the scan. In the case of H2S, the echoes are returned from the ground and objects on it. That means the very first signal that would normally be received would be from the ground directly beneath the aircraft, as this is the closest to the aircraft. Since the echo from this location took some time to return to the aircraft, the time needed to travel to the ground and back at the aircraft's current altitude, the H2S display naturally had an empty area around the centre of the display, with its radius representing the altitude of the aircraft. This was known as the centre-zero. Normally the operator used a dial that delayed the start of the sweep in order to reduce the size of this centre-zero, and thereby increase the amount of the screen used for the ground display. When the centre-zero was not entirely dialled out, operators noticed that fleeting echoes were visible within this circle, and quickly concluded these were from other aircraft. This presented a simple way to see enemy night fighters as long as they were below the bomber and not far enough away that they would be hidden in the ground return. German night fighters normally approached from below as it helped silhouette the target aircraft against the Moon, and the lack of a gun position in that location made it safe to approach from that direction. This left them ideally positioned for detection by H2S. However, the display was very small, and this blank area on the screen only a small portion of that, so seeing these returns was difficult. In early 1943 German night fighter operations were improving. Between January and April 1943 Bomber Command lost a total of 584 aircraft to the defences. Although this represented only 4% of the sorties, this was nevertheless worrying because the increasing daylight length during the summer meant that the defences would inevitably be more effective. Several systems were already under development to help the bombers defend themselves, including the Monica radar (a simple adaptation of the original AI Mk. IV radar from the RAF's own night fighters) and the Automatic Gun-Laying Turret (AGLT), which was intended to automate defensive fire. However, the former proved almost useless in practice, and it was already clear the latter would not be available at least until 1944. Dudley Saward, the Bomber Command liaison with the radar teams, visited the