 On 24 November 1939 , Hanbury Brown wrote a memo on Suggestions for Fighter Control by RDF calling for a new type of radar that would directly display both the target aircraft and the intercepting fighter , allowing ground controllers to directly control the fighter without need for interpretation . The solution was to mount a radar on a motorized platform so it rotated continually , sweeping the entire sky . A motor in the CRT display would rotate the beam deflection plates in synchronicity , so blips seen when the antenna was at a particular angle would be displayed at the same angle on the scope display . Using a phosphor that lasted at least one rotation , blips for all targets within range would be drawn on the display at their correct relative angles , producing a map @-@ like image known as a PPI . With both the bombers and fighters now appearing on the same display , the radar operator could now direct an intercept directly , eliminating all of the delays . 
