs of this vintage were too inaccurate to give reliable plots, so that the role of the PPI was to monitor the position continuously while the aerial rotated steadily at 4revs/min. The PPI operator would check, on each sweep, that there was still the right number of ships in the convoy, and that no strange responses, especially those appearing at extreme range to seaward of the swept channel, appeared. Such echoes, especially by night, were likely to be E-boats. Any strange echo was reported at once, even if its position was not accurately known, and sufficed to alert, and often to alarm, the naval plotting room. From the PPI plot, a more accurate range and bearing plot from the A-scope was passed as soon as possible.
The reason for the inaccuracy of the early PPIs has been discussed earlier in this paper; because there was no error in bearing, and because the relative error in range between a night-fighter and its target was much less than the absolute error, the GCI controller could guide the fighter to within Al range. For ships, of course, absolute positional accuracy was essential to avoid the mine-fields.
The RAF response to naval demands for the more accurate (but time-consuming) plots of range and bearing was sympathetic until we made a liaison visit to the naval plotting room. Here we found a large plotting table, perhaps 3m x 2 m, on which ship positions could indeed be marked up accurately. However, routine plotting was done on a much smaller map, perhaps 0.5m x 0.3m, on which the standard 'Chinagraph' pencil made a blob covering several square kilometres! Nevertheless, in the interest of good inter-Service relations, the RAE stations continued to plot from the A-scope.
Quarterly Overhauls on Centimetre Stations
On returning to HQ, 73 Wing, then at Malton in Yorkshire, from the three-week Centimetre Course at Yatesbury, I found that my new job was to carry out quarterly overhauls (QOs) on the Wing's centimetre stations. To this end, and to learn what the job involved, I was dispatched to Bempton, a Type 52 station situated on the cliffs just north of Flamborough Head. Here a QO was already in progress, under a W/O Walker, about whom I had already been given some sage advice at Wing - 'go easy with Mr. Walker'. The advice was superfluous, since I treated any W/O, and especially a W/O radar mechanic, with the utmost respect and deference, acknowledging their years of experience. Mr. Walker was a small hard-bitten man, with prodigious experience, knowledge and work-rate: he was not averse to colourful language about anything, or anybody, if things did not go well. However, he treated me very correctly, although he must have despised my smattering of centimetre knowledge. He called me 'Sir' once a day and even got me to sign the QG report, which he had already carefully filled in.
