 Experience demonstrated that the final approach to the target required fast action , too quick for the radar operator to easily communicate corrections to the pilot . In 1940 , Hanbury Brown wrote a paper On Obtaining Visuals from AI Contacts which demonstrated mathematically that the time delays inherent to the interception system were seriously upsetting the approach . In the short term he suggested the fighters make their approach to dead astern while still 2 @,@ 500 feet ( 760 m ) out , and then fly straight in . For the longer term , he suggested adding a pilot 's indicator that directly demonstrated the direction needed to intercept . 
