Searchlights - 1940/41

Anti-aircraft searchlights were employed at night for one of two purposes:

  • To illuminate targets for anti-aircraft guns
  • To indicate targets to fighter aircraft
    • Searchlights were at first deployed in a grid layout with 6,000 yards interval between searchlight sites. The idea was that this spacing would sufficiently concentrate beams so at least one searchlight would be able to pick up and illuminate a target. When one searchlight picked up an enemy aircraft, two others from neighboring detachments would also concentrate on it (the so called ‘three beam rule’). The pyramid of light formed would signal the position of the enemy aircraft to the anti-aircraft guns or to the interceptor fighters. The actual illumination of the target was not essential for fighters as long as a beam was close enough to the target to act as a ‘pointer’. An intersection of two beams, preferably three was required as an adequate ‘pointer’.

      The spacing would never achieve the ‘ideal’ in reality due to the need to site searchlights in suitable locations. ‘Ideal’ positions would need to be converted into ‘possible’ positions by selecting positions on a map close to the ‘ideal’ position. A ground reconnaissance would then select the actual position as close as possible to the ‘possible’ position with reference to the following factors:

      • An all round field of view down to an angle of sight not greater than 15 degrees.
      • Level Ground
      • Sited clear of low lying ground and water subject to ground mist
      • Easy accessible
      • Free from extraneous sources of noise e.g. trees, running water, railways etc
      • Clear of sound reflecting surfaces such as buildings and cliffs
        • A typical site would consist of huts for accomodation, offices, workshop, ablutions etc while the searchlight would be sited in a circular earthwork to give some protection.

          Searchlight site personnel were also to act in co-operation with Home Forces to hold the rear belt of resistance – to the last man and last round. Searchlights were to maintain their primary role until prevented by enemy action. Strong points were to be constructed near all searchlight sites and organized as centers of resistance. They were to be wired and dug for all round defence – 60 Searchlight Regt issued orders for circular belts of double apron fences at 40 and 60 yards. Pillboxes were often sited at strong points. In 1940 searchlight detachments were also expected to take on an active role in the immobilization of enemy parachutists by manning a road control post during daylight or at night in an emergency. The Home Guard were to co-operate if possible.

          Type 23 Pillbox, Earl Soham Searchlight site

          It was not long before problems were found with the grid layout – bombers were flying higher and faster and non-reflective paint on the underside of aircraft made it harder to keep targets in the beam. Trials were held during October 1940 to improve the searchlight layout, concentrating on a group of searchlights drawn together (referred to as the ‘Cluster System’). The idea was to replace the current system of intersections along a grid with a system of well defined ‘pointers’ at wide spacing giving greater illumination. The grouping of three searchlights into a Cluster was finally decided upon with intervals in theory of 10,400 yards. It is debatable if the true reason was to improve efficiency or to save on man-power; a Committee set up under Air Marshal Sir Philip Joubert had a term of reference to “pay special attention to the need for minimizing, in the needs of production of munitions for the Forces as a whole, the calls of A.D.G.B {Air Defence Great Britain} upon the man-power of the country, so far as this is compatible with the efficient discharge of that task, and, with this in mind, at a very early stage in its deliberations to review especially the existing searchlight organization”. General Sir F Pile, AA Command, noted that the Cluster system achieved a 20% cut in man-power.

          Above: The original grid layout with Searchlight sites at 6,000 yards. Cluster sites (circled red) were formed by transferring equipment from adjacent sites

          Instructions were issued to 32 Searchlight Regt to group searchlights in the Cluster system from December 4th with six sites initially on an experimental basis. The additional equipment required at each ‘Cluster Site’ was to be provided by transferring equipment from sites most adjacent to the new Cluster Site. Ideally each Cluster site was to have one 150 cm searchlight and two 90 cm searchlights, although often only three 90 cm searchlights were available. Each Cluster site would have 25 personnel, with a Sergeant in charge. Only coastal sites, watching for mine-laying aircraft, were to remain as single station lights.

          There were several problems with the ‘Cluster’ system. The wide spacing often allowed low flying enemy aircraft to weave in-between the clusters. Also it was harder to pass on any target that was illuminated between clusters. Also the ‘Cluster’ system did not fit in well with new developments (e.g. Searchlight Control radar, Ground Control Interceptor radar). As a result, in under a year, instructions were received to ‘decluster’ and reorganize into a system based on ‘belts’ (Indicator belts, Killer belts and Gun zones). In terms of the ground layout this was more or less the same as the original grid intersection layout!

          Other improvements carried out during 1941 in Suffolk included the increased provision of AA defence for Lowestoft. Lowestoft had no HAA defence in 1940 but by 1941 HAA guns along with additional LAA guns and searchlights were provided due to the persistent ‘tip and run raids’ – 60 Searchlight Regt employing 20 single station lights and 69 Searchlight Regt six single station lights in the Yarmouth/Lowestoft area (this Force was known as Bloater Force) .

          The operational orders for Bloater Force stated that no light would expose at an angle of elevation of less than 25 degrees and was to douse when that angle was reached if engaging a target. Lights were also not to expose if cloud cover was low and there was a danger of reflected illumination of the Vulnerable Point. This would be under the instructions of the Force Commander who would stand the Force down in such conditions and notify 41 AA Brigade HQ. Any number of lights could engage targets and searching was allowed provided it was not aimless. All aircraft were to be engaged until identified as friendly.

          Airfields were also provided with searchlights to work in co-operation with the LAA guns, again the number of lights increasing as they became available during the course of the war (see page on Scarecrow and Canopy). For example during May 1941 Wattisham received an additional five 90 cm searchlights in addition to the light already employed. These lights were to be controlled by spotters, sound locators not being employed. The lights were to be manned throughout the night and exposure was only to be on the orders of the Searchlight Troop Commander acting on orders of the Station Commander.

          Other work carried out by searchlights in addition to the two roles outlined above were:

          • To act as beacons for returning aircraft. The War Diary of 32nd Searchlight Battery noted in Dec 1942 an increase in the call for homing beacons as a result in the increased activity on the part of Bomber Command.
          • Illuminate barrage balloons if it appeared that friendly aircraft were in danger of flying into them. For example on the night of March 15th 1942 the barrage balloons at Harwich were illuminated for homing a Hudson which flew through the barrage at 22:08 hrs, hitting a cable but landing safely at Martlesham.
          • Increasing the effectiveness of decoy sites by working in conjunction with them.
            • Some Search lights were established as Coastal Searchlights to illuminate enemy mine laying aircraft and fixing the position of mines (which made the work of minesweepers safer). They could also aid Air Sea Rescue by passing back information back to Fighter Command resulting in a much quicker time that the rescue launch was sent out. Air Sea Rescue was established after the Battle of Britain as a result of the many airman that died of hypothermia after bailing out into the Channel, with little or no chance of rescue at the time.

              References
              Ack-Ack, General Sir F Pile G.C.B.,D.S.O,M.C, Panther edition, 1956
              AA Command, C Dobinson, Methuen, 2001
              Manual of Anti-Aircraft Defence Vol II Employment, Pamphlet No 1, WO 1940
              32 Searchlight Regt papers, TNA
              69 Searchlight Regt papers, TNA
              5th AA Brigade papers, TNA

Type 23, earl Soham, Suffolk
CLUSTER
Searchlights 1940

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