 The U @-@ shaped centring post was sized so the tips of the U were the same width as the range indication line when the target was at 2 @,@ 500 feet ( 0 @.@ 76 km ) , which indicated that the pilot should throttle back and begin his final approach . Two vertical lines to the sides of the display , the goal posts , indicated that the target was 1 @,@ 000 feet ( 300 m ) ahead and it was time to look up to see it . Two smaller lines indicated a range of 500 feet ( 150 m ) , at which point the pilot should have seen the target , or had to break away to avoid collision . 
