The Dowding system and the Filter Room

The so called “Dowding System” consisted of several parts:

• The British radars could pick up enemies as far away as 200 km. This gave enough time to assess size and tracks of the attacking units and to prepare and scramble the defending fighters.
• Visual observers would refine this with information on aircraft numbers, types and altitude, using triangulation based on radar distance and the observed angle.
• The radar and visual observations passed through a filter room before reaching control rooms.
• The information was received, filtered and relayed quick enough to make scrambling of fighters possible for them to meet the enemy before they reached their targets.
• Once airborne, the fighters were directed towards the enemy and kept on receiving updated information during the engagement.
• After each mission fighter’s were quickly refueled and armed for the next sortie.

The system was created using standardisation. Every person in the system had a specific task and worked according to standardised procedures. Anyone in the system (a plotter, pilot, controller or observer) could this way be transferred to another unit and continue working exactly the same way.

Let’s have a look at the “filter room”.  This picture above is a war-time photo of the filter room at Bentley Priory.

The idea of a “filter room” came from Squadron leader Raymond Hart. He created the first “filter room” at an experimental radar station at Bawdsey in July 1937. It was quickly realised how well it worked and already in October the same year it was moved to the Bentley Priory.

A full-size museum model of the original “filter room” has been created in the room where it was originally located. In the middle of 1940 the equipment was however moved to an underground bunker in the Bentley Priory grounds.

The role of the filter room was to collect, and as accurately as possible analyse, information coming from radar stations along the British coast. The “filtered” information was then forwarded to the “Command and Group Operations Room” at Bentley Priory but also to the different “Sector Control Rooms” where the information was used to give orders to the fighter squadrons.

The information from all radar stations was plotted on a gridded map table. Each plotter placed coloured counters on the map to describe the received information.

The activity around the table often became frenetic with lots of highly skilled staff engaged in the plotting.

The filterer’s job was the most difficult. It was about gathering all information quickly, try to see a pattern and determine which scenario was the most likely.  The filtering of this information included to assess the information (was it enemy or friendly aircraft or even birds?) and for enemy aircraft to determine the track and then to pass this information to the Operations Rooms and Control Rooms.

What is most interesting is that Germany had even more advanced equipment but did not use it to support the Luftwaffe units. The German radar-system was more advanced but not used and the Luftwaffe pilots had no comparable ground control system. The pilots only communicated with pilots in their own squadron. German fighters were not even communicating with the bombers they defended!

The German intelligence was also insufficient. Instead of attacking the fighter airfields they often attacked bomber airfields. During the Battle of Britain only one fighter airfield was knocked out for more than 24 hours. The German tactics for their fighters were not efficient. The Me109 was superior to the British fighters; it had better fire-power and diving capabilities. Still it was ordered to fly as close escort to slow bombers in a way that took away the advantage of high speed.

You might say that the Battle of Britain was a battle between an attacking Germany with more advanced technology and greater numbers versus a defending Britain with a coherent system and excellent tactics.