Fishpond
Fishpond was the code name given to an extension to the British H2S airborne radar system fitted to Royal Air Force (RAF) Avro Lancaster and Handley Page Halifax heavy bombers during World War II. It was designed to give early warning of German night fighters approaching in the hemisphere below the carrying aircraft out to a range of 30 miles (48.2 km).
The H2S radar was a navigational aid designed to give a map-like display of the ground below the aircraft. The display, known as a Plan Position Indicator (PPI), used a scanning electron beam in a Cathode Ray Tube which scanned from the centre of the display to the edge in a direction corresponding to the direction in which the radar's scanning head was pointing. 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. This delay was derived from a simple radio altimeter.
It was realised that during this undisplayed time echoes would be returning from any other aircraft within the vicinity of the bomber. Accordingly a second display was installed in the radio operator's position which displayed the image suppressed from the navigator's display. This display showed the relatively stationary blips from the bomber formation and, most importantly, the rapidly moving returns from the German night fighters.