 Tests were carried out starting in October 1940 , and quickly demonstrated a number of minor problems . One of the minor issues is that the crosshairs on the tube that indicated the center would block the spot . A more serious concern was the lack of range information , which the FIU pilots considered critical . Hanbury Brown went to work on these issues , and returned an updated version in December . A U @-@ shaped reticle in the center of the display provided a centre location that left the spot visible . Additionally , the circuitry included a second timebase that produced a longer signal as the fighter approached its target . The output was timed so the line was centred horizontally on the dot . This presented the range in an easily understandable fashion ; the line looked like the wings of an aircraft , which naturally grow larger as the fighter approaches it . 
