Radar, Gun Laying, Mark I, or GL Mk. I for short, was a pre-World War II 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 of its targets. The name refers to the radar's ability to direct the guns onto a target, known as gun laying.
The first GL set was an elementary design developed from 1936 onward. Based on the early Chain Home radar's electronics, 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 transmitted signal was quite wide, in a fan shape about 120 degrees across. This made it useful only for measuring slant range information; target bearing accuracy was approximately 20 degrees, and it could not provide any elevation information. Several were deployed with the British Expeditionary Force and at least one was captured by German forces during the Dunkirk evacuation. The subsequent German evaluation led them to believe that British radar was much less advanced than German radar.
Plans to address these shortcomings were under way even as the first Mk. I units were reaching service in 1939, but these Mk. II units would not be available until 1940 at the earliest. An expedient solution was the GL/EF attachment, which provided 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 tenfold 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 German 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. By the end of the production run, 1,679 Mk. IIs had been produced.
The introduction of the cavity magnetron in 1940 led to a new design effort using highly directional parabolic antennas to allow accurate ranging and bearing measurements from much smaller antennas. These GL Mk. III radar units were produced in the UK as the Mk. IIIB 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 of these were replaced by the superior SCR-584 starting in 1944.
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 as 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 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 possible uses in Army roles, leading to the 16 October 1936 formation of the Military Applications Section, 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 was 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 the chances of successfully engaging an aircraft. They were given the goal of producing a range measure accurate to within at a range of . In keeping with Army nomenclature, the concept was given the name "gun laying radar" due to its role in aiding gun laying (GL) of the anti-aircraft artillery.
That same year, an Airborne Group had been spun off from the main CH development team to develop a much smaller radar system suitable for mounting in large aircraft. This would become the aircraft interception radar role (AI), the intention being to detect bombers at night and allow the heavy fighters to find and attack them with their own radar. When these sets demonstrated the ability to easily pick up ships in the English Channel, the Army Cell started a second group to adopt these systems to the Coast Defence role (CD), providing both range and angle measurements with enough accuracy to blind-fire their shore batteries. This team was led by Butement, leaving Pollard as the primary developer of the gun laying systems.
Mk. I development The GL effort was started very early during Chain Home development, and was deliberately based on as much of its technology as possible. Unlike Butement's earlier experimental systems, CH was based on existing electronics from commercial shortwave radio systems, as these were among the highest power radio systems. The downside of this approach was that radio antennas generally have to be a significant fraction of the radio signal's wavelength to work with reasonable gain, the half-wave dipole being a common type. For the 50 metre wavelengths initially used by CH, antennas on the order of would be needed.
This was not practical for any sort of mobile system, but as newer electronics arrived through the late 1930s, the wavelengths being used by the radar systems continued to drop. By the time GL was ready to begin testing, the system was able to operate at wavelengths between reducing the antenna size to a more manageable several-metre length. Similar changes in electronics also produced smaller versions of CH, the Mobile Radio Units or MRU's, which were used both as a mobile early-warning system and a backup service in case a main CH station was knocked out.
CH-type radar displays use a time base generator to produce a sawtooth wave voltage that is fed to one of the inputs of a cathode ray tube (CRT). The time base is calibrated to move the CRT dot across the screen in the same time that echoes would be returned from objects at the radar's maximum range. The dot moves so rapidly that it looks like a solid line. The return signal is amplified and then sent into the CRT's other channel, typically the Y-axis, causing the spot to deflect away from the straight line being created by the time base. For small objects, like aircraft, the deflection causes a small blip to appear on the display. The range to the target can be measured by comparing the location of the blip to a calibrated scale on the display.
The accuracy of such a display is relative to the size of the tube and the range of the radar. If one might be expected to measure the blip to an accuracy of 1 mm on the scale along a typical CRT, and that radar has a maximum range of 14,000 yards, then that 1 mm represents, just over . This was far less accuracy than desired, which was about .
To provide a system able to make such an accurate measurement, and do so continually, Pollard developed a system that used the entire CRT display to provide a measurement showing only ranges a short distance on either side of a pre-selected range setting. The system worked by charging a capacitor at a known rate until it reached a threshold that triggered the time base. The time base was set to move across the screen in a time that represented less than a kilometre. A large potentiometer was used to control the charging rate, which provided a range offset. The range to the target was measured by using the potentiometer to move the blip until it was in the middle of the display, and then reading the range from a scale on the potentiometer. The basic system developed rapidly, and a test system was providing accuracy for aircraft between and by the summer of 1937. By the end of the year, this had improved to as accurate as .
As the original requirement for the system was to provide additional information to optical instruments, accurate bearing measurements were not required. The system did need some way to ensure that the target being ranged was the one being tracked optically, and not another nearby target. For this role, the system used two receiver antennas mounted about one wavelength apart vertically, so that when they were pointed directly at the target the received signals would cancel out and produce a null on the display, where the signal disappeared. This was sent to a second display, whose operator attempted to keep the antennas pointed at the target.
The transmitter, which produced short pulses of about 20 kW, was mounted in a large rectangular wooden cabin on a wheeled trailer. The single half-wave dipole antenna was mounted on a short vertical extension at one end of the cabin, with the "line-of-shoot" along the long axis. The antenna was only marginally directional, sending the signal out in a fan shape about 60 degrees on either side.
The receiver was considerably more complex. The operator's cabin was somewhat smaller than the transmitter, and mounted on the AA gun carriage bearing system which allowed the entire cabin to be rotated around the vertical axis. A short distance above the roof was a rectangular metal framework roughly matching the outline of the cabin. Three antennas were mounted in a line down one of the long sides of the framework; range measurements were taken off the antenna in the middle, and directionally by comparing the signal on the two antennas at the ends. Behind the two bearing antennas were reflectors mounted about a wavelength away, which had the effect of narrowing their reception angle.
In the field, the transmitter would be aimed in the expected direction of attacks, and the receiver placed some distance away to help protect it from the signal being reflected off local sources.
Initial deployment By 1939 the team was happy enough with the state of the equipment that production contracts were sent out. Metropolitan-Vickers won the contract for the transmitter, and A. C. Cossor the receiver. Mass-producing the GL set did not prove particularly difficult, and by the end of 1939, 59 complete systems had been delivered. Another 344 would be completed during 1940.
The system did exactly what had been asked of it; it provided range measurements with accuracy on the order of 50 yards. In the field it became clear that this was not enough. By late 1939 the spectre of night bombing was a major concern, and as the GL system could not provide accurate bearing information, and no elevation, it was unable to direct the guns at night. Instead the World War I style of operation was used, with searchlights hunting for targets largely at random, and conventional optical instruments being used to determine bearing and elevation once a target was lit up. In practice this proved just as ineffective as it had during World War I.
Despite spending considerable time, effort and money on the GL system, when The Blitz opened the entire Army air defence system proved to be ineffective. General Frederick Pile, commander of the Army's Anti-Aircraft Command, put it this way: