H2S was a critical part of the electronics war waged by the Pathfinders. The radar equipment transmitted a directional beam of high-energy impulses outwards and downwards towards the ground. Reflections of its own impulses received from the ground were accepted back into the H2S aerial, fed into a receiver, and showed up as bright spots on the screen in front of the navigator. Multiple signals joining up painted a map on the screen, showing details of the terrain within the equipment’s scanning range.
The radio waves, generated by electronic components, travelled at the speed of light and could pass through fog, rain, mist, snow and smoke. They were not affected by infrared energy from fire or heat haze, a vital factor during bombing raids.
The maps produced could be astonishingly clear, especially where there were large areas of sea, river or other water bodies.
H2S equipment on the left – the round clock-like dial was where the map appeared.
The effectiveness of H2S meant that the bombing campaign was freed from the constrictions of weather and daylight. Blind-flying became possible, and bomber aircraft could locate their targets and attack them at night even when they were completely cloud-covered.
H2S image, P/O Bartleman, flying from Warboys on 31 October 1944. Attribute unknown.
Bennett on the naming of H2S:
How H2S got its name is the subject of so many conflicting stories that I should hate to attempt to confirm or deny which is the most authentic. The one which I believe to be the best is simply that which Lord Cherwell, scientific adviser to the Prime Minister, gave. When somebody asked him what he thought of it, he said, ‘It stinks. Call it H2S.’ *
H2s is the formula for Hydrogen Sulfide, a colourless gas which smells like rotten eggs.
* Quotation from D C T Bennett, Pathfinder: Wartime Memoirs (Frederick Muller Ltd, London. 1958).