 In spite of best efforts , AI 's maximum range remained fixed at the aircraft 's altitude , which allowed Luftwaffe aircraft to escape interception by flying at lower altitudes . With a five @-@ mile ( 8 km ) accuracy in the ground direction , that meant anything below 25 @,@ 000 feet ( 7 @.@ 6 km ) would be subject to this problem , which accounted for the vast majority of Luftwaffe sorties . The lack of coverage over land was another serious limitation . 
