Halifax V9977, H2S test aircraft, bulge below the roundel houses H2S equipment

H2S 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 eventually showed up as bright spots on the screen in front of the navigator. Multiple signals joining up painted a map on that screen, showing details of the terrain within the equipment’s scanning range.

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, as H2S was not affected by cloud or fog. Bomber aircraft could fly at night locate their targets and attack them even when they were completely cloud-covered.  

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.