Indicator Unit Type 182.
(H2S Fishpond Indicator.)
Fishpond was the code name given to an extension to the British H2S airborne radar system fitted to RAF Lancaster and Halifax heavy bombers during World War II. It was designed to give early warning of German night fighters approaching from below the aircraft out to a range of 30 miles.
The H2S radar was designed to give a map-like display of the ground below the aircraft. In order that the centre of the display represented the ground immediately beneath the aircraft, the scan had to be delayed to allow for the transmitted radar pulses to travel from the aircraft to the ground and back again. It was realised that during this delay time, echoes would be returning from any other aircraft within the bombers vicinity. A second display was installed in the radio operator's position which displayed the image that was suppressed from the navigator's display. This display showed the relatively stationary echoes from the bomber formation and most importantly, the rapidly moving returns from the German night fighters.