ANSWER:  “The SCS 51 system, initially developed in the USA, was one of the new approach aids being introduced to service late in the war. Introduced to Britain early in 1944, it provided azimuth direction, the localizer, and glide path information to the user through a system of tone modulation discriminations. This was a great advance over the earlier beam approaches, as it provided a continuous indication of deviations from course. The civilian version of this equipment became famous as the Instrument Landing System (ILS). Also developed late in the war was the Ground Controlled Approach (GCA), which consisted of an extremely high precision microwave radar, which provided the position of a “target” aircraft in azimuth, range and elevation. By this means the position of a landing aircraft relative to a predetermined approach path was displayed on a ground scope, whereby a skilled operator could provide directions to “talk the aircraft down”. This equipment, however, was still in a developmental stage during the war years. Similarly, the use of short-range microwave Airfield Control Radar (ACR) for directing the flow of aircraft in the vicinity of airports was pioneered at several Bomber Command stations during the war years, and much valuable experience was gained for future applications.”

From  No Prouder Place – Page 185