 The Mk . IV display system consisted of two 3 @-@ inch ( 7 @.@ 6 cm ) diameter cathode ray tubes connected to a common timebase generator normally set to cross the display in the time it would take to receive a signal from 20 @,@ 000 feet ( 6 @.@ 1 km ) . The displays were installed beside each other at the radar operator 's station at the rear of the Beaufighter . The tube on the left showed the vertical situation ( altitude ) and the one on the right showed the horizontal situation ( azimuth ) . 
