Filter Room

The filter room at Fighter Command Headquarters lay at the top of the Dowding system - the integrated ground-controlled interception network that covered the United Kingdom.

A shortage of fighter aircraft, trained pilots and fuel created a need for filter room personnel to work with great speed.

They displayed it on a gridded map table by placing counters that indicated estimated height, number of aircraft, and Identification Friend or Foe (IFF) status.

The Filterer's job "was to decide the moment when a few successive plots, with all their possible inaccuracies, might be considered as a reliable track, fit for the operations room to act on for fighter interception".

The filterers constantly had to make a relatively difficult decision of "whether to put down an arrow on the strength of his (her) first guess, or to wait for another plot in the hope that it would confirm or confound his (her) suspicions.

Squadron Leader Mike Dean MBE, referring to the filterer's job, explained: "The mass of raw information generated by the Home Chain of radars had to be processed before it could be presented to the Operations Room.