Chain Home Radar - A Personal Reminiscence
Now
envisage the receiving dipoles connected to the goniometer coils as in fig.
8, where the stator coils also run N-S and E-W: in practice, of course, the
coils could lie in any direction, with suitable adjustments of the gonio pointer
and scale.
A target due east of the station would give a signal only in the N-S dipoles, and this would be transferred to the N-S coils of the goniometer. The goniometer would pick up a maximum signal when its search coil also lay in the N-S direction, and a minimum, (ideally, zero) signal when the search coil lay E-W Hence, if the gonio pointer is aligned with its search coil, it will point to the east, that is, to the bearing of the target, when the gonio is set for minimum signal. An ambiguity could arise here, since the goniometer would also give a minimum for a target bearing due west, or even when its search coil was 1800 from the true bearing. In practice, the ambiguity was resolved by the use of the sensing dipole before any attempt was made to take a bearing.
In
the height-finding mode, of course, the gonio measured, not so much an angle,
as the ratio between the signals on dipole arrays at different heights: this
ratio gives the angle of elevation of the target, and this, combined with the
range, gives the height a via simple formula.
The most important operator, by far, was the girl 'on the tube' (fig. 9). It was her duty to update plots on existing tracks at regular intervals, while at the same time watching for new echoes: to this end, the gonio was swung continuously. Since the bearing of a target was determined by swinging the gonio for a minimum (zero) echo, the station was blind on that bearing, or indeed on any bearing on which the gonio rested: hence, the first rule for operators was that the gonio must always be swinging over the full 1000 or so of the station coverage. The girl on the tube, together with the others of the team, were paralleled together on the telephone to the plotter in the filter room, who would accept the plots, allocate track numbers (H-for hostile, F- for friendly, X- for unknown), allot priorities ('another plot on X-, please', 'can you get a height on H- ?') and probe for information 'Staxton has a plot about 120 miles from you: can you see anything?' (Staxton being the neighbouring station).
The routine of establishing a new track was quite complex. If the operator noticed a new echo, which would first show as a tiny break in the noise, she would at once 'sense' it to determine that it was in fact in front of the station before announcing 'I have a new echo at such and such a range'. She would then sense again, to determine whether the target was north or south of the 'line of shoot'. She would then attempt, by swinging the gonio for a minimum, to take a bearing, and then a height.
