 Airborne Interception radar , Mark IV , or AI Mk . IV for short , was the world 's first operational air @-@ to @-@ air radar system . Early Mk . III units appeared in July 1940 on converted Bristol Blenheim light bombers , while the definitive Mk . IV reached widespread availability on the Bristol Beaufighter heavy fighter by early 1941 . On the Beaufighter , the Mk . IV arguably played a role in ending the Blitz , the Luftwaffe 's night bombing campaign of late 1940 and early 1941 . 
