The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

would light up from the transmissions from the pilot, which would then
indicate what bearing he was on. This was called High Frequency Direction
Finding, or HF/DF (pronounced ‘huff-duff’), while the automatic
transmissions were christened ‘Pip-Squeak’.
With information on enemy aircraft’s position from radar and the
Observer Corps, it was then a case of applying simple trigonometry.
Drawing a line from the enemy to the fighters and making this the long base
of an isosceles triangle, the fighter would then be vectored along an angle
that was the same as that of the bomber. Where the two equal angles met to
form the apex of the triangle would be where the two would meet, known as
the interception point. If the bombers changed course, a new triangle would
be visualized and if the fighter reached the apex before the bomber, he
could circle and wait. A further refinement was the ‘Pip-Squeak’ system, by
which a pilot’s radio gave off transmissions automatically for fourteen
seconds in every minute. In the control room there would be a clock with its
face divided into four coloured quarters, with a hand that rotated once a
minute. Four aircraft could thus be controlled in rotation, each pilot being
told the quarter in which he was to set his control. The code word for Pip-
Squeak was ‘cockerel’ if a pilot forgot to switch it on, as often was the case,
the controller could ask, ‘Is the cockerel crowing?’ It was simple but
ingenious.
The final cog was the telephone network, which was run and maintained
by the General Post Office, the GPO, then still part of the Government.
GPO engineers were responsible for laying vast numbers of extra lines
between 1937 and 1940. Most RDF stations, for example, were built on
comparatively remote farmland, which provided a big logistical challenge.
At each, two lines were needed for signalling between stations and the
appropriate fighter group headquarters Filter Room. These also needed to
be taken through different routes as an insurance against damage. Another
line was needed for general operational control, while a further two lines
were needed for communicating between neighbouring stations – thus each
station needed five new and separate lines in all. At Bentley Priory, all lines
went through the Stanmore exchange at the bottom of the hill, but, in case
of damage there, an entire duplicate set of lines was built through to the
Bushey exchange to the north.
In addition to this incredible and complex amount of engineering work
to and from fighter stations, radar stations, Observer posts and so on, the