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.

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.

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.

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.

[5] The group, officially known the Military Applications Section,[6] but referred to universally as the Army Cell,[7] 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.

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.

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.

[3] 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.

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).

[10] 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.

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.

Due to the way the operators had to "hunt" for the angle that gave equal returns between the two blips, the radar tended to lag the target's movements.

On 23 April 1941, a secret trial called the Newark Experiment was carried out to see if women of the Auxiliary Territorial Service (ATS) could fill these roles, as there was the concern they would not handle the stress of operating the machinery and living in desolate locations.

For instance, on the night of 8/9 September 1942, a Mosquito flown by Flight Lieutenant Henry Bodien was asked to follow a target, but given strict orders not to fire as it was likely a lost "friendly".

From sixty yards range it became possible to identify it as a Do217 with black crosses and a number visible on the green camouflaged underside of the wings.

The same problems that plagued the early GL units in UK service appeared here; ground reflections rendered the display largely useless, and the resolution was too poor to guide the guns.

These were the models placed on Mark IX sound locator frameworks, which originally drove the searchlight's direction through an electromechanical system.

By moving to microwave frequencies around 10 cm, the resolution of the radar was so improved that an antenna smaller than the 150 inch searchlights had enough accuracy to directly lay the guns.

[25] SLC had been rushed to service and proved somewhat unreliable at first as a result, but improvements were not carried out due to its expected outright replacement.

This would be smaller, less susceptible to jamming, and, due to its narrower beam, it would work better at low angles and able to pick out single targets in a group.

[26] Auto-follow, also known as radar lock-on, is a system that uses small differences in the signal received on two antennas or two shortly separated times in order to determine the location of the target within the beam.

Luckbrook experimented with the Mark VI mounting and added a system to use its signals to track the target automatically, reducing the crew to one and only as a backup.

[27] In July 1942 the Ministry of Supply inquired about two alternatives; in one the operator used a strobe control to pick out a target and then used a joystick to move the SLC according to the other two displays, the other was a full auto-follow system.

In September, they also suggested the ADRDE team consider using the unit built by their Air Ministry counterparts at the TRE for airborne radars.

[28] A further run of magnetron-based prototypes as Mark 8[a] was sent to BTH, but they initially refused to build them due to the small number of units and great demand for other systems.

The first examples of the Mark 8 did not begin to arrive until February 1945,[28] but were used by the 21st Army Group with some success in northern France, Belgium and Holland.

[29] The Army began to repurpose their SLC systems to support light anti-aircraft guns at airfields and other point targets.

As part of the general wind-down of AA in favour of guided missiles like the English Electric Thunderbird, all remaining SLC systems were removed from service in 1955.

Using a dial on the right side of the display, they could move a strobe back and forth along the signal, allowing them to position it under a single blip to select it.