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

although many never claimed their hourly rate, but apart from their tin
helmets they were not issued with any uniform.
Each area was divided into groups. Within each group were a number of
posts, whose observation area was a concentric ring around the post, and
which overlapped with neighbours so that every part of the sky above was
covered. These would be given a letter and a number dependent on where
they were on the group grid, such as ‘R2’ or ‘J3’, for example. There were
usually thirty to thirty-four posts in a group, each manned by around
fourteen to twenty observers. Each post consisted of a hut, in which there
were a telephone, binoculars, logbook, tea-making facilities and a
pantograph that looked a bit like a giant sextant. With a height bar and
sighting arm, it also had a device for correcting height estimates and was
mounted on a gridded circular map of ten miles radius. When an aircraft
was sighted, the observer manning the pantograph made his calculation,
then another of the observers rang through to the group centre. Plots were
then followed at the group centre and forwarded on to RAF Operations
Rooms. Where radar warned of aircraft approaching Britain, the Observer
Corps provided information inland, as well as a back-up to what was being
provided by CH and CHL. It was also incredibly quick. By breaking the
system down to area, group and post, and by having different people
concentrating on different tasks, the Observer Corps network could manage
over a million reports during a twenty-four-hour period, each of which
could reach Fighter Command Headquarters in under forty seconds.
Information from the radar chain and the Observer Corps was all very
well, but useless unless the controllers on the ground were able to use the
information to direct fighters towards their targets. Radio telegraphy – R/T
– was standard in most aircraft at this time and allowed pilots to
communicate with each other once airborne. What had not been common
practice, however, was for pilots to be able to communicate with ground
controllers, or for ground controllers to be able to direct (or ‘vector’ as it
was termed) fighters towards targets from a control room – but this is
precisely what Dowding introduced. Not only did Fighter Command pilots
have radios that enabled them to listen to ground controllers, but there were
also networks of antennae radio receivers on the ground that picked up
transmissions from the pilot. Cables ran from these antennae to the control
room, where, on a cathode ray tube screen, the direction of the transmission
could be picked up. With the receivers at the centre of the screen, a line