The sectors of 11 Group from an
original map. HQ Fighter Command (RAF Stanmore) and HQ 11 Group (RAF
Uxbridge) are marked in red, the sector stations are marked in blue.
First level control
Group was split into Sectors with RAF stations in each, one of which was
the Sector Control Station. All the Sector Control Stations reported to
the Group Headquarters, and they in turn reported to Fighter Command
Headquarters at Stanmore in Middlesex, near London. The Headquarters acted
as a filter and communications centre.
The Sector Control Rooms at the Sector
Stations reported to Group Headquarters, who in turn reported to Fighter
Command Headquarters. This report would initially include Fighter Squadron
status. For a larger view, click on any of the diagrams.
All round the coast were radar (Radio Detection and
Ranging) stations, which could 'see' an enemy raid, in some cases while it
was still over France. The raid was reported to Fighter Command HQ (FCHQ)
where it was plotted on a large map. This information was then passed to
the Group Headquarters, who passed it on to the Sector Control Rooms
affected by the plot.
The RDF Chain reported to the Filter
Room at Fighter Command Headquarters, this information was filtered then
passed to Group then Sector level.
RDF information was crude by modern standards, but was more than
sufficient to give bearing and range information on an incoming raid. The
system consisted of two dials and a cathode ray tube that gave a screen
presentation of the raids range. The two dials supplied time and bearing
information. The height of the raid could only be accurately provided once
the raid came within visual sighting range of the Observer Corps posts.
With experience, RDF Operators could judge the size of the raid by the
size and shape of the blip on the screen.
The RDF screen. The readout at the top
shows the ground return blip, from trees, buildings and any terrain near
the station, at zero miles. The blip at 40 miles is the raid. The left
hand dial shows a bearing to the raid of 89 degrees. The time on the right
hand dial reads 11 hours, 10 minutes and 27 seconds from the inner dial
Observer Corps Reporting
over the country were Observer Corps Posts. Their job was to report the
raids once they had crossed the coast and were behind the radar. They
reported to Observer Corps Centres, who passed the information on to their
Sector Control Rooms, thence to Group Headquarters, who in turn sent it to
FCHQ and the plot of the raid was kept up to date.
The Observer Corps Posts reported to
their respective Observer Corps Centres, which reported in turn to their
local Sector Control Rooms. This information then went to Group and HQ
level, for onward transmission to the other sectors.
Primary Defence Control
this information was passed up or down to the Sector Control Rooms, which
actually directly controlled the defences. The Sector Controller then knew
exactly where the enemy were and alerted the balloon sites in possible
target areas to put up a balloon barrage. The balloons forced the German
pilots to fly their bombers higher, which made bomb-aiming more difficult.
The Sector Control Rooms then alerted
the Balloon Barrages and Anti-Aircraft guns, as well as controlling the
Fighter Squadrons and giving them radio information about the incoming
Controlling the Fighters
In order to vector the defending fighters on to an
incoming raid, the controller had to know exactly where they were. The
last link in the defence system, keeping track of the RAF fighters, were
Direction-Finding or D/F radio stations. These also reported to their
local Sector Control Room, and so up the chain of command in the same way
as the Observer Corps. Again, this information was transmitted to every
sector to keep the "Big Picture" spread throughout the command system. By
doing this, the loss of a single Sector Control Room did not destroy
Fighter Command's ability to function as an effective defence.
The Direction Finding stations
provided the last link by tracking friendly fighters and reporting to the
This control system is the source of many of the terms
still used in air traffic control today. To assist the RDF and D/F
operators, British fighters were equipped with a simple form of
transponder, which automatically replied to interrogation from the ground
systems with an identification signal. Known as IFF, or Identification
Friend or Foe, it identified friendly aircraft on the screens. A secret
device, it was code-named 'Parrot'. The instruction to switch it on was
therefore "Squawk your Parrot". That term is still in use today as modern
transponder codes are known as 'Squawks'. Steer is also still in use, and
these remnants are a testimony to the basic soundness of the system of