Gee-H, sometimes written G-H or GEE-H, was a radio navigation system developed by Britain during World War II 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. 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 onboard aircraft and while somewhat less accurate, could direct as many as 80 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. Development history Distance measuring navigation Determining your location in 2D space requires two measurements of angle or range - two angle measurements, two distance measurements, or one angle and one distance. Early radio navigation was typically based on taking two angle measurements using radio direction finders, but these were accurate only to a few degrees and provided limited accuracy on the order of tens of miles. The development of range-based systems had to wait until the invention of accurate time measurement of radio signals were possible, which came about as a result of the development of radar. The Luftwaffe pioneered the use of distance-measuring radio navigation systems with their Y-Gerät system in 1941. Y-Gerät used a single Knickebein-like beam for steering the bomber in the proper direction, and an onboard transponder for distance measurements. A special signal was