Gee-H, sometimes written G-H or GEE-H, was a radio navigation system developed by Britain during the Second World War to aid RAF Bomber Command. The name refers to the system's use of the earlier Gee equipment, as well as its use of the "H principle" or "twin-range principle" of location determination.[1] Its official name was AMES Type 100.
Gee-H was used to supplant the Oboe bombing system which worked along similar lines. By measuring and keeping a fixed distance to a radio station, the bomber could navigate along an arc in the sky. The bombs were dropped when they reached a set distance from a second station. The main difference between Oboe and Gee-H was the location of the equipment; Oboe used large displays in ground stations to take very accurate measurements but could only direct one aircraft at a time. Gee-H used much smaller systems on aircraft and while somewhat less accurate, could direct as many as eighty aircraft at a time.
Gee-H entered service in October 1943 and first used successfully in November against the Mannesmann steel works at Düsseldorf on the night of 1/2 November, when about half of the sets failed leaving only 15 aircraft to bomb the factory. Gee-H remained in use throughout the war, although it was subject to considerable jamming from the Germans. It also remained a standard fixture of post-war RAF aircraft like the English Electric Canberra. Gee-H was adapted by RCA into the US wartime SHORAN system with improved accuracy. The same basic concept remains in widespread use today as the civilian DME system.