page_title: GL Mk. I radar
section_title: Calibration problems
hierarchical_section_title: GL Mk. I radar / Development / Calibration problems
caption_attribution_description: English: Aerial photo of the gun laying mat on the south side of Moor Lane at Cleadon, Jarrow taken on 27 March 1946 by the RAF. The GL mats were installed starting in early 1941 to correct a problem noticed when using the British Army's early GL Mk. I radar systems. The signal was sent out in a broad pattern and would strike the ground in front of the station to be reflected back in the sky. The same problem occurred on reception, with signals received directly as well as reflecting from nearby ground. The mixed signal was randomized by the lay of the land, which made calibration almost impossible. The mats provided an optically flat area that solved this problem. The mats consisted of 240 rolls of 2-inch chicken wire suspected on the ground about 5 feet on a cats-cradle of metal wires supported on wooden poles. The outline of the mat is the octagonal shape centred in the image. The white square object in the centre is a platform that raised the receiver antenna to the right altitude above the mat. The rectangular portion running to the top right is a ramp that allowed the carriage-born antenna to be backed up onto the platform. The round objects in the upper right are the AA gun emplacements. The radar equipment is missing in this photo. Either the site has been deactivated, or it has been replaced by a more modern radar system that did not require the mat. The grey rectangular object in the upper centre-left may be a GL Mk. III, or more likely a US SCR-584.
context_page_description: Gun Laying radar, 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 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.
context_section_description: It was at this point that serious problems with calibration appeared. After considerable study, using reflectors hung from balloons and testing against the occasional aircraft, it became clear that the main problem was the levelling of the ground around the station. The long wavelengths used in these early radars strongly interacted with the ground, causing the beams to be reflected forward as opposed to absorbed or scattered. These reflected signals sometimes reached the targets and were returned to the receiver, along with the ones direct from the transmitter. Interference between the two caused nulls to appear in the reception pattern, which made it difficult to find the target.
  In practice, these nulls, especially in elevation, would move about when the antennas rotated to track a target. At first, it was believed that this would not be a serious problem and that it could be addressed by developing a calibration table for each site. But even the very first tests demonstrated that the calibration changed with wavelength. This meant that they would either have to make multiple calibration tables, one for each radar, or that if a single table of corrections for different bearings was desired, the antennas would have to be moved vertically as the wavelength was changed.
  Once again, it was Bedford who suggested a solution; instead of calibrating the radar, he suggested calibrating the ground itself, flattening the area around station through the use of a metal wire mat. Actually designing such a system fell to Nevill Mott, a physicist who had recently joined the Army Cell. The proper dimensions were ultimately found to be a 130-yard (120 m) diameter octagon of 2-inch (5.1 cm) square wire mesh. This was supported in the air by hundreds of tensioned wires running over wooden stakes about 5 feet (1.5 m) in the air. To get the proper clearance between the antenna and the wire ground mat, the radar system had to be raised into the air on blocks, and was accessed via a wooden catwalk.
  The effort to equip UK-based GL sets with these ground mats was enormous. Each mat consumed 230 rolls of wire mesh, each one 4 feet (1.2 m) wide by 50 yards (46 m) long. In total they covered an area of about 15,000 square yards (13,000 m²) and used up 650 miles (1,050 km) of wire – not including the 10 miles (16 km) of wire used in the support structure below the mesh. They initially planned to install the mats at 101 sites immediately, but by December 1940 they had consumed over 1,000 miles (1,600 km) of galvanized wire, using up the entire nation's supply of the material and causing a countrywide shortage of chicken wire.
  Construction of the mat took about 50 men four weeks to complete. By the end of January 1941 only 10 sites had been upgraded, and all the while new AA emplacements were being set up so that the number of prospective sites was increasing more rapidly than they could be completed. By April, Pile had concluded that 95% of the AA sites would need the mats, and they expected 600 sites to be operational by March 1942. The program ultimately ran on for years, petering out as new systems were introduced that did not require the mats. The mat program formally ended in March 1943.
  Another problem, never wholly solved, was that any balloon barrage in the area would form a powerful reflector rendering anything behind it invisible. This was particularly annoying as the balloons were often placed nearby the AA guns as the two systems were used together to protect high-value targets. A solution was considered in the form of a system that would allow low-lying reflections to be eliminated, but this was not fully developed.
caption: Aerial photo of a gun laying mat installed on the east coast, north of Sunderland. The ramp and platform at the centre are prominent.
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