Searchlight Control, SLC for short but nicknamed "Elsie", was a British Army VHF-band radar system that provided aiming guidance to an attached searchlight. By combining a searchlight with a radar, the radar did not have to be particularly accurate, it only had to be good enough to get the searchlight beam on the target. Once the target was lit, normal optical instruments could be used to guide the associated anti-aircraft artillery. This allowed the radar to be much smaller, simpler and less expensive than a system with enough accuracy to directly aim the guns, like the large and complex GL Mk. II radar. In 1943 the system was officially designated Radar, AA, No. 2, although this name is rarely used.
The sight of searchlights swinging about wildly during the Blitz led a group of British Army engineers to begin development of SLC in early 1940. It was built using the electronics from the 1.5 m wavelength ASV Mark I connected to new antennas and a unique lobe switching system. The American SCR-268 and German Würzburg were generally similar in concept, but the SLC was small enough to be mounted directly to the light, as opposed to requiring a separate carriage. This made the overall operation much easier. There were several Marks of the system, differing in their mounting system, not the electronics.
The effect of using SLC was dramatic; aided by sound locators, in 1939 a searchlight had about a 1% chance of tracking a target, while with SLC this improved to 90%. SLC's greatest success was during Operation Steinbock in early 1944; SLC was involved in the vast majority of interceptions of German bombers, helping guide the night fighters without the need to use their own radars. They proved less useful against the V-1 flying bomb attacks later that year, but by this time improved radars like the SCR-584 were rendering the SLC concept obsolete. The US made a largely identical copy as the SCR-768 while an all-new design based on the same concepts was SCR-668.
SLC also proved useful to the enemy; in early 1942 an SLC and GL Mk. II were captured by Japanese forces at Singapore, along with the US SCR-268 in the Philippines. The SLC's Yagi antennas were unknown to the crews that discovered them, and they were surprised to find they were a Japanese invention. NEC produced a slightly modified version known as Ta-Chi 3, but it did not enter service.
History Searchlight use before radar Before WWII, searchlights generally had two settings. One produced a wider beam a few degrees wide that was used for searching while the second narrowed the beam as much as possible in order to illuminate a single target. At the altitudes flown during World War I, the wider beam still produced enough illumination to detect a target, which the crews would do by swinging around the expected location. This was often aided with a Mark IX sound locator, which could help narrow the search area. When a target was seen, the beam would be narrowed to provide more illumination.
By the opening of WWII, such systems were effectively useless. As combat altitudes moved over , the illumination provided by the wide setting was no longer enough to see a target, but searching for a target using the narrow setting was very difficult. The greatly increased speeds, roughly double that of WWI aircraft, made the speed of sound a major problem for the sound locators. Offsetting these problems was the high quality of pre-war training, and the crews were still able to find their targets at a high enough rate that there did not seem to be a problem.
Things changed when the number of searchlights in service began to increase with the start of the war in 1939. The rapid enlargement of the force by newly trained crewmen had the effect of diluting the competence level. Another problem was that as the lights were moved from their training areas inland to various coastal deployments, they were invariably placed near the guns they would work with. This made the sound locators useless once the guns started firing. At the beginning of the war, searchlights had about a 1% chance of locating a target.
Early Army radar
The British Army was the first group in Britain to suggest the use of radar; a 1931 report by W. A. S. Butement and P. E. Pollard of the Army's Signals Experimental Establishment proposed using it for detecting ships in the English Channel. The Army proved uninterested in their proposal and the matter was forgotten. In 1935, the Air Ministry independently took up the radar concept and began rapid development of Chain Home. The Army was suddenly very interested, and sent Butement and Pollard to set up a lab at the Air Ministry's research site at Bawdsey Manor in 1936.
The group, officially known the Military Applications Section, but referred to universally as the Army Cell, was first set to the task of producing a mobile version of the Chain Home radar, but this effort was soon taken over by the RAF. They were then told to develop a radar for measuring the range to aircraft as an aid for anti-aircraft artillery. Resolution of an antenna system (or any optical system) is a function of its aperture and operational frequency; higher accuracy requires shorter wavelengths or larger apertures. At the time, the available electronics were only capable of working at about 5 m wavelength at a minimum, so while the resulting GL Mk. I had antennas many meters across, it still had an accuracy of only 20 degrees in azimuth.
As the utility of radar became evident, the Army's thinking changed about using the radar to directly guide the guns. This led to the GL Mk. II, which improved performance with larger antennas and a system known as lobe switching. This switched the signal back and forth between two closely spaced antennas, aimed slightly to each side of the centreline of the radar, or line of shoot. This produced two blips on the screen for each target, and the one from the antenna that is closer to the target would be slightly larger. Trained operators could produce accuracies on the order of degree, allowing it to guide the guns directly.
While GL Mk. I was being developed, there was some consideration given to using its outputs to guide a searchlight. Similar solutions had been adopted in the US and Germany, although this was unknown to the British at the time. As the accuracy of the GL system improved, especially with the Mk. II, the need for the searchlight was eliminated. Additionally, the underlying electronics were in short supply, and dedicating a radar set to a searchlight would mean one that not guiding the guns directly.
Meanwhile, the Air Ministry had been working on radar sets that operated at shorter wavelengths in an effort to make a system with antennas small enough to fit on an aircraft. After much effort, by 1938 they had units that were operating reliably at 1.5 m. During early testing, the team noticed that they could detect shipping in the Channel at several miles range, which led to the new concept of that air-to-surface-vessel radar, or ASV. The Army Cell took up this concept in a new radar known as Coast Defense, or CD. At these shorter wavelengths, CD's antennas were small enough to easily rotate to search for targets in a fashion not unlike a searchlight, something that was more difficult with the earlier sets using longer wavelengths. Before these CD sets were deployed, however, the Air Ministry took them over and used them as Chain Home Low to detect low-flying aircraft.
Prototypes With the opening of the war in 1939, Bawdsey's exposed location on the east coast was cause for considerable concern. The Air Ministry teams decamped to Dundee, Scotland, while the Army Cell left for the Christchurch, Dorset area. Christchurch was the location of the Air Defence Experimental Establishment, originally the Searchlight Experimental Establishment, which had moved there from their former location at RAF Biggin Hill in 1939. The radar and searchlight groups were now working more closely together, and administratively merged to form the new Air Defence Research and Development Establishment (ADRDE).
In April 1940, W. S. Eastwood, D. R. Chick and A. J. Oxford worked at the new Army Cell location in Somerford, outside Christchurch. They grew tired of the way "searchlight beams swung wildly about the sky but rarely found and held a target." They proposed developing a radar system for the searchlights, offering to work on it solely in their spare time. Their design was essentially a combination of the electronics from an obsolete ASV Mark I radar set with the display system of the GL Mk. II. Using the ASV's 1.5 m electronics meant they could have the same accuracy as the GL sets using antennas the size. But they did not need the same sort of accuracy; the system only needed to be accurate enough to get the searchlight within about two degrees, at which point the target would appear in the beam and the searchlight operator could guide it the rest of the way optically.
Their first system used a Yagi antenna mounted on the trainable platform from a Mark IX sound locator. The antenna was mounted in the centre of the platform, on a motor that caused it to spin. A Yagi antenna is polarized, meaning it will pick up signals only in one plane, so as the motor spun the antenna it was sensitive only in the plane it was currently aligned.
Switches located around the antenna base were triggered when a cam on the antenna shaft passed them. As it passed the 3 o'clock position the signal was sent into one of the channels of the horizontal display, and when it passed the 9 o'clock, the other. This produced two blips on the same display. Switches at 12 and 6 did the same for the vertical display. When the antenna was not pointed directly at the aircraft, one of the two blips would be larger because the antenna was pointed closer to it. For instance, if the target was slightly to the left of the antenna's line of shoot, the second blip would be larger and the operator would then swing the platform to the left until the blips were equal length.
There were three displays; one for horizontal, one for vertical, and a third that received the signal at all times and was used for measuring the range. In the case where there were several aircraft flying close together, notably in the case when a night fighter was approaching its target, multiple blips would appear and this could result in very confusing displays. This was solved using a solution adopted from the GL radars. The range operator would rotate a large knob on the side of his display, which caused a "strobe", a short line acting as a cursor, to move back and forth under the signal. They would position the strobe under the target they wanted to select, and only signals within that short time window appeared on the bearing and elevation displays. This normally allowed a single target to be picked out.
In practice, the rotating antenna was found to whip around and was not practical. A new version was built with five vertically polarized antennas, four of them as receivers, one in each of the positions formerly held by the sound horns of the Mark IX. The transmitter was a separate antenna placed above, and sometimes behind, the other four. The received signal was sent into a "phasing ring" which delayed each of the four signals a different amount and then changed those delays by rotating it with an electric motor at about 20 RPM. The relative phases of the output produced an angle that the system was sensitive in, which rotated at the same 20 RPM. The resulting output was then sent through a mechanical switch on the same motor, causing the signal to be sent to the four channels as in the rotating antenna case.
Production models