 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.
