H2S was airborne radar equipment designed for use as an aid to navigation and blind bombing. It was developed by Dr Bernard Lovell at the Telecommunications Research Establishment . It was designed to produce a map on a a CRT (Cathode Ray Tube) PPI (Plan Position Indicator) of the area over which the aircraft flew.The PPI display showed a circular map of the terrain with the radar beam illuminating prominent relief features and towns at their true ground positions and bearings. The top of the PPI display always represents true north and the position of the aircraft is represented by the centre of the PPI scan, which is normally at the centre of the map. This revolutionary system was introduced into service in December, 1942. Bomber Command's continued losses to German nightfighters led to a variety of warning devices being developed. Fishpond was introduced in 1943. It utilised that part of H2S's signal, not used for mapping, which radiated between the ground and the aircraft, and gave warning of other aircraft in the vicinity. Fishpond's PPI screen was fitted in the radio operator's position.
Fishpond Warning Device
IND 82A Ref No 10. QB/6037