 By 1939 , it was clear that war was looming , and the team began to turn their primary attention from ASV back to AI . A new set , built by combining the transmitter unit from the latest ASV units with the EMI receiver , first flew in a Battle in May 1939 . The system demonstrated a maximum range that was barely adequate , around 2 to 3 miles ( 3 @.@ 2 – 4 @.@ 8 km ) , but the too @-@ long minimum range proved to be a far greater problem . 
