When the night-time raids commenced, it was necessary to
modify the ground control arrangements using 1.5m systems
(Ames Type 15 for example) because they were more accurate.
The daytime arrangements involved the use of 'filter rooms'
where information was collated from a number of sources including
ground observers. At night however, the GCI units information
was fed directly to Fighter Command and one ground controller
would then speak by radio-telephone to a specific 'Wing Leader'
and direct him onto the optimum course and altitude to engage