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

pilots. On the wall opposite the controllers would be the five-minute colour
change clock. Each five-minute section of the clock was coded red, yellow
or blue. When a plot came through, it was given the colour indicated by the
minute hand at the time of its arrival. There were also a weather board, a list
of barrage balloon squadrons and their heights, and a series of panels, at the
top of which were written the Fighter Command squadrons in each, and
beneath them their state of readiness, such as ‘available’, ‘in position’ or
‘landing and refuelling’. The relevant state would be illuminated by a
maximum of four lightbulbs, each reflecting the relevant colour of each
squadron section, red, blue, yellow and green. Only when ‘enemy sighted’
was lit up would the bulbs all be coloured red. These state boards were
known as ‘totes’ because they looked rather like the lists of horses and odds
displayed at a race meeting. Below these there was also a list of the pilots
and aircraft available to each squadron, which would be updated first and
last thing each day. Thus with one glance a controller had a mass of
information about the current state of play at his fingertips.
Dowding had also issued a series of very simple code words for various
orders; they could be easily memorized and rarely misheard or mistaken for
something else. Thus ‘scramble’ meant take off as soon as possible, ‘orbit’
to circle, ‘vector 230’ to fly on a course of 230 degrees; ‘angels’ meant
height – ‘angels 15’ stood for 15,000 feet; ‘bandits’ was the code for enemy
aircraft. It was simple and it was standardized.
The hub of this network – the nerve centre – was Fighter Command
Headquarters at Bentley Priory. The pièce de résistance, however, was the
Filter Room, which, since March, had been housed in a concrete bunker
below the priory itself. All radar plots were received here, with a filter
officer for each CH and CHL station. Information was checked and
assessed and cross-referenced with other known flights. Another canny
invention was Identification Friend or Foe (IFF), which was a small
transmitter in all RAF aircraft that gave a distinctive blip if they flew near
the coast and were picked up by radar. When it was not clear, plots would
be labelled ‘X’. This was the filtering process. Once this was done – and
filter officers were highly trained to do this task quickly and accurately –
the plot was given a number and passed to the Command Operations Room
in the bunker next door, and simultaneously to the Group controllers, who
then passed them on to the relevant sector stations. Sector stations then
forwarded details of the plot back to Observer centres.