H2S - A self-contained scanning radar. It consisted of three parts, a generator driven by the starboard outer engine, a rotating radar emitting and receiving scanner mounted in a pod under the aircraft, and a Plan Position Indicator, a cathode ray tube, in front of the Observer. The scanner rotated once per second, and reflected from buildings, etc. directly below and forward of the aircraft. The scan remained on the screen long enough to be updated by the next rotation. No reflection was received over water, but the coastline could be identified, as could reflections from towns and villages along the route. From these, a bearing and distance could be calculated and plotted, and this would be passed to the Plotter for his use in determining course, wind velocity and ground speed, which info he would pass up to the pilot with any changes required to keep us on track.
"H2S" was an downwards pointing radar scanner in the rear belly of the aircraft; a large Perspex black-painted blister contained the rotating scanner. It gave a reasonable "picture" of the ground below; water, buildings and roads showed up clearly. It could not be jammed, but specially-equipped Luftwaffe night-fighters could home in on any aircraft using it. Consequently it was only used by a bomber for very short periods. RAF intruders (counter-night-fighters) homed in on Luftwaffe aircraft using airborne radar, and shot them down, often over their own bases.
By this time, all the NF.IIs had been converted into NF.XIIs and NF.XVIIs. NF.IIs had been performing night intruder sorties over Occupied Europe since mid-1943 to help Bomber Command deal with German night fighters. The British had decided that the Germans had little to learn from the old AI.V longwave radar, but worries that the Germans might learn the secrets of centimetric radar kept the NF.XII and later night-fighter marks out of enemy airspace until May 1944.
In fact, the Germans had pulled an H2S centimetric bombing radar out of the wreck of a British bomber over a year earlier and the secret was pretty much out of the bag anyway. It did the Germans little good, as they never managed to put a centimetric radar into large-scale service, though they did build a device named "Naxos" that could home in on centimetric emissions.
In February 1943, a Stirling bomber with the H2S radar was shot down near Rotterdam and the radar was found by the Germans. The Germans tested this radar.
A Stirling fitted with H2S was shot down near Rotterdam on Feburary 2, the H2S were retrived by the Germans. But the German Scientists were never able to make a receiver in 2-3 months, as suggested by Watson-Watt. It was 8 months’ time before a Naxos-U receiver was made against ASV III systems, thanks to the huge gap in technology between the British and the German.
Alan Blumlein was head of the EMI team which was responsible for developing electronic circuitry for the H2S radar programme. It was while working on H2S that Blumlein and several of his colleagues were killed during a demonstration flight in a Halifax bomber.
Despite the loss of Alan Blumlein and other key members of the H2S development team, the project was completed. H2S went on to become one of the most important radar developments of the Second World War, allowing accurate bombing of enemy targets with a precision never before achievable.