60-inch searchlight and M2 acoustic sound locator with their crews.  The sound locator detected the approach of an enemy airplane and helped train the searchlight in the proper direction for illumination of the advancing target.  The traditional searchlight and sound locator tandem would be divorced when new radar technology became available.

      No other component of Antiaircraft Artillery experienced such a change of mission during the course of World War II as did the searchlight units.  At the onset of the war, their mission was to detect, locate and render vulnerable enemy aircraft.  The searchlight battalions worked closely with the Antiaircraft Artillery Intelligence Service (AAAIS) to provide timely and valuable information on the position and movement of potentially hostile aircraft.  The bright beams of the searchlights illuminated the night-flying enemy, enabling aircraft to be visually tracked and engaged by antiaircraft guns.  Although the searchlights continued in this role throughout the war, new technologies brought alterations and permutations to their traditional employment.

SEARCHLIGHTS

     Manufactured by both General Electric and Sperry Gyroscope, World War II Army searchlights were high-intensity carbon arc units designed with a 60-inch parabolic mirror.  The searchlights were capable of yielding an 800 million candlepower beam.  The parabolic mirror focused the light in parallel rays, producing a narrow beam with a 1.25° arc. It was discovered that by moving the lamp head just a few inches closer to the mirror from the focal point, the searchlight's beam could be spread to a maximum arc of 15°.  Spreading the beam affected both the intensity and range of illumination.  This spread beam feature was useful for engaging fast, low-flying aircraft or utilizing the illumination for non-antiaircraft work.  The 15° spread was still able to illuminate targets at up to 1,000 yards.  The maximum slant range of a searchlight's narrowest beam was about 15,000 yards (roughly 8.5 miles).  Ultimately, the effective range of illumination was dictated by atmospheric conditions and the size, shape and reflectivity of the target, and not just the candlepower of the searchlight.

     The basic fire unit of the antiaircraft searchlight service consisted of a searchlight, control station, power plant, .50 caliber machine gun, control cables and the vehicles needed to transport the equipment and personnel.  Half of the fire units included a detector (either an acoustic sound locator or radar set) and were organized into detector-searchlight sections.  A headquarters section plus six fire units (three with detector sections) comprised a searchlight platoon.  There were two platoons per searchlight battery, along with a headquarters element.  A headquarters battery and three searchlight batteries constituted an AAA searchlight battalion.

     Searchlights did not randomly comb the skies in an attempt to illuminate an aircraft by chance.  A light was always paired with a detection device in the detector-searchlight section.  Early in the war, acoustic sound locators were utilized to find the position of approaching aircraft.  These mechanisms were soon displaced by the new SCR-268 radar sets.  The detector equipment, whether acoustic or radio, trained the searchlight's beam toward the most probable location of an airplane in anticipation of its arrival.  The searchlight section carefully examined the beam for any indication of an aircraft. 

     The visibility of a target was dependent on the contrast between the illuminated aircraft and the searchlight beam.  Detected airplanes were more visible to viewers standing farther away from the searchlight.  Therefore, a method of remote control, referred to as distant electric control, was developed to increase the efficiency of searchlight operation and target discovery.  In this technique, the searchlight and detection device (either acoustic sound locator or radar) were tied together through a central control station.  From this position, at least 50 feet from the light, the chief controller was able to govern the movement of the searchlight's beam in both azimuth and elevation.

     During action, the detection device would help train the searchlight in the general direction of the intended target by supplying position information to the control station.  By turning handwheels on opposite sides of the control station, azimuth and elevation controllers would carefully keep a pointer centered on the zero index of an indicator mounted above their respective handwheels.  By this action, the operators kept the searchlight in motion corresponding to the data being supplied by the sound locator or radar.  Since the beam seldom fell on the exact location of an enemy aircraft, the two controllers would, on command, move the pointers to either side of the the zero index.  This forced the beam to sweep the sky around the predicted position while the chief controller looked for the aircraft through night binoculars.  When the plane was spotted in the searchlight beam, the chief controller would call out, "Flick!"  From that moment, the azimuth and elevation operators would step back from the control station and allow the chief controller to guide the beam from his station.  The target, now illuminated, could be challenged by nearby antiaircraft gun batteries.

Distant electric control  (DEC) of searchlights was achieved through the DEC control station.  The station was setup within 50 to 500 feet of the light.  The azimuth and elevation operators are keeping the searchlight pointed toward the target, while the chief operator is watching through the binoculars for the enemy aircraft to appear in the beam.   The crew pictured on the left is on duty in Iceland in 1942.  The daytime photo on the right better illustrates the binocular mount and chief controller's position.  An open sight is fixed above the binoculars to aid in orienting the device with the other components of the searchlight system.

Rolling out a searchlight into position.  Towed by a 2-1/2 ton truck, the M1 searchlight trailer was equipped with hinged drawbar arms that allowed the trailer to tilt to facilitate easier loading and unloading of the light.  A hand-operated winch at the front of the trailer aided this process.

     A searchlight section was under the command of a chief of section and included an azimuth controller, elevation controller, chief controller, searchlight operator, power plant operator, and radio operator.  A machine gunner was designated from among these men. 

     The personnel of a detector section varied depending on the type of equipment used.  The various models of radar and acoustic sound locators had their own requirements.

     A typical antiaircraft searchlight defense consisted of an outer ring of pickup lights with detectors and an inner circle of carry lights.  The outer searchlights detected the approaching aircraft, then handed the illumination off to the units closer to the intended target.  This arrangement kept the hostiles illuminated longer and bought more firing time for the AA gun batteries.

The power plant supplied electrical power for the entire section.  Pictured here is the M1941 plant.  The generator trailer used standard automobile tires and could be towed for short distances over good roads.  Otherwise, the unit was placed aboard a truck for longer movements. The instrument panel is on the right and the gasoline-powered engine is housed in the compartment to the left.  Gasoline, oil , and coolant levels are all checked in preparation for action.

SEARCHLIGHT EMPLOYMENT

     As radar made visual tracking of the enemy unnecessary, searchlight units began to find jobs beyond their customary craft of illuminating enemy airplanes.  In fact, in the European Theater of Operations, there is not a recorded instance of a searchlight unit being employed to light up a hostile flight for 90mm antiaircraft gun engagement. 

     Many searchlight units were assigned to provide homing beacons to aid friendly aircraft in locating airstrips.  There were several configurations of beacons employed, including a daylight system that used amber, yellow, red and green filters over the searchlight lens to assist fliers hampered by low visibility and poor weather conditions.  Searchlights were also used to support the Corps of Engineers in construction of facilities and bridge crossings.  The engineers' work could continue all night with the proper placement of a few searchlights set at maximum spread.  In the Pacific Theater, at least one searchlight battery mounted a few of their lights on PT boats and successfully illuminated Japanese troops along hostile shorelines, thereby providing visible targets for Navy gunners. 

     One of the more innovative uses of antiaircraft searchlights was the creation of artificial moonlight to support infantry troops in night operations.  Searchlights would bounce their beams off a cloud base, reflecting diffused light back to the ground.  When this type of battlefield illumination was first attempted, infantrymen and their commanders recoiled and demanded that the lights be shut down.  They feared that the bright light exposed their locations to the enemy.  It was soon realized that the searchlights revealed enemy positions while providing shadows to conceal American troops.   Searchlights soon became one of the infantryman's best friends.  Artificial moonlight was a success.

The concept of artificial moonlight simply illustrated.  Searchlights illuminated the enemy while friendly forces moved in the shadows.  Battlefield illumination accounted for nearly one-third of the missions assigned to searchlight battalions in the European Theater. (Drawing taken from IX Air Defense Command : Historical & Statistical Summary, 1 January 1944 - 1 June 1945.)

SOUND LOCATORS

     Seconds mattered in antiaircraft work.  It was critical for antiaircraft gun batteries to immediately engage hostiles when they flew within range.  Accuracy on the firsts shots was paramount.  Searchlights had no time to sweep the vast expanse of sky, hoping to catch the flick of an enemy aircraft in their beams.  Searchlights needed to anticipate the appearance of hostile targets.  How was this seemingly impossible task accomplished in the days before operational radar?  The answer was a rather improbable device called the sound locator.

     An acoustic sound locator can be thought of as a giant hearing aid.  Horns were used to amplify and focus distant sounds and make them more audible to the operators.  In addition to this basic amplification, the sound locator was designed to utilize the binaural property of human hearing to render a spatial dimension to the sound.  This is the same concept that gives depth to a stereo recording and makes the soundtrack in a movie theater so enjoyable.  The auditory perception of the sound locator operators helped determine the position of an aircraft in space based on the distant drone from its engines.  The locator then remotely trained the searchlight in the proper direction through the fire unit's control station.

M2 sound locator in action.   The chief of section is receiving information from outposts and other searchlight sections via field telephone.  He directs his sound locator crew based on information received from these other elements.  "Listen in that direction." A platform acoustically decoupled the operators from the ground and enabled leveling of the locator.  It was important to maintain sound discipline in the vicinity of the locator.  The softest noises could distract the operators from their task of determining the apparent location of enemy aircraft. Two men listened while a third operated the compensating mechanisms designed to make the M2 more accurate.

     The M2 sound locator was the standard for most searchlight units at the beginning of World War II.  This three-horn unit was broken down into six components for easy assembly, disassembly and transport.  It incorporated newer materials and an updated design to reduce interference from ambient sounds, whether environmental or from noise induced by its own mechanisms.  Two operators stood on a platform wearing leather head caps with acoustic tubing attached to each ear.  The men tracked the perceived azimuth and elevation of the far-off sounds by turning handwheels.  Built-in mechanisms compensated for sound lag errors caused by the relatively slow speed of sound.  Corrections were also provided for distortions caused by wind and placement of the locator relative to the searchlight.

     The effective range of the M2 sound locator was considered to fall between 2,000 and 8,000 yards.  Range was very dependent on atmospheric conditions and the well-being of the human operators, and could vary widely from day to day.  With the advent of radio-based detection and the availability of the SCR-268 radar sets for searchlight control, the advantages of the M2 sound locator all but evaporated in comparison to the developing technology.  Radar had a greater reach, was not dependent on human senses, and was able to function in all but the most severe weather conditions.  The sound locator was rapidly replaced by radar, but some searchlight organizations retained the M2 throughout the war and occasionally used the old locators to backup or augment local radar sections.

Unusual fashion wear, except for a sound locator operator.  Large tubing that attached to a leather head cap allowed these men to listen for the approach of distant aircraft.  Keeping the sounds centered was the key to effective tracking.   Soldiers were selected for this job based upon their attention span and sensitive hearing.  Rural boys, who were accustomed to quieter surroundings, were often best for this specialty.  The constant background noise of the larger metropolitan areas rendered the ear less sensitive to the auditory clues vital to the accurate operation of the sound locator.   To aid the operators, binaural training devices were developed that used phonograph recordings to simulate the sound of approaching aircraft.  They gave the operator a good sense of what to expect while on duty.

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