Radar, Gun Laying, Mark III, or GL Mk. III for short, was a radar system used by the British Army to directly guide, or lay, anti-aircraft artillery (AA). The GL Mk. III was not a single radar, but a family of related designs that saw constant improvement during and after World War II. These were renamed shortly after their introduction in late 1942, becoming the Radar, AA, No. 3, and often paired with an early warning radar, the AA No. 4, which was also produced in several models.
The Mk. III began development shortly after introducing the cavity magnetron in early 1940. The magnetron allowed radar systems to operate at microwave frequencies, greatly reducing the size of their antennas and making them much more mobile and accurate. Having originally started work on the magnetron as part of the AI Mk. VIII air-to-air radar, the team was told to drop everything and develop a radar for AA use as quickly as possible. This turned into a fiasco; by the end of the year, very little progress had been made, and the team returned to working on airborne radars.
The magnetron has also been demonstrated to the Canadians and the US as part of the Tizard Mission in the fall of 1940. Immediately following the visit, the National Research Council of Canada began development of a GL radar based on the UK design. The first examples of these GL Mk. III(C) (for Canadian) arrived in the UK in November 1942. British units of slightly more advanced design, GL Mk. III(B) (for British) arrived in December. 667 of the Canadian models were produced, with about 250 of these seeing service in the UK while most of the others were sent to the continent or remained in Canada, while a small number were used by the Australian Army. 876 of the British models were produced and saw more widespread service. Fifty Mk. IIIs were supplied to the Soviet Union.
Several improved versions of the Mk. III(B) were experimented with, but none were widely produced due to the 1944 introduction of the SCR-584 from the US, which provided both scanning and tracking in a single semi-trailer unit. Mk. III units found themselves relegated to secondary roles, as diverse as artillery spotting, coastal surveillance and weather balloon observation. Several upgrades were carried out for these roles, and modified weather units remained in use until about 1957/58. More radical development of the design also led to the greatly improved post-war AA No.3 Mk.7 radar, which served as the Army's primary AA radar until the removal of AA guns from service in the late 1950s.
Development Earlier systems
The British Army began serious research in radar systems in 1936, after being introduced to the developments made by the Air Ministry at their experimental station at Bawdsey Manor. Among several possible uses for radar, the Army saw it as a way to address the pressing need to accurately measure the range to aerial targets. This was being handled by optical means that were difficult, time-consuming and error-prone, and a radar system could dramatically improve this task. The development team, nicknamed the "Army Cell", was set the task of building a system that would provide slant range measurements with an accuracy of or better.
The result was an unwieldy system known as the GL Mk. I radar. The Mk. I, like the Chain Home technology it was based on, used separate transmitter and receiver antennas that had to be moved in unison to track targets. The system did not provide an accurate bearing measurement and had no provision for measuring elevation. It did, however, meet the requirement for 50 yard accuracy in range, a measure that was automatically fed into the analog computers that handled the ballistics calculations. Performance of the guns improved immediately; before the arrival of the Mk. It was estimated that 41,000 shells had to be fired to achieve one destroyed aircraft; the introduction of the Mk. I, along with improved training, reduced this to 18,500 by late 1940.
Plans to add bearing and elevation measurements had been planned for a Mk. II version, which would be ready sometime in 1941. As it became clear that the need was more pressing, Leslie Bedford of A.C. Cossor suggested adding an elevation system to the Mk. I to get it into the field as soon as possible. This became the GL/EF system, which reached service in early 1941 and resulted in a huge decrease in rounds-per-kill to 4,100, making AA effective for the first time. Mk. II, which offered slightly higher accuracy, reduced this further to only 2,750 rounds-per-kill when it began to arrive in 1942.
Microwaves The main reason for the unwieldiness of the early GL systems was a side-effect of the radio frequencies they used. GL had been designed in an era when the only electronics available were being adapted from commercial shortwave radio systems, and operated at wavelengths on the order of 5 to 50 m. It is a basic outcome of radio physics that antennas have to be at least half the length of the wavelength being used, which in this case required antennas several metres long.
The Admiralty had been placed in charge of vacuum tube (valve) development for the war effort. They were particularly interested in moving to much shorter wavelengths as a way to detect smaller objects, especially the conning towers and periscopes of U-boats. The Air Ministry's Airborne Group, led by Edward George Bowen, had the problem of desiring antennas small enough to mount in the nose of a twin-engine aircraft. They had managed to adapt an experimental television receiver to 1.5 m, but this still required large antennas that had to be mounted on the wings. At a meeting between Bowen and the Admiralty Experimental Department's Charles Wright, they found many reasons to agree on the need for a 10 cm wavelength system.
Given the support for microwave development from both services, Henry Tizard visited the General Electric Company's (GEC) Hirst Research Centre in Wembley in November 1939 to discuss the issue. Watt followed up with a personal visit some time later, leading to a 29 December 1939 contract for a microwave AI radar set using conventional tube electronics. Meanwhile, the Admiralty's Communication Valve Development Committee (CVD) approached Birmingham University to develop entirely new tube designs that might lead to better results.
Magnetrons
Birmingham's Mark Oliphant initially attacked the problem by attempting to further develop the klystron, a pre-war invention that was one of the first successful microwave-frequency tubes. A klystron produces microwaves by sending electrons past resonant cavities, causing microwave radio energy to be deposited within. This process is not efficient, and multiple resonators are normally used to produce useful output, although this produces very long tubes. Despite many attempts, by the end of 1939 their best klystrons were generating only 400 watts, far below what would be needed for radar use.
Two minor members of the team, John Randall and Harry Boot, had been asked to look at another concept, but this approach also failed to mature. Left with little to do, they begin considering alternative solutions. They struck upon the idea of using multiple resonant cavities arranged in a circle outside a common central core, as opposed to the klystron's linear arrangement. The electrons were bent into a circular path using a powerful magnet. In this arrangement, the electrons pass the resonators many times, producing, in essence, a klystron with hundreds of resonators. Their very first cavity magnetron produced 400 W, and was pushed to over 1 kW within a week. Within months, GEC had models producing pulses at 10 kW. These were soon being used in the design of a new airborne radar system initially known as AIS, for Airborne Interception, Sentimetric [sic].
The Army, meanwhile, had visited GEC on several occasions during 1940 and seen their progress using conventional tube electronics. In a series of steps, GEC had managed to reduce the operating wavelengths from the original 1.5 m to 50 cm, and later to 25 cm. These could be used in a directional system with an antenna a metre or less in size. In contrast to the Airborne Group, which needed even shorter wavelengths to make very small antennas that could fit in the nose of an aircraft, or the Navy, which needed a system with enough resolution to pick up periscopes, the Army was looking only for a practical improvement in accuracy and smaller antennas. This could be met with GEC's solution. The power output of these tubes was too low to initially see the targets as a reasonable range, so the idea emerged to use the new system just for tracking, while another would be used for early warning.
In August 1940, the Army issued a specification for a new GL radar that combined a VHF set like the Mk. II with a microwave-frequency tracking system with high accuracy. It required:
initial pick-up for early warning at 30,000 yards pick-up for guidance at 22,000 yards accurate range to any target between 2,000 and 17,000 yards, although a shorter maximum of 14,000 yards was acceptable a beamwidth of 10 degrees ideally, but 14 degrees maximum elevations from 10 to 90 degrees above the horizon, with tracking possible up to 70 degrees
P. E. Pollard of the Air Defence Research and Development Establishment in Christchurch, Dorset had been one of the first people to consider radar as early as 1930, and had been working with the "Army Cell" at the Air Ministry Experimental Station for some time before joining other Army researchers at Christchurch, Dorset. They selected British Thomson-Houston (BTH), who built the earlier 5 m GL sets, to build a prototype. Pollard moved to BTH's plants in Rugby, Warwickshire to work on the new system.
Klystron failure When Albert Percival Rowe, director of the Air Ministry's radar teams, heard of the Army's efforts in September 1940, he set about creating his own GL development effort using the magnetron. After a 22 September meeting with Philip Joubert de la Ferté, a senior commander in the RAF, Rowe built a GL team under the direction of D. M. Robinson using several members of the AIS team, telling them that they would have to focus on the GL problem for the next month or two.
This led to increasing friction between Philip Dee, who ran the AI team's magnetron efforts, and Rowe, in overall command of the Air Ministry researchers. Dee claimed that Rowe was "seizing this opportunity to try and filch the GL problem from the ADEE" (the Army Cell) and that "only Hodgkin is carrying on undisturbed with AIS, and Lovell and Ward are fortunately engaged upon basic work with aerials and receivers and are therefore relatively undisturbed by this new flap." According to Lovell this did not represent as much of a disruption as Dee believed. Moreover, to some extent, the klystron work at Birmingham had continued because of the Army's GL efforts.
The main issue for adapting the AI radar concepts to the GL problem was that of angular precision. In the case of AI, the radar operator was able to track the target with an accuracy of about 3 degrees, improving to as much as 1 degree at close range. This was more than enough for the pilot to see the target close to dead-ahead once they approached to within about . For gun laying at long range the operators might never see the targets, so precision has to be at least degree, and would allow the guns to be directed solely by the radar.