Radar, Gun Laying, Mark I, or GL Mk. I for short, was an early radar system developed by the British Army to provide range information to associated anti-aircraft artillery. There were two upgrades to the same basic system, GL/EF (Elevation Finder) and GL Mk. II, both of which added the ability to accurately determine bearing and elevation.
The first GL set was an elementary design developed from 1935 onward. Based on Chain Home, GL used separate transmitters and receivers located in wooden cabins mounted on gun carriages, each with its own antennas that had to be rotated to point at the target. The antenna produced a signal that was semi-directional and was only capable of providing accurate slant range information; target bearing accuracy was approximately 20 degrees, and it could not provide target elevation information. A number were deployed with the British Expeditionary Force and at least one was captured by German forces during the Dunkirk evacuation. Their evaluation led them to believe that British radar was much less advanced than German radar.
Plans to introduce the Mk. II with accurate bearing and elevation were underway from the start, but these would not be available until 1940. An expedient solution was the GL/EF attachment, providing bearing and elevation measurements accurate to about a degree. With these improvements, the number of rounds needed to destroy an aircraft fell to 4,100, a ten-fold improvement over early-war results. About 410 of the Mk. I and slightly modified Mk. I* units had been produced when production moved to the Mk. II, which had enough accuracy to directly guide the guns. Higher accuracy and simpler operation lowered the rounds-per-kill to 2,750 with Mk. II. After the invasion of the Soviet Union in 1941, about 200 Mk. II units were supplied to the Soviets who used them under the name SON-2. 1,679 Mk. IIs were ultimately produced.
The introduction of the cavity magnetron in 1940 led to a new design effort using highly directional parabolic antennas to allow both ranging and accurate bearing measurements while being much more compact. These GL Mk. III radar units were produced in the UK as the Mk. IIIB (for British), and a locally designed model from Canada as the Mk. IIIC. Mk. II remained in service in secondary roles as Mk. III's replaced them at the front. Both were generally replaced starting in 1944 by the superior SCR-584.
Development
Army Cell
The first mention of radar in the UK was a 1930 suggestion made by W. A. S. Butement and P. E. Pollard of the Army War Office's Signals Experimental Establishment (SEE). They proposed building a radar system for detecting ships to be used with shore batteries, and went so far to build a low-power breadboard prototype using pulses at 50 cm wavelength (600 MHz). The War Office was uninterested and did not provide funding for further development. The matter was mentioned in passing in the January 1931 issue of the Inventions Book of the Royal Engineers.
With the Air Ministry's successful demonstration of radar and rapid progress on the system that would become Chain Home (CH) in 1936, the Army suddenly became interested in the topic and visited the CH radar team at their new headquarters at Bawdsey Manor. Here they were introduced to smaller versions of the CH system intended for semi-mobile deployments. This appeared to have a number of uses in Army roles, leading to the 16 October 1936 formation of the Military Applications Section, but referred to universally as the Army Cell. This group was given room at Bawdsey, and included Butement and Pollard from the SEE.
The Cell was initially given the task of improving anti-aircraft fire, and had been told that the main problem to address was the accurate measurement of range. Optical instruments were used to detect aircraft and accurately determine their bearing and elevation, but rangefinding through optical means remained difficult, slow and open to simple errors in procedure. A radar system that could provide accurate and rapid rangefinding would greatly improve their chances of successfully engaging an aircraft. They were given the goal of producing a range measure accurate to with at a range of .