 Homing on the enemy 's broadcasts meant that there was no accurate way to calculate the range to the target ; radar ranging measurements are based on timing the delay between broadcast and reception , and there was no way to know when the enemy 's signal was originally broadcast . This meant that the homing device could only be used for the initial tracking , and the final approach would have to be carried out by radar . The extra range of the Mk . VIII was not required in this role as Serrate would bring the fighter within easy tracking range , and the loss of a Mk . IV would not reveal the secret of the magnetron to the Germans . For this reason , the Mk . IV was considered superior to the newer radars for this role , in spite of any technical advantages of the newer designs . 
